sustainability and success monitoring in protection forests2 sustainability and success monitoring...

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> Sustainability and success monitoring in protection forests

Guidelines for silvicultural interventions in forests with protective functions

> Protection forests> Environmental studies

2707

Environmental studies Protection forests

> Sustainability and success monitoring in protection forests

Guidelines for silvicultural interventions in forests with protective functions

Authors: Monika Frehner, consulting firm, Sargans; Brächt Wasser, consulting firm IMPULS, Thun; Raphael Schwitter, Centre for Mountain Forest Management, Maienfeld

Translated by: Peter Brang and Christian Matter, Federal Research Institute WSL, Birmensdorf; Assisted by Silvia Dingwall, Nussbaumen

Project part-financed by the European Union

> >

Published by the Federal Office for the Environment FOENBern, 2007

2 Sustainability and success monitoring in protection forests (NaiS)

Impressum

Signifi cance of this publicationThis publication is a partial translation of the implementation guide „Nach

haltigkeit und Erfolgskontrolle im Schutzwald NaiS“ published by the Federa

Offi ce for the Environment (FOEN). The translation has been realised in the

frame of the Interreg III C project „Network Mountain Forest“, and with its

fi nancial support.

Issued byFederal Offi ce for the Environment (FOEN), CH-3003 Bern

www.umwelt-schweiz.ch

FOEN is an offi ce of the Federal Departement of Environment, Traffi c, Energy

and Communications (DETEC)

AuthorsMonika Frehner, consulting fi rm, Sargans

Brächt Wasser, consulting fi rm IMPULS, Thun

Raphael Schwitter, Centre for Mountain Forest Management, Maienfeld

Original publication with contributions by:Jacques Burnand (Appendix 2, site classifi cation)

Gabriele Carraro (Appendix 2, site classifi cation)

Rolf Ehrbar (Appendix 8, case study Weesen – Amden SG)

Hans-Ulrich Frey (Appendix 2, site classifi cation,

including drawings of idealised stand profi les)

Werner Frey (Appendix 1, Avalanches)

Werner Gerber (Appendix 1, Rockfall)

Peter Lüscher (Appendix 2, Soil science basics)

Fredy Zuberbühler (Appendix 8, case study Ritzingen VS)

Kaspar Zürcher (Appendix 1, Landslides and Torrents / Floods)

FOEN consultantAndré Wehrli, FOEN

Arthur Sandri, FOEN

Steering group for the original publication- Harald Bugmann, Professor for Forest Ecology, ETH Zürich

l Werner Frey, Swiss Federal Institute for Snow and Avalanche Research SLF,

Davos (until 2001)

Walter Schönenberger, Swiss Federal Institute of Forest, Snow and Landscape

Research WSL, Birmensdorf (since 2002)

Jean-Jacques Thormann, Federal Offi ce of the Environment, Forests and

Landscape

Jürg Walcher, Cantonal Forest Administration, Glarus

Suggested form of citationFREHNER, M.; WASSER, B.; SCHWITTER, R., 2007: Sustainablity and success

monitoring in protection forests. Guidelines for managing forests with protec-

tive functions. Partial translation by Brang, P.; Matter, C. Environmental Stu-

dies no. 27/07 Federal Offi ce for the Environment (FOEN), Bern, 29 p. + 26 p.

Appendix.

TranslationPeter Brang and Christian Matter, Swiss Federal Institute of Forest, Snow and

Landscape Research WSL, Birmensdorf

Assisted by Silvia Dingwall, Nussbaumen

OrdersFOEN

Documentation

CH-3003 Bern

Fax +41 (0)31 324 02 16

[email protected]

www.environment-switzerland.ch

Order number and price:

UW-0727-E/CHF 10.– (incl. VAT)

The original publication is available in German, French and Italian.

(Order number VU-7005-D/F/I)

© FOEN, 2007

3NaiS

AbstractPreface Acknowledgements

1 Introduction 1.1 Overview 1.2 The project Sustainability and success monitoring in protection forests

2 What does sustainable management of protection forests mean? 2.1 Forest management and protection effect 2.2 Seven principles

3 Determining the need for action 3.1 Principles 3.2 The target profi les 3.3 Target and treatment types 3.4 Decision-making procedure on indicator plots 3.5 Deciding on interventions for a planning area

4 Requirements for a planning scheme 4.1 The network of indicator plots 4.2 Bases and preconditions for planning

5 Success monitoring 5.1 Goal and overview 5.2 Implementation assessment 5.3 Effectivity analysis 5.4 Silvicultural monitoring 5.5 Target review

6 Legal bases

Appendix

1 Natural hazards

Contents

Contents

5NaiS Abstract

Abstract

This publication is a partial translation of the implemen-tation guide Nachhaltigkeit und Erfolgskontrolle im Schutz-wald NaiS. This guide is a practical tool intended to ensure a permanently effective protection forest at a minimum cost. The partial translation is intended to promote the basic principles of silvicultural decision-making in protection forests. These principles are explained in the fi rst section. Based on the assumption that the state of a forest is cru-cial to its ability to provide effective protection against natural hazards, silvicultural target profi les are described for different site types and natural hazards. The target profi les for natural hazards are explained in some detail in Appendix 1. The profi les for the site types have not been

translated since they are very specifi c for mountain forests found in Switzerland. The procedure for determining the need for action in indicator plots can also be used as an aid in the planning of protection forest management. Success monitor-ing includes an effectivity analysis on indicator plots to test silvicultural interventions as well as a target review to ensure that new insights from research and practice are included in the target profi les.

Keywords: Sustainability, protection forest man-agement, natural hazards, risk management, success monitoring, controlling

7NaiS

Preface

Preface

This publication is a partial translation of the implemen-tation guide Nachhaltigkeit und Erfolgskontrolle im Schutz-wald NaiS (Sustainability and success monitoring in pro-tection forests) that has been realised within the frame of the Interreg project III C Network Mountain Forest. The partial translation is intended to promote the basic principles of silvicultural decision-making in protection forests as described in the implementation guide.

The NaiS system conveys current knowledge about forest effects and the protection from natural hazards provided by forests, in a form suitable for practical use. It was developed in close collaboration with researchers and practicioners.

Success monitoring in a protection forest is diffi cult because growth processes in mountain forests are slow and natural hazards have irregular frequencies. The NaiS system recommends a form of success monitoring based on three aspects. These are: an implementation assessment, an effectivity analysis on indicator plots to check the long-

term effects of silvicultural measures, and a target review. With these tools, it should be possible to demonstrate the effects of protection forest management and the effi cient use of public funding, thus justifying better the need for such funding. However, the guidelines function only if im-plemented on site by trained individuals. They cannot replace know-how, observation, professional judgment and deci-sion-making skills.

Many thanks to all the authors, those working in the fi eld, in research, in teaching, in administration as well as other areas, for contributing to these farsighted guidelines. I also wish to thank those who carefully translated these guidelines into English to make them available to an inter-national audience.

Federal Offi ce for the Environment Andreas GötzDeputy director

9NaiS

Acknowledgements

The project Sustainability and success monitoring in protection forests has been carried out at the request of FOEN with support from the Cantons and the Centre for mountain forest management. The resulting guide became possible through extensive teamwork involving many experts and institutions during four years. Colleagues working in the

fi eld, administration, education and research have attended conferences, workshops and fi eld excursions, with the aim to draw from practical experience and research and develop a product for practical use. The editor would like to thank all those who have contributed to this project, in particular:

the project team: Dr. Monika Frehner, Brächt Wasser and Raphael Schwitter

the advisory working group: Prof. Dr. Harald Bugmann, Werner Frey, Dr. Walter Schönenberger, Jean-Jacques Thormann and Jürg Walcher

many experts and colleagues from administration and practice: Dr. Frederic Berger, Dr. Uehli Bühler, Dr. Jacques Burnand, Gabriele Carraro, Dr. Hansueli Frey, Heinz Nigg, Dr. Dani Rüegg, Dr. Reinhard Schnidrig and Kaspar Zürcher

many researchers at WSL and SLF, in particular: Dr. Peter Bebi, Albert Böll, Dr. Peter Brang, Marco Conedera, Dr. Philippe Duc, Werner Gerber, Dr. Peter Lüscher, Christian Rickli and Dr. Josef Senn

those responsible for the case studies in Amden-Weesen and Ritzingen: Dr. Rolf Ehrbar and Fredy Zuberbühler

the members of the Swiss Mountain Forest Management Group (Schweizerische Gebirgswaldpfl egegruppe GWG)

the members of the Jura Mountain Silviculture Group (Groupe Jurassien de Sylviculture GJS)

the members of the Expert Group for Natural Hazards (Fachgruppe für Naturgefahren FAN)

the members of the Swiss Working Group for Forest Planning (Schweizerischer Arbeitskreis für Forsteinrichtung SAFE)

the forest rangers and forest engineers who supported the fi eld workshops.

Acknowledgements


Protection forest Success monitoring

Sustainability and sucess monitoring in protection forests

Site typesNatural hazards

Target profi les

Target types

Treatment types

Indicator plots

Deciding about the need for action

on indicator plots

Implementation of interventions

Sustainably effective protection forest Silvicultural monitoring

Implementation assessment

Effectivity analysis

Target review

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These guidelines are intended to help practitioners interested in ensuring that pro-tection forests are sustainably effective at minimal cost.

Protection forest management is based on the assumption that there is a direct link between the state of the forest and the level of risk. Therefore, depending on the natural hazards and on the local site conditions, target profi les for forests are defi ned to provide the best possible protective effect.

All stands with the same target profi le belong to the same target type.Target types are subdivided into treatment types according to the current state of the forest. An indicator plot is a forest area representative of a treatment type.

The need for action is derived on indicator plots from comparing the current state of the forest with the target profi le, taking into account natural forest development.

Interventions are implemented according to the results of the assessments of the indicator plots. Goal-oriented protection forest management requires a network of indi-cator plots and basic information from forest planning.

The aim of success monitoring is to make a protection forest sustainably effective as effi ciently as possible.

The implementation assessment ensures that the planned measures have been professionally carried out at the right sites.

The effectivity analysis uses the indicator plots to check how the measures implemented and the intentional omissions have affected the state of the forest. This is therefore a form of process control.

Silvicultural monitoring checks to what degree the state of the forest meets the target profi le.

The target review helps to clarify whether the target profi les used are appropriate or not.

Chapter 2

Chapter 3.2Natural hazards, Appendix 1Site types, Appendix 21

Chapter 3.3

Chapter 3.4Forms, Appendix 41

Chapter 4Use of timber, Appendix 71

Chapter 5

Chapter 5.2Implementation assessment, Appendix 31

Chapter 5.3Effectivity analysis, Appendix 31

Chapter 5.4

Chapter 5.5

1 Introduction

1.1 Overview

Introduction

1 Appendices 2–7 are unavailable in English.


1.2 The project Sustainability and success monitoring in protective forests – NaiS1

These guidelines are intended to help prac-titioners interested in ensuring that protec-tion forests are sustainably effective at minimal cost. Forest managers and authorities should be able to use these guidelines as an instru-ment for employing public funding effi ciently.

The Swiss Law on Forests (Waldgesetz, WaG) enacted in 1991 obliges the Cantons to ensure that forests with a protective function are managed to guarantee protection (Article 20, § 5 WaG). According to Art. 19, § 4 of the Ordinance on Forests (Waldverordnung WaV), minimal interventions are those limited to conserving the protective function and to ensuring permanent stand stability. To imple-ment this, FOEFL (Federal Offi ce of Forests, Environment and Landscape, now the Federal Offi ce for the Environment) issued the guidelines Minimal forest management for forests with a protective function in 1996. These guidelines were well received, and increasingly used for planning and im-plementing silvicultural projects. In the meantime this pub-lication has gone out of print. This new edition is an extended and improved version of the previous guidelines and follows the same principles. The project Sustainability and success monitoring in protection forests – NaiS paid particular atten-tion to the following aspects in the revised edition:

The guidelines were originally developed for application in Alpine regions. However, the methods are in princi-ple applicable everywhere, and can be directly adopted. In the revised edition, many additional site types have been added so that the guidelines are now applicable to protection forests throughout the whole of Switzer-land.

When managing mountain forests, issues related to regeneration usually have most priority. Reference values need to be established for sustainable levels of regeneration to determine silvicultural targets. The target profi les have therefore been extended to include measurable regeneration values.

Basic principles for dealing with natural hazards have been considerably improved in recent years. These have been incorporated in the new guidelines with supplementary explanatory text.

The importance of timber as a resource cannot be ignored even in protection forests. While where possible advantage should be taken of economically viable solu-tions, sometimes at least some of the cut trees must be left in the stand for ecological and protective reasons. In such situations, the guidelines offer improved decision-making support.

Establishing forest reserves (protected forests) in pro-tection forests may give rise to confl icts. The guidelines show for which site types ecological objectives are con-sistent with protection forest requirements.

Silvicultural monitoring can play a decisive role in managing protection forests effi ciently and effectively. This section of the guidelines has been considerably improved and supplemented. The procedure has been tested by many forest managers and found very suitable for application in mountain forests.

The guidelines specify the requirements for forest plan-ning. They overlap in ways with forest planning and are compatible, for instance, with the Forest Develop-ment Plan, the management plan and with inventory methods.

The aim of the revised guide is to provide an update on the relevant issues based on new fi ndings and recent experience. It is the result of close cooperation with both researchers and practitioners. The resulting guidelines are therefore up-to-date and have already met with high acceptance.

The guidelines are extensive and informative, but at the same time user friendly. The main part provides an over-view of the main goals and principles, and explains the most important steps in managing protection forests sustainably and effectively. The more applied section has been divided into 10 appendices so that practitioners can quickly fi nd the relevant information for specifi c problems in the area they are managing. Only Appendix 1 has been translated into English.

1 NaiS in German stands for Nachhaltigkeit im Schutzwald

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2.1 Forest management and protective effect

Forests often protect people and material assets from natural hazards, by preventing hazards or by reducing their impact. Protection forests are delineated on the basis of an assessment of the hazard potential, the damage potential and the potential effect of the forest. Deciding on the pro-tective status of a forest is up to the authorities and not dealt with in these guidelines.

Protection forest management is based on the assumption that there is a direct link between the level of

2 What does sustainable management of protection forests mean?

risk and the state of a forest. The goal of protection forest management is to ensure a forest is as effective as possible in reducing potential damage due to hazards.

The state of the forest aimed for is defi ned in so-called target profi les which are based on what is known about natural hazards and the local site conditions. These profi les describe stand conditions which should have a strong pro-tective effect (Fig. 1). The target profi les incorporate the attributes tree species composition, stand structure, stability carriers and regeneration.

It is diffi cult to provide direct proof that protection forest management improves safety since it takes considerable time for a silvicultural intervention to affect a forest’s state. More-over, hazard events occur rarely and at irregular intervals. The success of silvicultural interventions is therefore best assessed by monitoring its effects on the state of a forest, and not

Target profi les describe stand conditions which should have a strong protective effect.

Figure 1: The goal of protection forest management is to ensure a forest is as effective as possible in reducing hazards.

Sustainability in a protection forest implies that the target forest state can be guaranteed in the long term and in the right areas. A rockfall protection forest, for instance, is only effective if it is located directly upslope of the object at risk, and if, in the long term, there are always the required number of stems.

What does sustainable management of protection forests mean?

Protection forest management

State of the forest

Effect of the forest


2.2 Seven principles

The cantons can prescribe protection forest management if it is in the interests of the public (Chapter 6, legal bases). Any prescribed intervention will be subsidised according to the legislation. However, public funding should be used as effi ciently and effectively as possible. Therefore, prescribed silvicultural interventions which are subsidised with public funding must comply with the following seven principles. They must be:

1. With a focus on the protective targetSilvicultural interventions in protection forests serve exclusively to reduce natural hazards.

2. In the right placeSilvicultural interventions are carried out in areas where the forest can prevent or reduce the effects of natural hazards on people and material assets.

3. At the right timeSilvicultural interventions are carried out at that point in time when an optimal effect can be attained with minimal effort.

4. Consistent with natural life processes Silvicultural interventions are tailored to site conditions to make use of the forces of natural forest dynamics.

5. Tailored to each stand, transparent, replicable and controllable

Silvicultural interventions are determined by experts on the spot. This makes it possible to adapt them to small-scale variation in site factors. A standard decision-making procedure is followed and docu-mented. This makes it transparent, replicable and con-trollable.

6. EffectiveThe silvicultural interventions are very likely to lead to the targets.

7. With reasonable effortThe silvicultural interventions have a reasonable cost-benefi t ratio.

These guidelines describe requirements and suitable instruments for protection forest management to help put these principles into practice.

directly on hazard occurrence, taking into account what the natural forest development would have been without inter-ventions.

Success monitoring should aim at ensuring that pro-tection forest management is efficient and effective. It should be understood as a monitoring system to help conti-nually improve management practice, and thus steer forest dynamics in the right direction with the least possible effort. Checking the effects of the forest is also part of success monitoring, which thus becomes an instrument for ensuring sustainability in protection forests.

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3.1 Principles

The assessment of the need for action is based on a comparison of the current state of a forest with the target profi le, taking into consideration the natural forest dynamics.

The minimum profi le, i.e. the minimum targets related to natural hazards and the site (Chapter 3.2), serves as a benchmark. This is compared with the predicted probable development in 50 years of the stand without interventions,

3 Determining the need for action

which accounts for the natural forest dynamics. The com-parison is conducted for all important stand characteristics.

There is a need for action if the predicted state of the forest does not meet the minimum profi le and if it is possible to improve the situation by taking effective and reasonable action. Deciding which measures are adopted requires a profound analysis of the situation. This analysis is carried out on selected, representative areas, i.e. on the so-called indicator plots.

Figure 2: Scheme for deciding about the need for action.

Minimum profi le based on natural

hazardsAppendix 1

Current state of the forest

Forecast of forest development without interventions

for the next 10 and 50 years

Minimum profi leExpected state of the forest

in 50 years

The need for action is the result of a comparison between the expected state of the forest in 50 years with the minimum profi le

Minimum profi le based on site

conditions Appendix 21

Determining the need for action

1 Not translated into English


Stand and tree characteristicsMixture Mixture Type and degree

Structure Structure dbh variation

horizontal structure

Stability carriersStability carriersCrowns

Coeffi cient of slenderness

Stand/anchoringStand/anchoring

RegenerationRegenerationSeedbed

Small saplings (10 cm to 40 cm tall)

Large saplings (40 cm tall to 12 cm dbh)

3.2 The target profi les

The target profi les describe the states of the forest that are expected to have a clear protective effect against natural hazards and that can be permanently maintained with mini-mum effort. The profi les incorporate both site-related targets and targets related to natural hazards. They provide infor-mation about the requirements for the stand (mixture, stand

structure, stability carriers), the regeneration (new growth, saplings) and the seedbed. There are two target profi les: fi rst, the long-term silvicultural target (ideal profi le) and second, the benchmark for the need for action (minimum profi le). The target profi les are established mainly on the basis of research results, fi eld observations and practical experience.

Natural hazard: Rockfall in the transit zoneRelevant rock size about 50 cmTarget profi le see Appendix 1

Site type:Typical Silver fi r-Beech forest on carbonatic bedrock Target profi le see Appendix 2B (unavailable in English)

Minimum profi leBeech 30–80 %Silver fi r 10–60 %Norway spruce 0–30 %Sycamore maple Seed trees

Suffi cient number of trees with develop-ment potential in at least 2 different dbhclasses per ha

Individual trees, possibly clusters

At least 300 trees/ha with dbh > 24 cm

Ideal profi leBeech 40–60 %Silver fi r 30–50 %Norway spruce 0–20 %Sycamore maple, ash 10–30 %

Suffi cient number of trees with develop-ment potential in at least 3 different dbhclasses per ha

Individual trees, possibly clusters, canopy closure open

At least 400 trees/ha with dbh > 24cm

In the case of openings in the fall line, distance between stems < 20 mLying logs and high stumps to supplement standing trees, if they are not in danger of falling

Crown length of silver fi r at least 2/3, ofNorway spruce at least 1/2< 80

Upright stems, well anchored, few treesleaning at extreme angles

Area with strongly competing vegetation< 1/3

At canopy closure < 0.6 at least 10 beech/silver fi r per 0.01 ha (on average one sapling every 3 m). In openings maple present

On each ha, at least 1 group (0.02 - 0.05 ha), on average 1 group every 100 m) or canopy cover at least 4 %Mixture in line with target profi le

Crown length at least 2/3

< 70

Upright stems, well anchored, no trees leaning at extreme angles

Area with strongly competing vegetation < 1/4

At canopy closure < 0.6 at least 50 beech/silver fi r per 0.01 ha (on average one sapling every 1.5 m). In openings maple present

On each ha, at least 3 groups (0.02–0.05ha), on average 1 group every 60 m) or canopy cover at least 7 %Mixture in line with target profi le

Figure 3: Example for a target profi le for rockfall in a typical silver fi r-beech forest on carbonatic bedrock.

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Site-related targets: The most stable states of a forest are assumed to be represented by the range in varia-tion of development of a natural forest. If the state does not comply with the range in natural forest dynamics (e.g. if there is a Norway spruce stand in a Melico-Fagetum forest, site association no. 8 according to Keller et al. 1998), the forest will be less resistant to disturbances (wind, insects, etc.). This does not imply that all the conditions which can be encountered in a natural forest are advantageous in a pro-tection forest. In particular, extensive pioneer phases mostly offer poor protection.

The site-related targets should include all important tree species of the climax stand. The stand structure should be diverse, with single trees or clusters able to resist distur-bance, and regeneration should be continuous. The self-regulating processes of the natural forest should be utilised to an optimum so that disturbances to the ecosystem can be avoided or kept to a minimum and the silvicultural interven-tions in the long term can also be as small as possible. The targets for the individual site types are listed in Appendix 2 (unavailable in English).

Hazard-related targets: The targets for the stand and the single tree to avoid or reduce the effects of dangerous natural hazards are specifi ed. These requirements mainly concern the stem number, the size of the openings in the stand and the canopy density. The targets for avalanche, rockfall and fl ood prevention forests and for forests in active landslide and debris fl ow areas are listed in Appendix 1.

The minimum profi le: The minimum profi le is made up of the minimum targets of the relevant natural hazard (see Appendix 1) and the minimum targets of the applicable site type (see Appendix 2B, unavailable in English). A forest fulfi lling the requirements of the minimum profi le is expected

to provide suffi cient protection in the long term. The mini-mum profi le serves as a benchmark to decide whether or not there is a need for action. Applying the same standard to all protection forests enables us fi rst to identify where inter-ventions in protection forests are necessary, and second to set priorities for public funding. The decision-making process thus becomes transparent.

The ideal profile: The ideal profile is made up of the ideal targets of the relevant natural hazard (see Appendix 1) and the ideal targets of the applicable site type (see Appendix 2B, unavailable in English). The ideal profile describes a forest condition which is expected to have the greatest protective effect in the long term.

Long-term silvicultural target: The long-term silvicultural target normally corresponds to the ideal profi le (greatest protective effect in the long-term). Should there be other important interests (e.g. providing a habitat for caper-caillie), the long-term silvicultural targets can lie between the ideal profi le and the minimum profi le (suffi cient pro-tective effect in the long-term). The leeway between the ideal profi le and the minimum profi le can also be used to minimise the long-term silvicultural intervention costs.

The target profi les were drawn up by researchers and practitioners in cooperation. They refl ect the current state of knowledge. In view of their importance for decision-making, the profi les must be reviewed periodically as part of a target review (Chapter 5.5). The characteristics and categories were chosen so as to correspond wherever possible with those of the Swiss National Forest Inventory.

The profiles should be modified when applied only if local site features make it absolutely necessary. In this case the site-related targets should be adapted to the local features.


Source: Keller W., Wohlgemuth T., Kuhn N., Schütz M., Wildi O. 1998. Waldgesellschaften der Schweiz auf fl oristischer Grundlage. Statistisch überarbeitete Fassung der “Waldgesellschaften und Waldstandorte der Schweiz” von Heinz Ellenberg und Frank Klötzli (1972). Mitteilungen der Eidgenössischen Forschungsanstalt für Wald, Schnee und Landschaft 73, 2: 91–357.


Treatment typeAll stands within a target type in similar condition and requiring the same intervention. Stands belonging to the same treatment type do not necessarily lie in a contiguous area.

Target typeCompilation of stands with the same target profi le. Stands belonging to the same target type do not necessarily lie in a contiguous area.

Indicator plotArea representative of a treatment type. Its size depends on the homogeneity of the stand (0.5 to 1 ha).

Determining target types on a map (Figure 4) gives an overview of the long-term targets for the protection forest interventions in the whole area. The treatment types provide the bases for planning and implementation measures.

For the assessment of the need for action, a so-called indicator plot, which is as representative as possible of every target type or treatment type, is selected.

Indicator plots allow exploration of silvicultural questions. Later on, they can be used for success monitor-ing. Test results from indicator plots can be transferred to the whole forest area within the same treatment type.

Figure 4: A planning area is subdivided in target and treatment types. An indicator plot is representative for a particular treatment type.

Target type B

3.3 Target and treatment types

The target profi les established on the basis of natural hazards and site types not only apply to individual stands but also to larger areas with similar conditions. All areas to which the same target profi le applies are considered to belong to the same target type.

Within one target type, stands in very different states needing rather different interventions can occur. Areas within a target type which require the same type of intervention to a similar extent are called treatment types.

Target type A

Target type C

Indicator plot 5

Indicator plot 3

Treatment type 5

Treatment type 3

Indicator plot 2

Treatment type 2

Indicator plot 1Treatment type 1

Indicator plot 4

Treatment type 4

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In areas with very diverse site conditions, the number of target and treatment types can become very large. For the sake of clarity, it may then be necessary to group the site types before specifying the target types. In this case, only similar site types with similar target profi les should be assign-ed to the same group.

If a stand map and a detailed stand description are available, target profi les can be directly assigned without delineating target and treatment types. A prerequisite for this is that the site conditions and the hazard potential are known.

3.4 Decision-making procedure on indicator plots

Deciding what is the need for action on the in-dicator plots is the most important procedure in the planning of sustainable protection forest management. To this end a treatment concept for the most important treat-ment types must be elaborated. Both the planned measures and the intentional omissions should be transparent, traceable and controllable. To achieve this, the following conditions must be fulfi lled:

It must be easy to relocate the indicator plots. The targets, the objectives and the scope of the indicator plots must be defi ned.

Decisions about the need for action and the correspon-ding information must be recorded in such a way that another professional can follow the decision-making process.

The bases for estimating costs and deciding on the utilisation of the timber must be transparent.

The documentation must be available for an effectivity analysis (Chapter 5.3).

To select indicator plots within a planning area, various factors as described in Chapter 4.1 need to be considered. It is advisable to establish each indicator plot and to collect the necessary information before doing the silvi-cultural analysis.

It is important to involve the local forest managers in the decision-making procedure on the indicator plots. They are familiar with the local conditions and also responsible for the implementation of the treatments. Form 2 (Figure 5) helps make the decision-making procedure transparent and traceable.



Figure 5: Form 2 serves to decide about the need for action. It lists the requirements of the minimum profi le (tree mixture, stand structure, stability carriers, seedbed, seedlings and saplings). It documents the assessment and the decisions for the later success monitoring.

Explanation of the decision-making procedure in Form No. 2The minimum profi le is derived from the identifi ed natural hazard (Appendix 1) and the site type (Appen-dix 2B, unavailable in English). Appendix 2A (unavailable in English) also includes some tips for identifying the site type. The next step is to record the same characteristics as those used in the minimum profi le (tree mixture, stand structure, condition of the stability carriers, seedbed, small and large saplings) on the indicator plot. In many cases it is useful and necessary to record additional information regarding the stand’s condition (Appendix 4, Form 3, unavailable in English).

Since the forest continually changes even without inter-ventions, forecasts are made for all the characteristics for the next 10 and 50 years assuming a natural forest development. The expected development is marked

with arrows. With this procedure the natural dynamics of the forest can be taken into account when deciding whether or not an intervention is necessary.

In making this decision, the expected condition of all the characteristics in 50 years is compared with the mini-mum profi le. If the conditions are forecast to be worse than the minimum profi le, effective interventions should be considered to improve the development. Provided the recommended interventions can be assessed as reason-able, there is a need for action. Should there be a need for action, the necessary interventions are normally fi ne-tuned to the ideal profi le as a long-term silvicultural target. For later success monitoring, it is important that any intentional omission is also documented, i.e. it must be noted when interventions are not carried out and why.

To assess the urgency of an intervention, the current state of the forest is considered as well as the speed and the direction in which the stand could develop without

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interventions. According to Principle 3 (Chapter 2.2), silvi-cultural interventions should be made at the point in time when the required impact can be achieved with a minimum input.

Stage targets are intermediate targets as steps towards realising the long-term silvicultural targets. They are to be reached within a time span of 5 to 10 years. The stage targets serve later as important references for a later effecti-vity analysis (Appendix 3, unavailable in English).

There is some leeway when defi ning the stage targets. In principle a stage target should not be below the minimum profi le. Usually, it is a step in the direction of the ideal pro-fi le. If the initial condition is bad, this requirement cannot always be complied with. Some leeway can also be given to consider other interests (e.g. nature conservation, timber production) in the forest besides effective protection (cf. Appendix 4, Form 2, unavailable in English).

Following this decision-making procedure can also help in estimating costs (Appendix 4, Form 4, unavailable in English). In this connection the decision about how to use the obtained timber is important. First it must be clarifi ed whether the timber is to be left in the stand for ecological or protective reasons (cf. Appendix 7, un-available in English).

Normally the procedure described above serves both planning and success monitoring. Experience shows that cost estimations often require information about other treatment types. In such cases it is advisable, with the help of Forms 2 and 4, to process other areas. These are not normally subject to long-term monitoring.

A stand description can be used to quantify the inter-ventions and expenditures more precisely.

3.5 Deciding on interventions for a planning area

The information gained from studying the indicator plots provides a good overview of the interventions in a particular planning area and their costs. The accuracy of these estimates, however, depends on the variety of the treatment types, the information contained in the stand maps and the selected net of indicator plots (Chapter 4.1).

For the implementation of the interventions within a planning area, the targets and the interventions must be clearly identifi able on the basis of a recent analysis for all intervention units, i.e. contiguous areas which receive the same silvicultural treatment.

As a rule the decisions made on the indicator plots serve as a reference for all intervention units within a specifi c treatment type. The corresponding information, especially the type and the scope of the interventions (Form 2), can be taken over directly. Naturally the practitioner will nonetheless have to tailor the intervention to the specifi c local situation.

If an intervention unit is not represented by an indicator plot (i.e. if there is no indicator plot with the same target profi le and similar condition), the decisions can be made immediately before the intervention, analogous to that on the indicator plots (Form 2). Unlike the indicator plots, no long-term monitoring is done in this case. Therefore, the standards regarding the level of detail and precision of the survey are lower. This method has the big advantage that the silvicultural planning is always up-to-date because the planning and execution of the interventions are temporally close together.

This procedure results in the collection of the most important information required for an annual work program and the budget, which permits an easy analysis of the inter-ventions in the fi eld (Chapt. 5.2 and Appendix 3, analysis of interventions, unavailable in English).



4.1 The network of indicator plots

Establishing and managing indicator plots is relatively costly. It is therefore advisable to select them carefully so that their long-term utility can be assured.

The indicator plots are representative of many other stands, so they can be used to determine the appropriate tar-get profile, to analyse the need for action and to assess the effect of certain interventions. The fi ndings and experiences gained from the indicator plots serve as guide-lines for all stands within the treatment type.

The signifi cance of the indicator plots: The need for action (Chapter 3) is determined on the basis of just a few areas, but with an in-depth examination of the relevant silvicultural questions.

Important information is obtained that provides a basis for planning (targets, priorities for interventions, cost estimation, effectivity analysis).

Detailed assessments, observations and documentation of the forest development on the indicator plots are pre-conditions for the effectivity analysis, which is part of success monitoring (Chapter 5).

The management of the indicator plots promotes manager skills (on the job training) and ensures that the knowledge gained is rapidly implemented. The indicator plots form the basis for success monitoring in protection forest management (controlling).

Indicator plots can be used as a basis for teaching and further education, and are also useful for performing convincing public relations campaigns.

Selection: Indicator plots are selected by dividing a planning area into target and treatment types. In practice, it is rarely possible to select an indicator plot for every treat-ment type. In selecting the areas, it is advantageous to pre-pare a table with the most important target types in an area, and with the most silviculturaly delicate treatment types. If the stands within a target type have a similar structure, just one single indicator plot can, in some circumstances, be sufficient. If there are clear differences between the stands, it may be necessary to delineate several treatment

4 Requirements for a planning scheme

types within the same target type, and to select the cor-responding number of indicator plots. A stand map with a detailed stand description facilitates the well-targeted selection of the relevant areas.

The selection can also be used to inform the public about the concrete silvicultural targets for an area (e.g. in a region used for establishing a Forest Development Plan) and, accordingly, about what actions are to be taken in protection forests.

Number of plots: Basically the diversity of the natural conditions determines the number of target and treatment types and thus the number of indicator plots. However, determining the number of indicator plots is a process of optimisation. On the one hand, the areas of pro-tection forest represented by the indicator plots should be as large as possible and, on the other hand, the manage-ment efforts required must be reasonable. The following con-siderations can help make the appropriate selection:

The number of target types can be reduced if stand types with similar target profi les are grouped together.

Treatment types covering a proportionally large area have more weight.

Treatment types where the effect of the silvi-cultural intervention is uncertain are important for the effectivity analysis.

Preference should be given to selecting indicator plots in areas where it is suspected that alternative interventions or even no intervention can also lead to the target.

Additional areas can be used to determine the need for action in silvicultural planning (Form 2, Figure 5), without these subsequently being used as indicator plots.

A network of indicator plots across several manage-ment units can be shared if managers cooperate across borders in establishing indicator plots.

Experience shows that a forester with a traditional range of about 1000 ha size can manage, on average, three to seven indicator plots. Moreover, for every 50 to 100 ha forest area, one indicator plot is necessary. This framework

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should make it possible to focus on the most important problems in the protection forests with well-selected indicator plots. The managers should have enough opportunity to monitor the effects of their interventions without excessive extra work.

Indicator plot size: The size of an indicator plot is governed mainly by the stand structure. As a rule smaller plots are chosen in homogenous stands and larger plots in heterogeneous stands. Experience has shown that the ideal size lies between 0.5 ha and 1.0 ha. Areas of approximately 1 ha are suitable in e.g. mountain selection forests, whereas areas of approx. 0.5 ha are adequate for young stands up to pole size. As the indicator plots will be monitored over a long period with in general a heterogeneous structure as the goal, the indicator plots should not be too small in homo-genous treatment types.

Quality and input: The indicator plots are impor-tant for managing protection forests, which means the surveys and observations must be reliable, and it should generally be clear where and when they were made. This requires accurate surveys and documentation. This is only possible if suffi cient time is available. Experience has shown that a two-person team takes one to two days to establish an indicator plot (incl. documentation). The time required for the follow-up surveys varies according to the characteristics to be recorded and the observation cycle. On average, half a day per plot and year (incl. documentation) is required.

Continuity: The explanatory power of the effectivity analysis increases the longer the period of observation. Many questions can only be answered after a number of years or even after decades. It is therefore important to ensure that observations on the indicator plots can take place in the long term, for example, by linking them with forest planning.

Maintenance: The forest services of the cantons are responsible for ensuring that capable personnel carefully and competently establish, maintain and analyse the indicator plots. Decisive here is involving the local forest manager in this process. The supervisors of the indicator plots must be well trained.

4.2 Bases and preconditions for planning

The cantons are responsible for forest planning. For this reason this manual specifi es only the minimal preconditions which must be met in order to practise sustainable protec-tion forest management. The defi nition and the delineation of protection forests are decided on a higher level and cannot be dealt with here.

Planning area: The planning of the protection forest intervention should involve large units. The planning areas should be chosen in such a way that the operators (forest owners, forest service) and the benefi ciaries (local com-munities) of the area can identify with the targets of the planning and also feel responsible for it. The planning of protection forest management should be combined with general forest planning. Experience has shown that there is a tendency in small project areas to carry out a relatively large number of severe interventions within a short time. If plan-ning areas are larger and more extensive, there is a better chance that interventions will be carried out at the right time and place. Moreover, selecting target and treatment types is best done with larger planning areas, where the cor-responding network of indicator plots can be laid out and maintained for the long term.

Prerequisites for planning: The selection of the target and treatment types requires information about the protection forest, site conditions and forest conditions.

Protection forest area: When planning protec-tion forest management, a map of the protection forests with information about hazard potential is necessary. The assessment of the hazard processes, the selection of the catchments areas and the evaluation of the damage potential is done during higher-level planning. This manual is intended to help in assessing the potential effectivity of the forest. In Appendix 1, the relationships between hazard pro-cesses and forest infl uences are summarised.

Site: The selection of the target types and the goal-oriented implementation of the planned interventions are based on an overview of the site associations in the whole

Requirements for a planning scheme


area. Ideally this should be a site map. At least all the site types in each stand should be known. The advantage of the site map is that it contains all the information needed for planning and implementation. If this is missing, the local manager should be in a position to assess the site con-ditions every time an intervention is necessary and to select the correct target profi les.

State of the forest: The selection of treatment types within a target type requires an overview of the state of the forest in the whole area, for instance, in stand maps. Structure-type maps that include the tree species mixture, basal area, top-height and stand layers give a good overview of the state of the forest. The recording of the forest state must be coordinated with management planning. The more accurate the available data, the easier and more exact the planning can be. When implementing the planning, it must be adapted to the current state of the stand to be treated.

Target: With the selection of the target type, the long-term targets for the protection forest are set, based on the target profi les. In view of the relevance for the public, it is advisable to include the target types in the higher-level plan-ning, e.g. in the Forest Development Plan.

Priorities: Priorities can be set at various levels. The point is to decide which area has precedence over others. At the higher level the delineation of the protection forests is already an assessment, but such high-level decisions are not included in this manual.

Within the planning area, the priorities can be deter-mined according to the following possibilities:

Target types of varying importance: Indications of the potential contributions of the forest in providing protection against natural hazards (large, medium, poor, see Appendix 1), and information concerning silviculture in the various site types (Appendix 2, unavailable in English) allow a differentiated assess-ment of the importance of the silvicultural interventions for the target types.

Treatment types with varying urgency: The selection of the treatment types in combination with the deriva-tion of the urgency on the indicator plots gives an indi-cation of the current state of the forest and allows the identifi cation of areas with an above-average need for action. This information, together with that on current variables such as timber prices, available means or operational factors, makes it possible to stagger the interventions expediently.

Continuity: Long-term continuity is essential. Pro-tection forest management cannot be seen as a once-off improvement of neglected stands. Managers must be able to reconsider their decisions and set priorities anew. Tradi-tionally, funding was provided for a period of 5 to 10 years to implement interventions. It was unknown if the protec-tion forest interventions would be further subsidised at the end of a project, so many managers tended to maximise the interventions. If continuity, however, is ensured, it is easier to await the natural development and to shelve interventions. This manual is oriented towards long-term and continuous protection forest management.

Planning periods: Management experience in pro-tection forests, and in mountain forests in general, show that the infl uence of the managers actually is less than is often thought. Analyses in the experimental plots of the Swiss Mountain Forest Management Group showed that many of the changes are the result of natural infl u-ences (storms, avalanches, bark beetles, ungulates, etc.). Effective protection forest management must therefore be tuned to what can be expected in a natural development. The situation must be re-assessed for every intervention before it is carried out.

Experience also shows that detailed planning is often overtaken by natural development after a few years. It is ap-propriate to determine the long-term targets as recom-mended with the selection of target types. It is possible to estimate the need for action and the costs in the medium term (5–10 years) with the help of the indicator plots. The

25NaiS

cost estimates should be considered as credit rates for a long-term protection forest management. The implementation, i.e. operational planning and carrying out the interventions, is done at short notice, within the framework of the annual programme and the budget.

Success monitoring: Success monitoring involves monitoring the effects of protection forest interventions (Chapter 5). A suitable method must be integrated in a plan-ning concept.

Integral risk management of natural hazards: Forests provide an important, but not the only form of protec-tion against natural hazards. Protection forest management is therefore to be seen as part of an integral management of natural hazards, which includes organizational measures, development planning and technical measures.

Requirements for a planning scheme


5.1 Goal and overview

The goal of success monitoring in protection forests is to achieve a high protective effect as effi ciently as possible.

According to the seven principles described above, carrying out interventions must be monitored and their effectiveness verifiable in subsidised protection forest management. Appropriate monitoring should ensure that new find-ings and experiences are fed into practical im-plementation as fast as possible.

Success monitoring includes the following four stages:

1. Implementation assessment: Were the planned interventions completed at

the correct location and were they executed professionally?

2. Effectivity analysis: What effect do the completed interventions

or the selected omissions have on the forest’s state?

3. Silvicultural monitoring: To what extent does the forest’s state cor-

respond to the target profi les? 4. Target review: Are the target profi les adequate and appro-

priate?

Monitoring is part of a closed loop of planning, im-plementing, monitoring and steering. It is mainly done on two levels of monitoring: the analysis of the interventions and the analysis of effectivity. Silvicultural monitoring pro-vides information for the higher level of forest planning and the target review assesses how appropriate the targets, and especially the target profi les, are.

Success monitoring is challenging and requires con-tributions to problem solving from professionals from vari-ous fi elds on the different control levels. As it may not be im-mediately clear why four different levels of monitoring are necessary, what must be monitored and who is responsible for what, each level will be described separately. The four levels have not evolved from a preconceived theory but have resulted from a close examination of the relevant issues. To illustrate this, and to emphasise the importance of the four monitoring levels, these issues, phrased as questions, are given before the relevant description.

5 Success monitoring

5.2 Implementation assessment

The implementation assessment checks whether the planned interventions have been carried out at the correct location and with due professional care.

Question: How can we ensure that the im-plementation of effective and target-oriented silviculture is performed at the correct location?

Solution: Ensure that the implementation of protection forest management can be checked in the fi eld using a simple sampling method.

The goal of the success monitoring is to ensure that the forest management is as effective as possible. The know-ledge gained from the assessment of the indicator plots and the later effectivity analysis must therefore be implemented throughout the area as fast as possible.

The implementation assessment is needed to enable the cantonal and federal forest authorities to inform third parties reliably about whether the forest management has been implemented at the correct location, according to the planned framework and professionally. It should be possible to do on the spot checks that require little documentation. An implementation plan, and a basic intervention description for every intervention unit, will, however, be needed.

5.3 Effectivity analysis

Checks made with the effectivity analysis will show whether the interventions realised or deliberately omitted have the wished for effect on the state of the forest.

Question: How can the manager decide which interventions can be applied under which circum-stances?

Solution: The manager monitors and documents the effects of the interventions or the deliberate omissions on the indicator plots. The experiences gained from these operations allow the manager to manage the protection forest increasingly more effectively.

The currently valid target profi les, based on the nature of the natural hazards and the sites, can be defi ned by the federal authorities. In contrast, the interventions must be tuned to the state and potential development of each stand

27NaiS

and to the local conditions (e.g. hazard potential, topo-graphy and operational conditions). This means that the inter-ventions are not predetermined but must be defi ned by trained individuals on the spot. As it is often uncertain which inter-ventions or deliberate omissions are correct or which level of intervention intensity is the most effective, practitioners need an instrument to analyse the effectivity of their silviculturalinterventions.

The effectivity analysis is fi rst of all the task of the local managers. The cantonal forest services provide support by ensuring the conditions are suitable. They provide, in par-ticular, for the long-term continuity of the monitoring and documentation and support the managers in carrying out the effectivity analysis (analysis and interpretation).

When applying expert knowledge to the whole pro-tection forest area, the local managers must be the key people for the effectivity analysis. They can observe which inter-ventions or which omissions are successful and thus make sure there is no untimely delay between analysis and im-plementation and no losses due to lack of local acceptance.

The effectivity analysis on indicator plots is the core element in the silvicultural monitoring in a protection forest. It promotes the professional com-petence of the manager and therefore enables pro-tection forest management to be highly effective and tuned to local conditions based on up-to-date knowledge. The effectivity analysis is very important, which means the managers must be well-trained and backed by the cantons and the federal government.

5.4 Silvicultural monitoring

Silvicultural monitoring involves checking to what extent the status of the forest corresponds to the target profi les. It serves as an important link to higher planning and monitor-ing levels.

Question: How can an overview of the condition and development of the protective function of the forests be obtained for a large region (cantons, the whole of Switzerland)?

Solution: The level of protection provided can be monitored by comparing the actual state of the forest with the target profi les. The target profi les are broadly based and take recent fi ndings into account, which makes them therefore suitable measures for silvicultural monitoring.

The maintenance and promotion of the forests’ pro-tective functions are anchored in Swiss forest law. The federal government and the cantons use taxpayers’ money to manage protection forests. It is therefore just a matter of time until reliable data on protective levels will be needed on the cantonal and federal levels. Performing silvicultural monitoring is, however, not the subject of this manual.

Doing an effectivity analysis on the indicator plots also involves a form of selective silvicultural monitoring and familiarises the managers of the protection forests with this monitoring instrument. This is an important precondition should, at a future date, compensation be based not on the interventions executed (managed area, m3 of timber harvested,etc.), but on achieving a particular state of the forest.

The target profi les provide the criteria and the thresholds for silvicultural monitoring at a higher level.

Success monitoring


5.5 Target review

The adequacy and the appropriateness of the target profi les established are checked in a target review.

Question: What infl uence does the state of the forest have on natural hazards and therefore on risks to people and material assets?

Solution: Protection forest management is based on the assumption that there is a direct link between risk reduction and the state of the forest. This link has been partially demonstrated by research, but should be subject to further study.

Normally it is not known where and when dan-gerous natural hazards will test the protective function of a managed forest. Moreover, it is hardly likely that both a managed and an unmanaged forest will be put to the test by the very same natural hazard. For this reason it is almost impossible for practitioners to prove the direct effect of the forest and silvicultural management on the safety of people and material assets.

Research can help here by investigating the effect of the forest on the hazard processes through specifi c monitoring and appropriate experimental designs.

The closer the forest comes to the ideal state, the better is its protective effect and the smaller the risk to people and material assets. This assumption is not really contested.Should, however, the question arise about e.g. the ideal number of stems in a rockfall protection forest or how much cover an avalanche protection forest must have, no defi -nite answer can be given. What still needs to be investiga-ted is whether the minimal and ideal standards aimed for, which are based on the natural hazards as defi ned in this manual (Appendix 1), can substantially lower the risks in

practice. Here the target analysis is used, which is that part of silvicultural monitoring that presents above all a challenge for science.

Question: How can the cost and effort involved in protection forest management be kept to a minimum?

Solution: If protection forest management can optimally use natural forest dynamics, it will require minimum efforts in the long term and be the most effective.

Minimum does not mean as cheap as possible in the short term, but rather at least cost in the long term.

The effort required to maintain a protection forest is thus assumed to decrease the closer to nature the forest becomes. Those states of the forest that are as close to those of natural forests are defi ned in the manual. The assumption is not disputed in principle, but a periodic check is necessary because of the following three problematic areas:

1. We still have a great deal to learn about the natural dynamics of our forests.

2. How much scope there is for action within the natural dynamics is often unknown (e.g. what should the target diameter be to ensure in the long term there is the required number of stems with an effective mini-mum diameter?).

3. How strongly does climatic change infl uence forest dynamics?

Because of these uncertainties, the requirements based on site types (Appendix 2C, unavailable in English) must periodically be reviewed in the target analysis. Forest research and, in particular, practical experience provide the bases for the target analysis. The most important resources for the practitioner will, in future, be the effectivity analysis on the indicator plots.

29NaiS

6 Legal bases

Forest legislation (WaG1 and WaV2)

The forest legislation differentiates between- Minimal management in forests with a protective function (silviculture type B), and - Silvicultural management in forests with a direct protective function (silviculture type C). Three conditions must be fulfi lled to be able to receive

compensation for such interventions in protection forests from the federal government and the cantons. The inter-vention must:

- conserve and promote the protective function of the forest. - be directed by the authorities.- be limited to the sustained conservation of stand stability.

Subsidy legislation (SuG3)

The subsidy legislation stipulates that fi nancial aid and compensation must be adequately justifi ed and their targets must be achieved by economical and effective means ( Art. 1 Paragraphs 1 a and b SuG).

Furthermore, there is an obligation to provide infor-mation (Art. 11) and the appropriate authorities must be able to audit the assignment (Art. 25).

To make this possible, the decisions must be trans-parent and traceable, and the effect of the interventions must be controllable.

Circulars4

In Circular 8 from the Swiss Federal Forest Agency, the specifi c requirements for sustainable protection forest management are subject to the following goals:

Forests with protective functions (silviculture B and C) reduce the risk to people and valuable material assets in their sphere of infl uence down to an acceptable level.

The necessary silvicultural interventions to achieve a goal differ in silviculture B and C as follows (next page):

Silviculture type BArticle 20, Paragraph 5 WaG 5 Where a protective function requires a minimal manage-

ment, the cantons ensure it.

Article 19, Paragraph 4 WaV4 Minimal intervention measures for the conservation of

the protective functions are those interventions which are limited to the sustained conservation of the stability of the stand; the accrued timber is to be used in constructions on the spot or it is left in the stands if it poses no hazards.

Article 38, Paragraph 1 a WaG 1 The confederation pays up to 70 percent of the costs of the

following interventions: a. minimal silvicultural interventions of limited duration which

are necessary for the conservation of the protective function and are ordered by the authorities;

Article 47, Paragraph 3 a WaV3 Compensation is made according to Table 1 of the Appendix

for:a. minimal silvicultural interventions according to Article

19, Paragraph 4, which are necessary for conserving and promoting the stability in forests with a protective function.

Silviculture type CArticle 38, Paragraph 1 b WaG 1 The Confederation pays up to 70 percent of the costs of the

following interventions: b. silvicultural interventions in open, unstable and destroyed

forests with direct protective function when the total costs are not covered and these interventions are ordered by the authorities.

Article 19, Paragraphs 1–3 WaV1 All interventions which contribute to conserving or

restoring the stability and quality of the stands are considered silvicultural interventions.

2 Interventions in young forest management are:a. tending of young growth and thickets and thinning in pole

stage stands in order to create a stable standing crop;b. the specifi c interventions to tend regeneration in single-tree

selection forests, in other multi-storied forest, in coppices with standards and coppices as well as in multi-storied forest edges,

c. protective measures against game damage;d. the construction of trails through areas of diffi cult access.3 Interventions involving thinning and regeneration are:a. the removal of logging slash and the establishment of

a new stand as well as all the necessary accompanying measures;

b. logging operations and hauling of the timber.

Article 47, Paragraph 3 b WaV3 Compensation is made according to table 1 of the Appendix

for:b. silvicultural measures according to Article 17, Par. 1 a and

Article 19, Par. 2 and 3, which are necessary to conserve forests with a direct protective function (Art. 42, Par. 2).

1 Federal Law on Forests of 4 October 1991 (Waldgesetz, WaG), SR 921.02 Ordinance on Forests of 30 November 1992 (Waldverordnung, WaV), SR 921.013 Federal Law of 5 October 1990 on Financial Contributions and Indemnities (Subventionsgesetz, SuG), SR 616.14 Circular 8 of the Federal Offi ce of the Environment, Forests and Landscape of 30 October 2003

Legal bases

NaiS Appendix 1 1

1 Introduction2 Avalanches3 Landslides, erosion and debris fl ows4 Rockfall5 Torrents and fl oods

Appendix 1: Natural hazards

1 Introduction

Erosion (landslides, debris fl ow, rockfall) is a natural geological process on mountain slopes. Even the best forest cannot fully prevent it. A stand, however, can infl uence the speed of such erosion and dampen the resulting energies.

This documentation is based on the current state-of-the-art, with many important questions still to be addressed through sound research. New fi ndings may mean the recom-mendations will need revision.

The information provided about each natural hazard should help in the assessment and management of stands affected by these hazards.

This text does not cover the following topics:

The delineation of protection forests, though the information should help to establish priorities within protection forests.

Whether a forest is more effective in preventing fl oods and landslides than, for instance, a pasture, needs to be assessed separately, as does the decision to enlarge the forest area or not (e.g. high-elevation afforestation to reduce avalanche activity or forest expansion into mid-elevation pastures and meadows to reduce the risk of landslides).

Assessing whether the protective effect of a forest is suffi cient or whether further protective measures are needed may require additional investigations, de-pending on the case.

Introduction

Sustainability and success monitoring in protection forests (NaiS) Appendix 12

2.1 Target profi le for avalanche protection forests2.2 Formation of avalanches2.3 Forests providing protection against avalanches2.4 Forest infl uence

2.1 Target profi le for avalanche protection forests

2 Avalanches

Locality

Zone of originZone of origin

Sub-alpine and upper montane coniferousforests


Broadleaved and mixed forests of theupper and lowermontane zones

Potential contribu-tion of the forestLargeLarge

In larch forests if incline ≥ 30°(58 %)In evergreen coniferousevergreen coniferous forestsforests1 if incline ≥ 35°(70 %)

IntermediateIntermediateif incline ≥ 35° (70 %)

Hazard-related target profi le:ideal requirementsHorizHorizontal structureontal structureIncline Opening lengthIncline Opening length2 in the fall linein the fall line≥30° (58 %) → smaller than 50 m≥35° (70 %) → smaller than 40 m≥40° (84 %) → smaller than 30 m≥45° (100 %) → smaller than 25 m

If opening length2 is greater than indicated above, opening width mustbe < 15 m

Canopy cover > 50 %

Ideal requirements of site-relatedprofi le accomplished

HorizHorizontal structureontal structureIncline Opening lengthIncline Opening length2 in the fall linein the fall line≥35° (70 %) → smaller than 40 m≥40° (84 %) → smaller than 30 m≥45° (100 %) → smaller than 25 m


Canopy cover > 50 %

Ideal requirements of site-relatedprofi le accomplished

Hazard-related target profi le: minimum requirementsHorizHorizontal structureontal structureIncline Opening lengthIncline Opening length2 in the fall linein the fall line≥30° (58 %) → smaller than 60 m≥35° (70 %) → smaller than 50 m≥40° (84 %) → smaller than 40 m≥45° (100 %) → smaller than 30 m


Canopy cover > 50 %

Minimum requirements of site-relatedprofi le accomplished

HorizHorizontal structureontal structureIncline Opening lengthIncline Opening length2 in the fall linein the fall line≥35° (70 %) → smaller than 50 m≥40° (84 %) → smaller than 40 m≥45° (100 %) → smaller than 30 m


Canopy cover > 50 %

Minimum requirements of site-relatedprofi le accomplished

An active promotion of surface roughness (e.g., leaving high stumps and lying logs) in gaps and at the edge of avalanche tracks reduces the probability of avalanche release.

If the surface is suffi ciently rough, the minimal target profi le related to gap length in the fall line can be used as the ideal profi le.

1 In evergreen conferous forests, the canopy cover and the general surface roughness usually prevent avalanche release unless the incline exceeds about 35°. Pure larch forests, in contrast, have often a ground vegetation rich in grass which reduces the general surface roughness. Therefore, potential release should be taken into account already at inclines of about 30°.

2 Opening length measured from crown edge to crown edge in pole and old timber stands.

NaiS Appendix 1 3

2.2 Formation of avalanches

In the snowpack on a slope, creeping movements occur and, depending on the characteristics of the ground-snow interface, additional gliding may take place on the ground surface. These movements can also cause the sliding of the whole snowpack, depending on:

Slope incline Snow depth Surface roughness Snow consistency

Local changes in these factors cause zones of enhanced tensile, compression and shear stress in the snowpack.

Slab avalanches develop mainly in the following situation:

Slope of at least 30° (58 %) Weak layers and/or gliding surface (e.g.,snow-covered hoar, smooth ground surface)

Snow cover with continuous layers Snow with high cohesion Snowdrifts due to wind promote local snow depots and the occurrence of snow with high cohesion

Loose snow avalanches develop mainly in the following situation:

Slope with incline between 40° (85 %) and 60° (170 %). On steeper slopes (>60°), avalanches discharge con-tinuously

Snow with low cohesion

Avalanches starting in the forestIf the starting zone of an avalanche is located within the

forest, it is called a forest avalanche. Gap size in a stand is a decisive factor in determining the extent of snow movements. As openings are part of a near-natural stand structure and required for regeneration, especially in the sub-alpine and upper montane zones, snow movements cannot be com-pletely excluded. This is why only incidents which are able to damage trees of at least pole size are considered forest avalanches. Trees in the new growth and sapling stages are usually damaged by snow gliding, creeping and settlement rather than by avalanches.

Slab avalanche

Loose snow avalanche

The following meteorological and snow conditions favour the formation of forest avalanches:

Air temperature below –4°C, more than 80 cm new snow within 2 days, slight wind during snowfall, snow depth exceeding 120 cm, and in addition a slight increase in temperature on the day the incident happens.

Air temperature below –4°C, more than 60 cm new snow within 3 days, slight winds during snowfall, strong warming on the day the incident happens.

More than 50 cm new snow within 3 days, snow depth exceeding 120 cm, rain.The colder and the less windy it is during snow fall,

the less new snow is required for the release of forest avalanches.

Figure 1: Avalanche types (after Salm 1982).

Avalanches


2.3 Forests providing protection against avalanches

In regions and elevations with snow conditions which enable the development of large slab or wet snow avalanches, forests on slopes with more than a 30° (58%) incline have the potential to provide protection against avalanches.

In the region of coniferous forests at elevations ranging from 1600 to 2200 m a.s.l., starting zones are often to be found on slopes with north-east to north-west aspects. Here, mostly dry slab avalanches are released. Often release occurs where there are changes in incline of at least 10°.

In the region of broadleaved and mixed forests below 1200 m a.s.l., wet snow avalanches or moist loose snow avalanches are released, mostly on sunny slopes.

At the upper tree line, the forest is often open and concentrated along ridges. No forest can grow in gullies

be-cause of snow movements and long-lasting snow cover. The uppermost part of the forest is very important for the stability of the whole forest. The ecological conditions are mostly extreme. Regeneration is often only possible under the protection of old trees. If there is no such protection, technical measures are needed.

The situation at the tree line needs to be taken into account in deciding about the forest located below it. Under certain circumstances, high-elevation afforestation can improve the situation. At the upper tree line, the poten-tial forest cover decreases and its protective effect against avalanches thus also tends to diminish. Below an avalanche release area located above the potential tree line, the estab-lishment of forest is restricted to favorable sites such as ridges (see also Fig. 2).

Figure 2. Potential avalanche protection forest. On the left, a closed forest covers the slope up to the ridge, and the tree line is above the ridge. In the middle, the ridge extends over the tree line, and there is a starting zone for avalanches above the tree line. The forest is restricted to particularly favorable sites. This must be taken into account when making decisions about the forest.

Frey / SLF

NaiS Appendix 1 5

2.4 Forest infl uence

The forest influences snowpack structure und thus avalanche formation by intercepting snow, maintaining a stand climate and increasing surface roughness through the presence of trees, stumps and lying logs (Fig. 3).

In the forest, an average statistical recurrence interval for avalanches of 30 years is assumed (on open land with avalanche barriers, the interval used is 100 years) since, in many cases, openings become so overgrown within 30 years that they prevent forest avalanches from releasing.

Factors impeding avalanche release Interception reduces snow depth in a forest in com-parison to open land. The difference between the forest and open land is more pronounced during light snowfall (70 % interception) than during heavy snowfall (30% interception). If temperatures are low during snowfall, interception is less.

Transpiration and wind =

interception loss

Figure 3: Sketch of snow ablation (after Meyer 1987 and Cemagref).

In the forest, the continuous layering of the snowpack is disturbed, e.g., by snow falling to the ground or tree wells around stumps.

Less longwave radiation is emitted particularly in ever-green forests, i.e. warming in the daytime is redu-ced and the longwave radiation emission at night is reduced. This leads to a special climate in the forest which infl uences snow transformation. Therefore, less surface hoar frost and depth hoar are built, and higher snow temperatures stabilise the snowpack. If the snow is moist, small avalanches can be released in the forest and, if the ground surface is smooth, so can wet snow avalanches.

In the forest, wind speed is reduced close to the ground, which is why there is less wind-transported snow. In openings and at stand edges, however, the wind can result in larger snow deposits.

Direct deposition

Interception =intermediate depots

Delayed down-fall and roll-down on the

slope

Direct deposition

Snow precipitation

Delayed deposition Direct

deposition

Avalanches

Wind transport and turbulence

Additionalsnow


In the forest, surfaces are generally rougher than in the open, which reduces the risk of snow movements.

Upright stems and stumps, as well as lying logs, enhance surface roughness and act therefore as stabilising elements in the snowpack. The supporting effect of trees only, however, is usually insuffi cient to prevent avalanche release. To be effective as avalanche barriers, the following stem numbers (dbh > 8 cm) are required: 500 stems/ha at an incline of 30° (58 %), and 1000 stems/ha at an incline of 40° (84 %).

Factors promoting avalanche release In shady openings and at stand edges, surface hoar frost can develop and persist for a long time. When snowed on, this can promote snow gliding.

Importance of tree species and stand structureThe presence of forests generally reduces the probability

of avalanche release on slopes with an incline of at least 35° (70 %). In open areas or in larch stands, the critical incline is only 30° (58 %).

Trees help to prevent avalanche release if their height is at least double the snowpack depth.

Evergreen tree species intercept more snow than deciduous trees, especially if temperatures are low. Short- and long-wave radiation emissions are reduced by up to 90% below a dense canopy of evergreen trees, but only by up to 30% under a canopy of deciduous trees. The recom-mended proportions of evergreens, which will depend on the site association, take this into account.

Deciduous tree species are effective in preventing avalanche release with light snowfall, but their effect is limited during heavy snowfall. Moreover, snow glides easily over beech leaves.

Small trees which are entirely covered by snow (e.g., green alder, dwarf mountain pine) can promote avalanche formation as their branches may move elastically. Moreover,such sites are prone to depth hoar. If such stands extend over vast areas, avalanches can be less frequent but larger than in open land.

Deciduous tree species often occur at the edge of avalanche tracks where evergreen tree species cannot survive as they cannot withstand the drag forces. In the central Alps, larch is often found on these sites (and the mineral soil facilitates its regeneration), whereas in the Pre-

Alps sycamore maple or beech are more common. Here, ever-green tree species should not be specially promoted.

In trees with a high crown base, snow falling to the ground can cause avalanche release. In trees with branches on the lower stem (e.g., trees in clusters), this risk is smaller.

Tall trees with large crowns infl uence the snow cover over a larger area than small trees.

Braking effect of the forestIf the fl owing depth of an avalanche is only 1–2 m and

therefore only affects the stems, it can be slowed down by the forest. If the fl owing depth is greater and the avalanche speed is high too (e.g., powder avalanches), a forest will be destroyed. In the deposition zone, avalanche speed is often low so that the forest is more effective in slowing it down and in diminishing its reach.

Dead wood on windthrow areasOn most uncleared windthrow areas, the timber is

initially highly effective in providing protection against snow movements. The surface structures consist of snags, stumps, root plates and lying logs, which form a dense and tight entanglement. This nails the snowpack effectively to the ground and favorably affects snow deposition for several decades. For typical avalanche release zones (about 30 to 40° incline) and normal snow depth in the forest, such timber provides good protection. However, on very steep slopes and occasions with exceptionally high snow depth, the timber may be unable to resist the strain and the snow-pack, including the timber, may be set in motion. As timber decays, this danger gradually increases. This should be taken into account if the damage potential is large. Clearing an area reduces the protection provided against snow movements from the beginning.

Planting was done in potential avalanche starting zones on areas devastated by the hurricane «Vivian» where there was little regeneration. This advanced the density and size of trees by at least ten years over that of newly establis-hed natural regeneration. Planting can thus shorten or even eliminate a period during which there is a lack of protective effect, which may occur if the new forest cannot compen-sate for the reduction in protective effect caused by timber decay. Planting is also possible in uncleared windthrow areas, although it is more diffi cult.

NaiS Appendix 1 7

Source: Bebi P. 2000. Erfassung von Strukturen im Gebirgswald als Beurteilungsgrundlage ausgewählter Waldwirkungen. Beiheft Schweiz. Zeitschrift für Forstwesen 90.Berger F. 1997. Interactions forêt de protection - risques naturels, Détermination des Zones d‘interventions forestières prioritaires, l‘exemple du département de Savoie, Thèse de doctorat, CEMAGREF Grenoble.De Quervain M. 1978. Wald und Lawinen. In: de Quervain M. (ed.), Mountain forests and avalanches. Proceedings of the Davos Seminar, September 1978.Eidg. Institut für Schnee- und Lawinenforschung (Hrsg.) 2000. Der Lawinenwinter 1999. Ereignisanalyse. Davos, Eidg. Inst. Schnee- und Lawinenforschung.Frey W. 1977. Wechselseitige Beziehungen zwischen Schnee und Pfl anze – eine Zusammenstellung anhand von Literatur, Mitt. Eidg. Inst. Schnee- und Lawinenforschung 34.Frey W., Frutiger H., Good W. 1987. Openings in the forest caused by forest deperishment and their infl uence on avalanche danger. Pp. 223–238 in: Fujimori T., Kimura M. Human inpacts and management of mountain forests. Forestry and forest products research institute, Ibaraki, Japan. Frey H.U., Preiswerk T. 1993. Waldstandorte und Waldgesellschaften im Kanton Schwyz. Vegetationsschlüssel, Kurzbeschrieb und detaillierte Vegetationstabellen. Unveröffentlichtes Typoskript, Oberforstamt des Kantons Schwyz.Frey W., Leuenberger F. 1998. Forstlicher Lawinenschutz. Bündnerwald 51, 1: 21–33.Frey W., Thee P. 2002. Avalanche protection of windthrow areas: A ten year comparison of cleared and uncleared starting zones. For. Snow Landsc. Res. 77, 1–2: 89–107.Imbeck H., Ott E. 1987. Verjüngungsökologische Untersuchungen in einem hochstaudenreichen subalpinen Fichtenwald, mit spezieller Berücksichtigung der Schneeablagerung und der Lawinenbildung. Mitt. Eidg. Inst. Schnee- und Lawinenforschung 42.Kaltenbrunner A. 1993. Methodenbeitrag zur Ermittlung der Lawinenschutzfunktion subalpiner Wälder. Diplomarbeit, Abt. für Forstwirtschaft ETH Zürich.Meyer-Grass M., Imbeck H. 1985a. Waldlawinen: gefährdete Bestände, Massnahmen. Eidg. Inst. Schnee- und Lawinenforschung (SLF) Davos. Meyer-Grass M., Imbeck H. 1985b. Waldlawinen: Anleitung für die Meldung von Waldlawinen. Eidg. Inst. Schnee- und Lawinenforschung (SLF) Davos. Meyer-Grass M. 1987. Waldlawinen als Folge immissionsgeschädigter Gebirgswälder. Massnahmen. Verhandlungen Ges. für Ökologie (Graz 1985) XV, 257–265.Meyer-Grass M., Schneebeli M. 1992. Die Abhängigkeit der Waldlawinen von Standorts-, Bestandes- und Schneeverhältnissen. Internationales Symposium INTRAPRAEVENT 1992 Bern, Tagungspublikation, Band 2.Munter W. 1997. 3x3 Lawinen, Entscheiden in kritischen Situationen. Garmisch Partenkirchen, Agentur Pohl und Schellhammer.Pfi ster R. 1997. Modellierung von Lawinenanrissen im Wald. Projektarbeit Nachdiplomkurs in Angewandter Statistik ETH Zürich.Salm B. 1978. Snow forces on forest plants. Pp. 157–181 in de Quervain M. (ed.), Mountain forests and avalanches. Proceedings of the Davos Seminar, September 1978. Salm B. 1982. Lawinenkunde für den Praktiker. Schweizerischer Alpen-Club, Bern.Schwitter R. 2002. Sturmholz als Lawinenschutz – ein Erfahrungsbericht. Wald und Holz 83, 6: 31–34.

Avalanches


3.1 Target profi le for forests providing protection against landslides, erosion and debris fl ows3.2 Landslides3.3 Surface erosion3.4 Debris fl ows

3.1 Target profi le for forests providing protection against landslides, erosion and debris fl ows

3 Landslides, erosion and debris fl ows

Locality


Infi ltration zoneInfi ltration zone

Potential contribu- tion of the forestLargeLargeIn the case of shallow landslides (depth of slide surface at most 2 m) and of surfaceerosion

IntermediateIntermediateIn the case of intermediate to deep landslides(depth of slide surface at least 2 m), if it is possible to infl uence the water balancein the slide surface

SmallSmallIn the case of intermediate to deep landslides(depth of slide surface at most 2 m), if the po-tential infl uence on thewater balance in theslide surface is small

Hazard-related target profi le: ideal requirementsHorizontal structureMaximum opening size3 0.04 ha, if secured regeneration1 exists 0.08 ha.

Horizontal structureCanopy cover2 permanently and at a small scale ≥ 60%Ideal requirements of site-related target profi le accomplished

MixtureIn areas of transition between sitetypes, the tree species composition of the moister / wetter site should bethe target

Stability carriersNo heavy trees and no trees prone to windthrow

Horizontal structureCanopy cover2 permanently ≥ 50%Ideal requirements of site-related target profi le accomplished

RegenerationPermanent regeneration guaranteedIdeal requirements of site-related target profi le accomplished

Hazard-related target profi le: minimum requirementsHorizontal structureMaximum opening size3 0.06 ha, if secured regeneration1 exists 0.12 ha.

Horizontal structureCanopy cover2 permanently ≥ 40%Minimum requirements of site-related target profi le accomplished

Mixture In areas of transition between site types, the tree species composition ofthe moister / wetter site should bethe target

Horizontal structureCanopy cover2 permanently ≥ 30%Minimum requirements of site-related target profi le accomplished

RegenerationPermanent regeneration guaranteed

1 Secured regeneration: Young growth or thickets with a mixture meeting the requirements. In the sub-alpine zone, larger areas are possible if cut in elongated form; opening width ≤ 20 m.2 The canopy cover is related to trees of at least pole size (i.e. young growth and thicket stages are ignored).3 Openings are measured from crown edge to crown edge in pole and old timber stands.

NaiS Appendix 1 9 Natural hazards Landslides, erosion and debris fl ows

3.2 Landslides

The depth of the gliding surface is one way of dif-ferentiating between landslides. When considering the potential effectiveness of a forest, it is important to

differentiate between shallow and intermediate to deep slides. In all types, the water infi ltrating the soil is, in most cases, a very important trigger (Fig. 4).

Shallow landslides:• Depth 0–2 m• In most cases frequent landslide activity with short duration (minutes to months)• Slide areas small (mostly < 0.5 ha)• Develop mostly on slopes with incline above approx. 25°, but

can also occur in clearly less inclined terrain• Frequently with characteristic starting zone pockets from

recurring landslides

Sachseln, Canton Obwalden, 15th of August 1997Sachseln, Canton Obwalden, 15th of August 1997

• About 100 m3 volume in each landslide • Duration of precipitation two hours; landslides occurred

within a couple of minutes• Transition to slope debris fl ows due to heavy water

saturation

Landslides with intermediate to large depth:• Depth 2–10 m or > 10 m, respectively• Landslide activitity mostly in the range of cm to dm / year• On large areas (mostly > 0.5 ha, up to several km2)• Landslide processes going on for years to centuries, often in phases with varying activity• Characteristic traces in the terrain: Extensive rupture edges

in the starting zone, leaning trees or trees with a bend close to the ground, soil cracks, roots under tension, compression bulges, infi ltration zones for surface water, water-logged zones, roads or buildings with cracks and deformations

Sörenberg, Canton LucerneSörenberg, Canton Lucerne

• Material several million m3 in volume• In motion for more than 100 years; alternating active and

passive phases, depending on weather conditios• Has consequently led to additional debris fl ows and shallow

landslides

WSL

Thormann/SHL

Figure 4: Landslide examples.


Areas prone to landslidesAreas prone to landslide (in particular those with deep

slide surfaces) are often well known and documented. The following documents are important for assessing land-slides:

Hazard map / hazard index map

Map of soil and slope instabilities (map of phenomena)

Event register/records of past incidents

Geological map

Shallow landslides may also develop sponta-neously in the interior of the forest, particularly after a com-plete stand collapse.

A number of factors determine whether landslides occur and, if so, where. The most important factors, how-ever, are the slope and the type of loose material. The decisive criterion for assessing the instability of loose material is the angle of internal friction specifi c to each material, which designatesthe critical incline value of a slope.

In the following table, the types of loose material found have been roughly subdivided into three classes. For each class, a threshold value is given for the incline above which shallow landslides should be anticipated (Table 1). If a forest area is less inclined than indicated, spontaneous landslides are unlikely3.

Table 1: Threshold values for critical incline.

Type of loose material Threshold value for critical incline1 Soils rich in marl Soils rich in clay ≥ 25° (47 %)2 Intermediate soils, without signs of heavy water saturation ≥ 30° (58 %)3 Highly permeable soils Soils with small proportion of fi ne- grained material (clay, silt) Sandy soils, gravel ≥ 35° (70 %)

Infl uence of forests on landslide releaseShallow landslides: Such landslides are located

within the reach of tree roots, so that forests can greatly infl uence landslide intensity by:

Mechanically reinforcing the soil throuh the root system

Positively influencing the water balance of the soil through interception, transpiration and through en-hanced soil permeabilityA forest with an ideal structure can improve soil stability

and thus reduce landslide activity. However, landslides cannot be completely eliminated even in an ideal forest. Moreover, the effect of the forest decreases markedly if the incline exceeds about 40°.

Large windthrown trees can tear open the soil, which in turn may increase landslide risk and surface erosion.

Strong winds can cause cracks in the soil through tree movements.

Intermediate to deep landslides: The stabilising effect of a forest through root reinforcement is of primary importance for shallow landslides, but is much less pro-nounced on slopes with intermediate to deep gliding sur-faces. The forest has considerable indirect influence on these, however, since it creates a storage zone which pre-vents the infi ltrating water from percolating through the soil down to a potential rupture zone. This effect, however, is lost if the soil is completely saturated with water.

For landslides with intermediate to deep surfaces, an infi ltration zone can be defi ned. This zone encompasses the area where water infi ltrates the soil and penetrates the potential sliding material. The forest can partly retain this water through storage. However, the underground water flow is often unknown, which makes it very difficult to delineate the infi ltration zone. If this is the case, it must be assumed that the infi ltration zone is usually the surface catchment area located directly above the slide foot.

The weight of trees does not infl uence landslides with intermediate to deep surfaces. «Relief cutting» is therefore not helpful.

Unstable trees, however, can cause problems in the infl uence zone of a channel if drift wood forms congestions (cf. target profi le for torrents and fl oods).

3 Under certain circumstances, landslides can occur at smaller inclines. Special attention should be paid to previous incidents.

NaiS Appendix 1 11 Natural hazards Landslides, erosion and debris fl ows

Importance of the tree speciesTree species able to form a deep and intensive root

system are important for landslide prevention. They can not only strongly reinforce the soil but also make optimal use of the storage zone. While most tree species are able to achieve this on highly permeable soils, the species-specifi c reaction to clay soils, compacted soils and temporarily soaked sites is decisive. A couple of tree species root relatively deeply in compacted, wet, clay soils. These are:

Broadleaved trees: Ash, elm, oak, aspen, black alderConiferous trees: silver fi r, Scots pine

Silver fir, as a widespread species in natural Alpine mountain forests, plays a central role here.

Importance of stand structureEffective landslide prevention is furthered by having

as deep-reaching and intensive a rooting system as possible.

Uneven-aged stands with the highest possible canopy cover are best at sustaining such root penetration in the long-term. A multi-layered stand structure can be assumed to be refl ected in the root system in the soil. Such a structure also ensures sustainable tree regeneration, which speeds up reforestation after a po-tential stand destruction (e.g., by windthrow).

In contrast, large clearcuts are the least favourable forest condition for preventing landslide since the stabilising role of the dead roots diminishes after a few years, when the new forest is still at a young stage.

Openings should therefore be as small as possible and only as large as necessary for suffi cient regeneration.

Large trees vulnerable to windthrow can negatively affect slope stability. When windthrown, they cause large soil wounds. This can increase water infi ltration and intensify the weathering of the underlying material so that starting points for erosion and landslide may develop.

Effects of drainage systemsDrainage ditches can have very different effects. If an

active landslide area is well-drained, this can slow down any sliding movements. Large-scale drainage systems, however, often cause other diffi culties:

Maintenance costs of drainage ditches are very high. Drainage systems can act counterproductively if their maintenance is neglected.

In particular in areas with landslides with intermediate to deep surfaces, there is a high risk that the drainage system will be disrupted by slope movements.

Large-scale drainage systems can, in certain circum-stances, contribute to larger peak discharges.

Often, the water stored cannot fl ow off without putting a strain on other potential landslide areas.

For these reasons, the purpose of drainage systems must be carefully considered and a maintenance plan estab-lished.

Leaving timber on siteLeaving timber on sites poses a problem in landslide

areas if the timber could fall into a torrent channel where it might lead to congestion or become drift wood in debris fl ows (cf. the target profi le for torrents and fl oods).

Timber harvestingInappropriate timber harvesting techniques can cause

massive soil compaction, especially on vulnerable soils. This impairs the rooting zone, which is decisive for stand stability and thus preventing landslides, for decades. Damage caused by careless harvesting may well outweigh the intended benefi t! When looking for the most cost-effective harvesting practice, a careful approach must be taken to conserve the stand and the soil. This applies in particular to salvage log-ging, which can do large-scale and permanent damage in a short time.


3.3 Surface erosion

Surface erosion is the gradual loss of loose material on the soil surface, in particular due to water. The transition between surface erosion and shallow landslide is blurred. In contrast to landslides and slope debris fl ows, surface erosion alone does not present a hazard potential. However, it can in the long term deposit loose material in channels which may be mobilised by debris fl ow. Moreover, progressive erosion of fi ne-grained material will reduce the water storage capacity of the soil and the rooting zone for the vegetation.

Erosion as such is a natural process which cannot be completely prevented. However, it can be speeded up or slowed down by particular forms of land use.

The positive effect of forests in hindering surface erosion is well known. It is essentially the result of the soil being reinforced by the root systems of the trees and other vegetation, which reduces the removal of soil material by surface runoff. Moreover, a closed vegetation diminishes the ongoing weathering and destabilisation of the granular soil. Weathering leads to a reduced shearing strength and thus promotes erosion and landslide processes.

Vegetation cover of the soil that is extensively closed is therefore of primary importance in preventing surface erosion. The state of the forest plays a special role in this. Thus:

To ensure that the vegetation cover is permanently closed in the long-term, phases of stand destruction (e.g., by windthrow) must be prevented. First and foremost this means that the manager should aim for a stand which makes large-scale collapse unlikely, and here a multi-layered stand structure plays a central role.

To prevent surface erosion, silvicultural interventions should aim to reduce the occurrence of landslides, which often initiate surface erosion.

3.4 Debris fl ows

Debris fl ows are rapidly fl owing mixtures of water and solid components, where the proportion of solid material is about 30 to 60%. They occur often in surges in torrent channels. Typically they have a high density, sometimes with high fl owing velocities, and a high transportation capacity with large volumes of solid material (with rocks several m3 in volume) transported.

Landslides and surface erosion lead to the accumu-lation of loose material in torrent channels and thereby con-tribute to the development of debris fl ows. Moreover, debris fl ows can be triggered by slope instability as so-called slope debris fl ows.

The forest can influence debris flows by reducing slope processes (landslides, surface erosion) and therefore slow-ing the supply of material that could be transported by debris fl ow. In the deposition zone of a debris fl ow, a forest can also have a certain braking function by promoting debris fl ow drainage.

Debris fl ows are not explicitly considered in the target profi le, but the triggering processes (landslides and surface erosion) are.

A potential negative infl uence of a forest on debris fl ow (drift wood in the channel) is considered in the target profi le for torrents and fl oods.

Source: Arbeitsgruppe Geologie und Naturgefahren. 2000. Ursachenanalyse der Hanginstabilitäten 1999. Bull. angew. Geol. 5, 1.Böll A. 1997. Wildbach- und Hangverbau. Ber. Eidg. Forschungsanstalt Wald, Schnee Landsch. 343.BRP/BWW/BUWAL. 1997. Empfehlungen: Berücksichtigung der Massenbewegungen bei raumwirksamen Tätigkeiten.BUWAL. 2000. Schlussberichte Projekt «Einfl uss des Waldes und minimaler Pfl egemassnahmen auf das Abfl ussverhalten der Gewässer und die Rutschaktivität in Flyschgebieten». BUWAL, Bern. Unveröffentlicht.Polomski J., Kuhn N. 1998. Wurzelsysteme. Eidg. Forschungsanstalt WSL, Birmensdorf.Rickli C. (Red.). 2001. Vegetationswirkungen und Rutschungen. Eidg. Forschungsanstalt WSL, Birmensdorf.

NaiS Appendix 1 13

4.1 Target profi le of forests protecting against rockfall4.2 Rockfall processes4.3 Zone of origin4.4 Transit zone4.5 Run-out and deposition zones4.6 Additional information regarding forest effects

4.1 Target profi le of forests protecting against rockfall

Locality


Transit zoneTransit zone

Run-outRun-out and and deposition zonedeposition zone

Potential contri-bution of the forestMediumMedium

LargeLargeRocks up to 0.05 m3 (diameter about 40 cm)

Rocks 0.05 to 0.20 m3

(diameter about 40 to 60 cm)

Rocks 0.20 to 5.00 m3 (diameter about 60 to 180 cm)Additionally for all rock Additionally for all rock sizes:sizes:

LargeLargeThe effective minimum diameter of trees is considerably smaller than in the transit zone, and lying logs are always effective

4 Rockfall

1 Opening size in pole and old timber stands is measured from stem to stem. 2 The target diameter should be chosen so as to ensure that the required stem number

with trees of the minimum effective diameter is permanently achievable.

Hazard-related target profi le:ideal requirements

Horizontal structureAt least 600 trees/ha with dbh > 12 cm



Ideal requirements of the site-related target profi le accomplishedHorizontal structureAt least 600 trees/ha with dbh > 12 cm

Ideal requirements of the site-relatedtarget profi le accomplished

Stability carriers No unstable heavy trees

Potentially also coppice shootsVertical structure

Target diameter2 appropriate

Vertical structureTarget diameter2 appropriate

Horizontal structureIn openings1 in the fall line, stem distance < 20 m

Lying logs and high stumps supplementing standing trees if no risk of fall

Horizontal structureIn openings1 in the fall line, stem distance < 20 m

Potentially also coppice shoots Vertical structure

Target diameter appropriateLying logs and high stumps supplementing standing trees

Hazard-related target profi le: minimum requirements




Minimum requirements of thesite-related target profi le accomplishedHorizontal structureAt least 400 trees/ha with dbh > 12 cm

Minimum requirements of thesite-related target profi le accomplished

Natural hazards Rockfall


4.3 Zone of origin

Role in the rockfall processIn this zone rocks are released. The size and form of the

rocks as well as the rockfall frequency are infl uenced by the bedrock type, bedrock stratifi cation, aspect and elevation.

Forest infl uenceTree roots hold rocks together. However, they can also

speed up weathering since organic acids from roots and coniferous litter corrode the rocks. The roots may also grow into cracks and lead to frost wedging. If the bedrock layers are parallel to the slope, weathering processes act more stronglythan if they run perpendicularly to the slope. Falling trees can

also dislodge rocks. Trees, particularly those taller than 20 m, can sway in the wind to such an extent that the roots move and thus release rocks.

The effects of the forest depend on the local geology and topography, the tree species, the trees’ weight, height and centre of gravity.

Effectiveness of lying logsWell-anchored logs hamper rockfall if there is no

risk of them falling. The risk of them falling increases with increasing incline, the way logging has been done and also depends on snow conditions.

4.2 Rockfall processes

A rockfall process is the movement of falling rocks and their interaction with the environment. The rocks roll, bounce or slide. These movement types can be well described. In their forward movement, the rocks hit the ground or obsta-cles such as logs or defence structures. This causes the rocks to lose energy.

Rockfall processes occur in several distinctive areas: the zone of origin, the transit zone and the run-out and deposi-tion zone (Fig. 5). Often, these zones overlap.

In addition to rockfall, icefall often occurs.

Zone of originIncline clearly exceed-

ing 30° (58 %),mostly very steep,

often bedrock exposed

Transit zoneIncline > 30° (58 %)

Run-out and deposition zoneIncline < 30° (58 %)

Figure 5: Schematic slope profi le.

NaiS Appendix 1 15

4.4 Transit zone

Role in the rockfall processOn slopes with inclines between 30° (58 %) and 35°

(70 %), rocks roll or slide. On slopes with inclines over 35° (70 %) they can also bounce. These movements can be calculated with relatively high precision. When hitting the ground or obstacles, rocks lose energy (energy = mass x velocity2), and they may also change their direction. It is still diffi cult to calculate the extent of energy loss. Rocks may be stopped as a result of such contacts, but they may also regain speed afterwards.

The following factors besides forest and protective structures will slow down rocks:

Topography: if the topography is rugged, rocks are deflected; the gentler the terrain, the slower rocks move.

Surface roughness: Rocks are greatly slowed down on rough surfaces, in particular if the size of the items con-tributing to surface roughness is of the same magnitude as that of the rocks (talus slopes).

Damping: soft soil has a strong breaking effect on rocks.Round rocks usually move faster than angular or elon-

gated rocks, assuming the conditions are otherwise similar.

Forest infl uenceContact with trees brakes rocks or stops them tempo-

rarily. Braking makes rocks not only reduce speed, but also bounce less high.

How large a braking effect trees will have depends on their diameter and rock size:

Very slim trees give way if hit by rocks, so their braking effect is slight.

Larger trees can be injured or broken by rocks, de-pending on the rocks’ energy (which in turn depends on their velocity and size). Such contacts reduce the velocity and the energy of the rocks to a considerable extent.

Forests are not so effective in preventing the fall of very large rocks (=boulders).

It is not possible to calculate precisely what the minimum diameter is that a tree must have to be effective against a particular rock. We know from experiments that living trees absorb more energy than wooden beams and that

rocks moving slowly contain little energy. Thus even a relatively slim tree is able to slow down small rocks.

The effective minimum diameter of a tree plays an important role since it is not always possible to sustain a stand structure which offers optimal protection against rock-fall, especially if the effective minimum diameters are large.

Therefore, an important question arises: for which situation should the forest be designed? In the case of moving objects at risk (i.e. where the damage potential consists, e.g., of hikers or cars), small but frequent rocks are often a pro-blem. In the case of a house, it is the rather rare, large rocks that are dangerous.

The effectiveness of a forest depends on the diameter and the number of trees as well as the size of gaps. Rocks can already reach maximum speed and bounce a long way after a distance of 40 m, although this varies with the terrain. This means that the forest above an opening 40 m long parallel to the slope has little infl uence on any rocks that move through it and then reach the forest below. The target profi les there-fore limit the length of openings in the fall line to 20 m.

If the stem number is high, there will be numerous contacts between rocks and trees. However, the stem density that is permanently possible in a forest is restricted. The stem numbers given in the profi les for large rocks are in the upper range of values to be found in virgin forests. For smaller rocks, slim stems serve same the purpose. The target diameter is therefore smaller but the stem number greater than in a virgin forest.

The target diameter exceeds the effective minimum diameter. It should be selected to sustain the required stem number with trees that have the effective minimum diameter.

The values for the effective minimum diameter given in the profi les are based on experiments and experience (Table 2).

Table 2: Rock size and assumed effective minimum diameters for the target profi les.

Rock volume Rock diameter Assumed (m3) (cm) effective minimum diameterup to 0.05 m3 up to about 40 cm up to 20 cm dbh0.05 m3 about 40 cm 20–35 cm dbhup to 0.20 m3 to 60 cm0.20 m3 over about 60 cm over 35 cm dbhup to 5.00 m3



The transit and/or deposition zones should have a minimum length if the forest is to be effective. Should the transit zone be short, without a deposition zone, the rocks small and the tree species in place able to sprout, coppice shoots can also be recommended for regeneration (see also Chapter 4.6).

In short transit zones, potential icefall should also be taken into account.

If the transit zones are long, the stands located close to the zone of origin are particularly important in stopping stones before they reach high speeds.

Effectiveness of lying logs Lying logs increase surface roughness. If the logs are at

an angle to the fall line, rocks will normally be slowed down. Logs lying along the contour lines brake the rocks and partly stop them. If there is a dense net of logs lying along the con-tour line, the danger of large rock accumulations is small since the rocks are well spaced out. In contrast, if there are only a few logs lying along the contour line in a forest, large accumulations of rocks may form.

Logs lying along the contour line are recommended if there is no run-out or deposition zone above the damage potential (e.g., a road). The rocks accumulated behind these logs need to be observed, and possibly secured before the logs decay, or alternatively new logs deposited below the decaying logs. Lying logs in the transit zone can also protect the stand itself from being injured. Logs lying at an angle to the fall line can help to canalise rocks. Where logs are weakly anchored, they must be observed in case they fall. This risk increases with increasing incline, certain logging practices (debranching and bark peeling) and snow.

Piles of branches improve damping effectsUprooted root plates increase the roughness of the

terrain and therefore help, at least in principle, to prevent rockfall. Problems may arise if large rocks are attached to the root plates. Such rocks are in most cases released with increasing root decay and thus become a rockfall source (which frequently occurs in the Jura mountains). Moreover, loose root plates are set in motion. If a log is bucked so that at least 4 m of the stem remain on the stump, this problem can largely be avoided.

High stumps favour braking and help to stop rocks.

4.5 Run-out and deposition zones

Role in the rockfall processThe speed of a falling rock will also diminish without

any contact with obstacles. At inclines between 25° (45 %) and 30° (58 %), rocks can roll over a long distance if they do not hit an obstacle. If the incline is less than 25° (45 %), rolling rocks will stop quickly. Once stopped, rocks do not usually start moving again.

Transit and deposition zones overlap. The same factors as in the transit zone serve to slow

down rocks.

Forest infl uenceContact with trees slows rocks down or stops them

completely. In principle, trees hit by rocks show the same reactions as those in the transit zone. As rock velocity is on average less than in the transit zone, the effective minimum diameters are accordingly smaller.

As in the transit zone, the larger the number of trees, the more contacts there will be between rocks and trees.

The stem numbers indicated in the profi les are higher than in a virgin forest, but the target diameter is accordingly smaller.

Effectiveness of lying logsLying timber increases surface roughness. Rocks that

have stopped moving remain immobile. There is a transition in rock movement from bouncing to rolling in the run-out and deposition zones, so that logs lying in these zones will be particularly effective obstacles.

Piles of branches improve damping effects.

Dead wood on windthrow areasOn uncleared windthrow areas, the timber provides an

effective protection against rockfall. For several decades, surface structures such as snags, stumps, root plates and lying logs remain densely entangled in layers several meters tall. These prevent any release of small to intermediate rocks and stop moving rocks. Only very large boulders can break through the entanglement due to their weight. Clearing considerably reduces the level of protection against rockfall.

NaiS Appendix 1 17

4.6 Additional information regarding forest effects

Silvicultural interventions compared to technical protective structures

The appropriate management of forest stands can replace technical constructions or allow them to be built more cheaply to accomodate lower bounce heights and less rock energy.

RotInjured trees can start rotting (Norway spruce and

beech after approx. 10 years). Wood grown after the injury will not be infected.

Coppice shootsSmall trees with a dbh of at least 12 cm are already

effective in the run-out and deposition zones as they are with small stones in the transit zone. In these cases it can be favourable to work also with coppice shoots if appropriate tree species are present. Coppice shoots grow very fast in their youth and reach the minimum dbh for effective protection after only a few years. When coppicing, the stumps must be cut cleanly and close to the ground so that not only the shoots but also the roots can renew themselves after the cut. As the space from stem to stem should not exceed 20 m in openings in the fall line, no large coppice cuts should be made, but only strips which have a maximum length of 20 m in the fall line. Coppice forests need intensive tending. The area must be tended regularly. Natural regulating forces cannot be relied on as much as in multi-layered forests. Therefore the areasbest suited for providing protection with coppice shoots are those where there is little space between the source of the rockfall and the objects at risk.

TopographyIn the rockfall process the topographical characteristics

must be taken into account. In the transit zone, small fl at areas can be used as deposition zones. Timber left lying in these areas is particularly effective. An eye must be kept on small isolated rockfall origin zones (e.g., unstable taluses, small rocky outcrops).

Determining the effective minimum diameterThe profiles in the transition zone are based on an

incline of about 35° (70 %) and average conditions in relation to rock shape, damping and surface roughness. The effective minimum diameter can be changed if these factors change, or if the tree species changes.

Factors which increase the effective minimum diameter:- Incline greater than 35° (70 %)- Round rocks - Poor damping (e.g. rocks at the soil surface)- Limited surface roughness - Tree species with soft wood (e.g. Norway spruce, silver

fi r, alder)- Tree species with wood that is prone to rot (e.g. Norway

spruce, beech)

Factors which decrease the effective minimum diameter:- Inclines below 35° (70 %)- Angular, elongated rocks - Good damping (e.g. soft soil, heaps of branches)- Very uneven surfaces (e.g. talus, lying wood, high

stumps)- Tree species with heavy wood (e.g. beech, locust,

hornbeam, ash, yew, oak)- Tree species that are rot resistant (e.g. silver fi r, larch,

valuable broadleaved species)

Source: Berger F. 1997. Interactions forêt de protection - risques naturels, Détermination des Zones d‘interventions forestières prioritaires, l‘exemple du département de Savoie, Thèse de doctorat, CEMAGREF Grenoble.Couveur S. 1982. Les forêt de protection contre les risques naturels, ENITEF - CEMAGREF Grenoble - DDAF Isère - ADRGT, Mémoire de fi n d‘études.Crenn R. 1999/2000. Infl uence exerceé par la forêt exploitée en taillis sur la dynamique du phénomène de chutes de pierres. Analyse rétrospective des traces d‘un événement récent sur la commune du Fontanil-Cornillon (38). Mémoire de DEA. Promotion. Cemagref Grenoble, Université Joseph Fourier.Gerber C., Elsener O. 1998. Niederwaldbetrieb im Steinschlaggebiet. Mitteilungen aus dem Gebirgswald. Wald und Holz 79, 14: 8–11.Gsteiger P. 1993. Steinschlagschutzwald. Schweiz. Zeitschrift für Forstwesen 144: 115–132.GWG/FAN. 1998. Waldwirkung und Steinschlag (inkl. Beiträge von W. Gerber, Geotest, W. Frey, R. Schwitter).Korpel` S. 1995. Die Urwälder der Westkarpaten. Fischer, Jena-Stuttgart-New York.Krummenacher B., Keusen H.-R. 1997. Steinschlag-Sturzbahnen: Modell und Realität. Mitteilungen der Schweizerischen Gesellschaft für Boden- und Felsmechanik Nr. 135, Herbstta-gung 7. Nov. 1997, Montreux.Leibundgut H. 1993. Europäische Urwälder: Wegweiser zur naturnahen Waldwirtschaft. Haupt, Bern-Stuttgart-Wien, 260 S.Mourer M. 1999. Forêt et phénomènes naturels: Les peuplements de bas versants face aux chutes de blocs en vallée de la pique (haute-garonne-31). Promotion. FIFENGREF, ONF, rtm.WSL. 2001. Schweizerisches Landesforstinventar LFI. Spezialauswertung der Erhebung 1993–95 vom 4. 12. 2001. Ulrich Ulmer. Eidg. Forschungsanstalt WSL, Birmensdorf.Zinggeler A. 1989. Die Modellierung der Steinschlaggefahr in Gebirgswäldern. Diplomarbeit, unveröffentlicht. Geographisches Institut der Universität Bern.



5.1 Target profi les in forests providing protection against torrents and fl oods5.2 Role of the forest in different situations5.3 The infl uence of forests on the hydrologic regime5.4 Importance of the site5.5 Signifi cance of individual factors affecting the state of the forest5.6 Forests on channel slopes5.7 Classifi cation of the site types

5.1 Target profi les in forests providing protection against torrents and fl oods

5 Torrents and fl oods

Locality

Catchment areaCatchment areaReduction of peakwater discharges in the whole catchment

ForestForest on channel on channelslopes slopes Prevention of negative impacts of timber inthe channel

Potential contribu-tion of the forestLargeLargeOn site types in class 11

IntermediateIntermediateOn site types in class 21

SmallSmallOn site types in class 31 Very smallVery smallOn site types in class 41

Small to largeSmall to largeDepends on channel characteristics(e.g., potential bottle-necks)

Hazard-related target profi le: ideal requirementsHorizontal structureCanopy cover2 permanently ≥ 70 %Ideal requirements of site-related target profi le accomplished

Horizontal structureCanopy cover2 permanently ≥ 50%Ideal requirements of site-related target profi le accomplished

Other requirementsNo unstable trees or stems prone to slideIdeal requirements of site-related target profi le accomplishedPioneer vegetation on areas whichare temporarily or permanentlyunstocked

Hazard-related target profi le:minimum requirements Horizontal structureCanopy cover2 permanently ≥ 60 %Minimum requirements of site-related target profi le accomplishedHorizontal structureCanopy cover2 permanently ≥ 50%MinimaI requirements of site-related target profi le accomplishedRegenerationPermanent regeneration ensured

No requirements

Other requirementsNo unstable trees or stems prone to slide

In areas where there are not only fl oods but also land-slide problems, the target profi les must be adjusted to each other. In the case of shallow landslide, as a rule, landslide

profi les should have priority. In contrast, intermediate and deep landslides must be assessed from case to case.

1 cf. site type classifi cation (Appendix 1, Chapter 5.7).2 The canopy cover relates to trees of at least pole size (i.e. young growth and thicket

stages are ignored).

NaiS Appendix 1 19

5.2 Role of the forest in different situations

Stable forests well-suited to the site are the most favou-rable for utilising the soil to store as much water as possible during heavy rain. Whether the status of the forest can also have a substantial infl uence on the runoff in the catchment area depends on the following basic circumstances:

Proportion of forest in the whole catchment area and its location

The infl uence on the total runoff in a catchment area is obviously the greater the larger the proportion of forest is in the catchment area. Additionally, the location of the forested area in the catchment area must be considered. Often forest is to be found growing on areas near watercourses (slopes of streams), which make the largest contribution to the runoff. Thus the forest can have a greater impact than one would expect it to have on the basis of its area alone.

Critical precipitation eventThe water regime is greatly infl uenced by the inten-

sity and duration of the precipitation that occurs. Short showers in a dry period are almost completely absorbed by inter-ception in a forest and only a small part reaches the ground. With heavier rainfall the impact of a short, heavy storm is very different from that of a long period of drizzle, even if the amount of rainfall is the same. The different intensities of the rain affect the infi ltration capacity of the soil, which may be insuffi cient after intense rainfall so that there is surface water runoff. This is less likely in the second case.

The time distribution of precipitation before an extreme event is also very signifi cant. Should the soil be very saturated due to snow-melt or previous precipitation, its storage capa-city will be diminished.

Thus we have, to put it simply, the following three typical scenarios3:

1 Short intense showers on a relatively small area 2 Long periods of heavy rain over a fairly large area 3 Rain spread over a wide area on soil with high water

saturation (e.g. during snow-melt)

The forest and vegetation in general have their most signifi cant protective role when the soil-water reservoir is at its emptiest at the time of the event. In the case of scenario 1, the signifi cance of the forest is therefore much greater than in scenario 3, with scenario 2 lying somewhere between the two.

When delineating forests that provide protection against fl oods, the likely scenarios for precipitation events need to be considered.

5.3 The infl uence of forests on the hydrologic regime

In the case of extreme precipitation, the forest has mainly an indirect impact on fl ooding by infl uencing soil characteristics and conditions in the long and medium term4. The soil characteristics also depend on bedrock, climate and topography. These three soil-forming factors enhance or reduce the potential infl uence of the forest.

The forest can have considerable impact by affecting the intensity and depth of root penetration. Rooting creates a fi nely branched system of cavities and thus pro-motes good soil permeability. The more intensive and the deeper the penetration of the soil is, the better the available water storage capacity of the soil can be utilised when precipitation is heavy.

In addition, the conditions on the soil surface, which affect the infi ltration capacity of the soil, can be infl uenced by the forest. When the soil is compacted on the surface, (e.g. due to machine use or cattle trampling), less water can infi ltrate within a useful timespan, so that the probability of surface runoff is increased. Conversely the infi ltration capacity can be signifi cantly increased by having a favourable humus and top-soil form and an intensive layer of forbs and moss.

4 While the aboveground vegetation considerably affects the annual course of discharge by interception and transpiration, its infl uence on individual extreme incidents is very small.3 after Zimmermann (2001), adapted

Natural hazards Torrents and fl oods

5.4 Importance of the site

The soil, particularly that in the rooting zone, is the key forest variable infl uencing the water regime. Soil character-istics can be easily determined at any point by boring or digging soil profi les, but the variations and distribution of the soil characteristics over a whole area are very diffi cult to determine. Every forest site type has a known range of soil characteristics, so that it can be used as a basis for assessing the soil characteristics over larger areas5.

In this way it is possible to assess how much the state of the forest can infl uence fl ooding in a particular location and if there is a need for silvicultural action. In Figure 6 this is shown schematically: on site A there is the highest absolute storage capacity, but the infl uence of the forest on the storage capa-city is greatest on site C. Sites of type C therefore have the highest priority in forest management. These sites are often sites which are periodically water-logged. In contrast to per-meable soils where the state of the forest plays a lesser role, deep-rooting tree species can increase the storage capacity substantially on intermittent watertable soils by improving the accessibility of the available storage space. The infl uence of the state of the forest is marginal on shallow soils and on soils with a very permeable subsoil (type D).

5.5 Signifi cance of individual factors affecting the state of the forest

Tree speciesTrees have typical, site-specifi c rooting patterns. The

rooting of different tree species in the soil can vary greatly, and also depends on the layering of the soil profi le and the development of the soil characteristics.

In the literature there are only few, rather inexact data concerning rooting in different soil types given, and they have mainly to do with maximum rooting depths. Some tree species have the special capability of accessing inter-mittent watertable soils, which is decisive for water storage. Of all the main tree species of the upper and lower montane altitudinal belt, this capability is the most developed in silver fi r. But beech also accesses these horizons better than Norway spruce. Of the secondary tree species, ash and maple have the best characteristics in this regard.

Regarding the species-specific rooting intensity, which is at least as decisive, Norway spruce also displays worse values than silver fi r and beech6.

From the point of view of the infiltration condi-tions, tree species with litter that decomposes well (broad-leaved trees, particularly ash and maple) are desirable. Non-absorbant organic surface matter that hinders the pen-etration of the water into the soil is unfavourable.

Stand structureRooting intensity obviously increases in the soil with

denser tree stocking. For the rooting to be as intensive as possible, a high canopy cover is best.

Additionally, an even distribution of the rooting throughout the entire potential rooting space is crucial in both horizontal as well as vertical directions.

In the horizontal direction this implies that there are as few gaps as possible. The size of the individual gap is not very important here, but the total area of the gaps is.

Figure 6: Range of storage effects in the case of heavy precipitation (schematically). The boxes designate the range for different site types: The lower edges designate the storage effect in the worst case (e.g., after large-scale windthrow), the upper edges the effects of stands with ideal structure.

A: large storage effect regardless of the state of the forest: sites with deep soils and normal permeability.B: intermediate storage effect regardless of the state of the forest: sites with intermediate soil depth and normal permeability.C: large storage effect if the state of the forest is good, and small effect if it is bad: sites with deep soils and limited permeability.D: small storage effect regardless of the state of the forest: heavily water-logged, very shallow or overly permeable sites.

Storage effect

large

inter-mediate

small

5 Studies on windthrow areas have demonstrated that the site class is a suitable instrument to assess the formation of surface fl ow (Badoux et al. 2003; Hegg 2004).

6 Lüscher (2000)


In the vertical direction rooting should be as regular as possible throughout the whole rooting zone. Pre-sumably multi-layered stocking that has a regular space distribution has a similar effect in the rooting space.The ideal stand structure is one that is multi-layered

on small areas with a dense canopy cover and regular distribution.

Soil compactionThe incorrect use of machines (especially during logging

operations) can lead to massive compaction of the soil. This causes a long-term deterioration of the infi ltration conditions, the permeability and storage capacity of the soil. The for-mation of continuous linear structures in the fall-line should, for example, be avoided when clearing damaged areas.

5.6 Forests on channel slopes

While forests have positive dampening effects on the water regime within the catchment area of a torrent, they can also act negatively through trees and wood falling in the channel. Logs within the range of a fl ood cross-section can be swept away by a fl ood or by a debris fl ow. At narrow points (rock spurs, stream bends, bridge culverts) congestion and bottle-necks can then occur.

Bottle-necks are particularly unfavourable because behind them bed load can accumulate, which can later be set in motion as debris fl ow. When a fl ood occurs, it can cause a channel displacement with fl ooding, overbank sedi-

mentation and debris fl ow deposition at blockage points. For this reason bottle-necks must be avoided where there is signifi cant damage potential.

Areas close to the stream bedThe relevant areas close to a stream bed are those which

would be affected by an extreme fl ood or debris fl ow.In most cantons monitoring these areas is the

responsibility of the water board or the public works department. Measures undertaken in the immediate proximity of the stream bed must therefore be coordinated with the appropriate authorities.

Managing forests on channel slopesThe relevant forest areas on channel slopes are those

from which wood can slide or be swept into the channel itself.On channel slopes the primary management target

is to maintain stable stocking so that no wood can reach the stream bed and cause blockages. The most important measure, therefore, is the selective removal of unstable trees (and root plates). As channel slopes are usually areas that are diffi cult to access, an option to clearing can also be cutting logs into short pieces. The size of the pieces depends on the conditions and the possible blockage points in the stream bed.

A destabilisation of the embankment and surface erosion can often be prevented by stable stocking. In this case the target profi le to prevent landslides must also be considered.

NaiS Appendix 1 21 Natural hazards Torrents and fl oods


In the following cases there may be some deviation from this classifi cation:

Sites that are diffi cult to infl uence silviculturallyThe possibility of infl uencing the water storage capa-

city of the soil through silviculture is not the same on all site types. At higher altitudes (sub-alpine zone) especially, the infl uence generally decreases. Norway spruce is usually the only tree species which can be used. The processes are generally slower, and the stand densities and, as a consequence, the rooting intensity are lower. But also on other sites (e.g. pure ash sites), the silvicultural possibilities are limited by the absence of other tree species. These sites are therefore allocated to a lower class.

5.7 Classifi cation of the site types

The site types are classifi ed in principle according to the three soil criteria: depth, water logging and permeabi-

lity (Figure 7). These are known for the soils of all site types (Table 3, the soil characteristics of each site type are descri-bed in Appendix 2A, unavailable in English):

Soil depth Water-logging Permeability

heavily water-logged limited normal high

very shallow very shallow heavily water-logged

shallow to intermediate

intermediate

to large

* impermeable underground

shallow to intermediate, with limited permeability

intermediate to large, with

limited permeability

shallow to intermediate, with normal permeability

intermediate to large*,

with normal permeability

highlypermeable

Legend: Class 1 Class 2 Class 3 Class 4 large silvicultural intermediate silvicultural little silvicultural very little silvicultural infl uence infl uence infl uence infl uence

Examples7: 26 Aceri-Fraxinetum 53 Polygalo chambaebuxi-Piceetum 57 Sphagno-Piceetum

calamagrostietosum villosae

Sites with highly variable soilSome important and common site types have highly

variable soil characteristics. One reason for this is the variation in regional (especially geological) conditions. There-fore, these site types cannot be assigned to a single class for all of Switzerland. They must be classifi ed separately for each region, and the classifi cation explained. Hence, they are assignedin the following list to a class of their own (class E = case-wise assessment). For these site types, instructions are given in Figure 8 to explain under what circumstances they can be assigned to which class. For the defi nite assignment, additional investigations of the soil in the particular forest will be necessary (Hegg et al., 2004).

Figure 7: Site type classifi cation based on soil characteristics.

7 Numbers refer to the site type classifi cation in Appendix 2A (see also Table 3)

NaiS Appendix 1 23

Site type

7a, 8a,18, 19,

50, 50P,51

1112S46

12a46M

(42)-34ARob

49

Class 1

if there is clear evidence of water-

logging (common on Flysch soils)

if the soil is deep (common in the Plateau

and in the Pre-Alps)

Class 2

if there is no or almost no evidence of water-

logging

if the soil is shallow (common in the Jura

mountains)if the soil is deep (com-

mon in the Plateauand in the Pre-Alps)

on Cambisols and(humic) Podzols

if the proportion of indented sites (with

vegetation indicating wet conditions)

< 60% of the area

Class 3

if the soil is shallow (common in the Jura

mountains)

on Leptosols



> 60% of the area

Class 4



< 80% of the area

Tips for handling the site type classifi cation

If a site map exists, a priority map can be produced with the help of the four classes described above. The classes based on the site type must be weighted according to the urgency and effectivity of the silvicultural inter-vention in the individual stand. In this way, is is possible to determine where silvicultural interventions will have the largest infl uence on the storage capacity of the soil.

When necessary, deviations from the classification according to the list can be made. This can be the case, for example, if the geology suggests there are differences in the depth of soil or the permeability of a particular site type.

Usually not only pure units are to be found on a site map. There are often transitions or a mosaic of different site types. In this case, the different classifi -cations of the unit must be weighted against each other.

Example: An area was mapped as transitional 18 (20). 18 (Abieti-fagetum festucetosum) can, depen-ding on circumstances, belong to classes 1 or 2, and 20 (Abieti-fagetum polystichetosum) only to class 1. This area is therefore assigned to class 1.

When deciding on silvicultural interventions to help retain fl oods, it is not the single stand or the single stand type area which is important, but rather the state of the forest in the whole catchment area. Thus, as well as taking into account the site type and the forest’s state, further factors, e.g. timber harvesting methods, must be considered during plan-ning to achieve the best possible result.

Figure 8: Classifi cation of the site types with highly variable soil characteristics.

Natural hazards Torrents and fl oods


Class 1 Sites with large silvicultural infl uenceSoils with limited permeability, intermediate to large soil depthSoils with limited permeability, intermediate to large soil depth7S Galio-Fagetum stachyetosum silvaticae8S Milio-Fagetum stachyetosum silvaticae8* Milio-Fagetum blechnetosum19f Luzulo-Abieti-Fagetum, variant on Gleysol20 Adenostylo alliariae-Abieti-Fagetum typicum20E Adenostylo alliariae-Abieti-Fagetum hordelymetosum20* Streptopo-Fagetum s.l. prov.

Class 2 Sites with intermediate silvicultural infl uenceSoils with limited permeability, shallow to intermediate soil depthSoils with limited permeability, shallow to intermediate soil depth9w Pulmonario- / Lathyro-Fagetum caricetosum fl accae10w Pulmonario- / Lathyro-Fagetum melittetosum, variant with Carex fl acca 18v Adenostylo glabrae Abieti-Fagetum calamagrostietosum variae, variant with Carex ferruginea18w Adenostylo glabrae Abieti-Fagetum calamagrostietosum variae46* Vaccinio myrtillii-Abieti-Piceetum sphagnetosum

Soils with normal permeability, intermediate to large soil depthSoils with normal permeability, intermediate to large soil depth3mL-4L Ilici-Fagetum typicum and dryopteridetosum4 Luzulo niveae-Fagetum dryopteridetosum9a Pulmonario- / Lathyro-Fagetum typicum18M Adenostyle glabrae-Abieti-Fagetum typicum19L Laburno-Abieti-Fagetum typicum25A-34mA Cruciato glabrae-Quercetum p.p. Luzulo niveae-Tilietum25Am-33m Arunco-Fraxinetum typicum; Luzulo niveae-Tilietum p.p. 25AB-33B Arunco-Fraxinetum; Luzulo niveae-Tilietum p.p.25AF Lunario-Acerion, Tilion, Arunco-Fraxinetum p.p. 50* Adenostylo glabrae-Abieti-Piceetum typicum51C Galio-Abieti-Piceetum coryletosum52 Adenostylo glabrae-Abieti-Piceetum caricetosum albae55 Veronico latifoliae-Piceetum

Class 3 Sites with little silvicultural infl uenceSoils with normal permeability, shallow to intermediate soil depthSoils with normal permeability, shallow to intermediate soil depth1h Luzulo-Abieti-Fagetum, variant poor in species3 Luzulo niveae-Fagetum typicum3VL Ilici-Fagetum typicum, variant poor in nutrients10a Pulmonario- / Lathyro-Fagetum melittetosum12w Mercuriali- / Cardamino-Fagetum caricetosum fl accae12*h Cardamino-Fagetum veratretosum13a Tilio-Fagetum typicum24* Ulmo-Aceretum33AV-33A Arunco-Fraxinetum vaccinietosum34B Cruciato glabrae-Quercetum p.p.36 Carpino betuli-Ostryetum37 Fraxino orni-Ostryetum47 Calamagrostio-villosae-Abieti-Piceetum typicum47D Calamagrostio-villosae-Abieti-Piceetum dryopteridetosum47M Calamagrostio-villosae-Abieti-Piceetum melampyretosum

Table 3: Classifi cation of site types in Swiss protection forests. The nomenclature follows that in Appendix 2A in the German version.

NaiS Appendix 1 25

Sites with little silvicultural infl uence, intermediate to large soil depthSites with little silvicultural infl uence, intermediate to large soil depth21 Aceri-Fagetum21* Alno viridi-Sorbetum aucupariae prov.26 Aceri-Fraxinetum26h Aceri-Fraxinetum, variant in high altitude47* Rhododendro-Abietetum54 Melico-Piceetum typicum57C Homogyno-Piceetum calamagrostietosum villosae57M Homogyno-Piceetum melampyretosum sylvatici57V Homogyno-Piceetum vaccinietosum myrtilli59V Larici-Pinetum cembrae vaccinietosum myrtilli60 Adenostylo-Piceetum typicum60A Adenostylo-Piceetum athyrietosum distentifolii60* Calamagrostio variae-Piceetum

Class 4 Sites with very little silvicultural infl uenceHighly permeable soils, intermediate to large soil depthHighly permeable soils, intermediate to large soil depth53 Polygalo chamaebuxi-Piceetum53* Erico-Piceetum57S Homogyno-Piceetum sphagnetosum57Bl Homogyno-Piceetum, variant on boulders58 Larici-Piceetum typicum58C Larici-Piceetum calamagrostietosum villosae58L Larici-Piceetum laserpitietosum halleri59 Larici-Pinetum cembrae typicum59A Adenostylo-Laricetum59C Cotoneastro-Pinetum cembrae59E Larici-Pinetum cembrae ericetosum59J Junipero-Laricetum59L Larici-Pinetum cembrae laserpitietosum halleri59* Rhododendro ferruginei-Laricetum60E Adenostylo-Piceetum equisetetosum silvaticae72 Sphagno-Pinetum cembrae

Heavily water-logged soilsHeavily water-logged soils27 Carici remotae-Fraxinetum27h Carici remotae-Fraxinetum, variant with Petasites albus 27* Adenostylo-Alnetum incanae33-27 Osmundo-Alnetum; Arunco-Fraxinetum p.p.49* Equiseto-Abieti-Piceetum caricetosum ferrugineae56 Sphagno-Piceetum71 Sphagno-Pinetum montanae Very shallow soilsVery shallow soils12e Mercuriali-/Cardamino-Fagetum caricetosum albae12* Cardamino-Fagetum insubricum s. l.14 Carici (albae)-Fagetum typicum14* Cephelanthero-Fagetum insubricum s. l.15 Carici-Fagetum caricetosum montanae17 Taxo-Fagetum / Seslerio-Fagetum calamagrostietosum variae18* Adenostylae glabrae-Abieti-Fagetum caricetosum albae61 Molinio-Pinetum sylvestris62 Cephalanthero-Pinetum sylvestris65 Erico- / Coronillo-Pinetum sylvestris65* Ononido-Pinetum sylvestris67 Erico-Pinetum montanae

Natural hazards Classifi cation of site types


68 Calluno-Pinetum sylvestris68* Vaccinio vitis-idaeae-Pinetum sylvestris69 Rhododendro hirsuti-Pinetum montanae70 Rhododendro ferruginei-Pinetum montanae Highly permeable soils, shallow to intermediate soil depthHighly permeable soils, shallow to intermediate soil depth13e Tilio-Fagetum caricetosum albae13eh Adenostylo-Fagetum seslerietosum13h Adenostylo-Fagetum typicum22 Phyllitido-Aceretum23 Sorbo-Aceretum25 Asperulo taurinae-Tilietum typicum25B Asperulo taurinae-Tilietum, insubric variant s.l. 25* Aceri-Tilietum / Asperulo taurinae-Tilietum tametosum42R Phyteumo betonicifoliae-Quercetum festucetosum variae; Quercion pubescenti-petraeae p.p.42C/Q Phyteumo betonicifoliae-Quercetum typicum42V Phyteumo betonicifoliae-Quercetum vaccinetosum47H Hypno-Piceetum48 Asplenio-Abieti-Piceetum55* Luzulo niveae-Piceetum

Class E Sites with variable silvicultural infl uence (case-wise assessment)Soils highly variableSoils highly variable7a Galio-Fagetum typicum8a Milio-Fagetum typicum11 Aro-Fagetum12a Mercuriali-/Cardamino-Fagetum typicum12S Mercuriali-/Cardamino-Fagetum circaetosum / allietosum18 Festuco-Abieti-Fagetum19 Luzulo-Abieti-Fagetum typicum(42)-34A Phyteumo betonicifoliae-Quercetum polygonatetosum multifl orii; Cruciato glabrae-Quercetum p.p.46 Vaccinio myrtillii-Abieti-Piceetum typicum46M Vaccinio myrtillii-Abieti-Piceetum melampyretosum49 Equiseto-Abieti-Piceetum typicum50 Adenostylo alliariae-Abieti-Piceetum typicum50P Adenostylo alliariae-Abieti-Piceetum petasitetosum51 Galio-Abieti-Piceetum typicumRob Chelidonio-Robinion; Carpinion s. l. p.p.

Source: Badoux A., Witzig J., Lüscher P., Hegg C. 2003. Einfl uss von Sturmschäden auf die Abfl ussbildung und den Wasserhaushalt von Wildbächen - aufgezeigt am Beispiel des Sperbelgrabens. FAN-Agenda 2/03: 5–10.Hegg C., Thormann J.J. (Hrsg.) 2004. Lothar und Wildbach, Schlussbericht eines Projektes im Rahmen des Programms Lothar Evaluations-und Grundlagenprojekte. Eidg. Forschungs-anstalt WSL, Birmensdorf.Kölla E. 1986. Zur Abschätzung von Hochwassern in Fliessgewässern an Stellen ohne Direktmessungen. Mitt. VAW 87, Zürich.Lüscher P. 2000. Untersuchungen von Wurzelsystemen (Fi, Ta, Bu) auf ausgewählten Waldstandortstypen im Gantrischgebiet. WSL, unveröffentlicht.Lüscher P., Zürcher K. 2003. Waldwirkung und Hochwasserschutz: Eine differenzierte Betrachtungsweise ist angebracht. - Ber. Bayer. Landesanst. Wald Forstwirtsch. 40: 30–33.Ott E., Frehner M., Frey H. U., Lüscher P. 1997. Gebirgsnadelwälder. Ein praxisorientierter Leitfaden für eine standortgerechte Waldbehandlung. Haupt Verlag Bern Stuttgart Wien.Polomski J., Kuhn N. 1998. Wurzelsysteme. Eidg. Forschungsanstalt WSL, Birmensdorf.Richard F., Lüscher P., Strobel T. 1978–1987. Physikalische Eigenschaften von Böden in der Schweiz. Bände 1–4. EAFV, Birmensdorf.Zimmermann M. 2001. Gefahren und Schadenpotentiale aus Überschwemmung und Übersarung im Kanton Bern. Referat zu einer Studie der Gebäudeversicherung des Kt. Bern (GVB), Medienkonferenz vom 11.5.2001.Zürcher K., Lüscher P., Wasser B. 2000. Anleitung zur Bewertung der Waldwirkung auf das Abfl ussverhalten der wichtigsten Standortstypen im voralpinen Flysch. Arbeitsgrundlage für die GWG-Tagung vom 21.–23. August 2000, Hotel Gurnigelbad BE, unveröffentlicht.Zürcher K. 2003. Wald - Hochwasser. Prioritäten bei waldbaulichen Massnahmen in hydrologischen Einzugsgebieten. Forschungsauftrag BUWAL (F+D). Schlussbericht, unveröffent-licht.

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