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Urban Forestry & Urban Greening 17 (2016) 158–165

Contents lists available at ScienceDirect

Urban Forestry & Urban Greening

journa l homepage: www.e lsev ier .com/ locate /u fug

lanting clonal shade-tolerant herbs in young urbanoodlands—Effects of compost on plant growth, flowering and

urvival

ustav Richnau a, Jörg Brunet b, Anders Busse Nielsen a,c,∗, Björn Wiström a

Department of Landscape Architecture, Planning and Management, Swedish University of Agricultural Sciences, P.O. Box 66, SE-230 53 Alnarp, SwedenSouthern Swedish Forest Research Centre, Swedish University of Agricultural Sciences, P.O. Box 49, SE-230 53 Alnarp, SwedenDepartment of Geosciences and Natural Resource Management, University of Copenhagen, Rolighedsvej 23, DK-1958 Frederiksberg C, Denmark

r t i c l e i n f o

rticle history:eceived 7 February 2014eceived in revised form 9 December 2015ccepted 13 April 2016vailable online 22 April 2016

eywords:fforestationmenity valueorest restorationabitat creationerbaceous layer introduction

a b s t r a c t

Rich herbaceous layers supply important ecosystem functions and amenity values in urban wood-lands. However, due to poor dispersal and recruitment, typical woodland herbs often remain absentin woodlands established on post-agricultural and post-industrial sites, especially in fragmented urbanlandscapes. Efficient methods for active restoration of the herbaceous layer are therefore needed. In aplantation experiment in an 11-year-old oak stand established on post-agricultural land just outsideMalmö City, southern Sweden, this study investigated the effects of compost addition and weed controlon post-planting survival, growth and flowering of three typical shade-tolerant woodland herb species:Galium odoratum, Lamiastrum galeobdolon and Stellaria holostea. Removal of competing weed vegetationdid not affect plant performance, but compost addition dramatically increased growth and flowering of allthree species during the first two growing seasons, and also increased survival in S. holostea. This positivetreatment effect probably derived from a more suitable top soil structure and a higher soil moisture in the

oil conditions compost treatment. We conclude that combined use of planting and composting is an effective methodfor rapid achievement of profusely flowering carpets of summer-green woodland herbs, which can pro-vide important amenity values in the many woodlands established in and around cities in north-westEurope in recent decades. The method may also be applicable in other woodland restoration projects,where it can promote colonisation by typical shade-tolerant woodland herb species.

© 2016 Elsevier GmbH. All rights reserved.

. Introduction

During recent decades, the pace of urbanisation has increased.his has resulted in most of the world’s population now living inrban areas, where the pressure on green spaces to provide bothiodiversity and human recreation services, with related health andell-being benefits, is higher than ever (Young et al., 2005). Being

mong the most natural components of the urban landscape, wood-and plays a key role in the provision of such services (Alvey, 2006;ilsson et al., 2011).

Expanding the woodland cover in urban areas has become clear, consistent political priority in the forest-poor, highlyrbanised countries of north-west Europe (Nielsen and Jensen,

∗ Corresponding author at: Department of Landscape Architecture, Planning andanagement, Swedish University of Agricultural Sciences, P.O. Box 66, SE-230 53

lnarp, Sweden.E-mail address: anders.busse.nielsen@slu.se (A.B. Nielsen).

ttp://dx.doi.org/10.1016/j.ufug.2016.04.009618-8667/© 2016 Elsevier GmbH. All rights reserved.

2007). In England alone, 10,000 ha of community forest have beenestablished in and around the twelve largest cities since 1990(England’s Community Forests, 2013). During the same period,more than 30,000 ha of post-agricultural land have been afforestedin Denmark, much of which is located at the fringe of urban areas(Danish Forest and Nature Agency, 1999, 2010).

However, numerous studies have documented that many typi-cal woodland herb species remain absent in woodlands establishedon post-agricultural or post-industrial land, so that the communitycomposition bears the imprint of past land use for many decadesor even centuries (Flinn and Vellend, 2005). There is general con-sensus that the low colonisation rate of woodland herbs is due inpart to their poor dispersal capacity and in part to recruitmentfailure in post-agricultural habitats (Flinn and Vellend, 2005). Ameta-analysis of studies across Europe has shown that the rate of

colonisation is especially poor in highly fragmented landscapes –as often found in urban areas – where the proportion of mature(and ancient) woodlands which act as seed sources is low and these

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ccur mostly in isolated patches (De Frenne et al., 2011). In addi-ion, soil conditions in new urban woodlands are influenced by aersistent legacy of intensive land use. The organic matter con-ent of the topsoil is often reduced, which decreases soil porositynd water-holding capacity (McLauchlan, 2006). As a consequence,esiccation has been identified as a potential reason for poor estab-

ishment of introduced woodland herb species (Cohn and Packham,993).

In most urban afforestations only trees and shrubs are plantedFrancis et al., 1992; Danish Forest and Nature Agency, 1999, 2010),espite the fact that the herbaceous layer is by far the most impor-ant contributor to overall plant diversity in mature woodlandsnd also provides important ecosystem functions and amenityalues (Gilliam, 2007). From an ecological perspective, wood-ands without an appropriate herbaceous layer can be regarded asincomplete” ecosystems (Gilliam, 2007). From a social perspective,uch afforestations may be perceived by visitors as unattractive andnsuitable for recreation (Gundersen and Frivold, 2008; Jensen andkovsgaard, 2009). While spring bulbs and vernal geophytes are aajor attraction for woodland visitors, Nielsen et al. (2012) found

hat the shade-tolerant summer-green species are also importantnd when absent, visitors perceive the woodland to be empty oracking some vital element.

Our current knowledge on introductions of typical woodlandlant species is mainly based on studies in basic plant or conser-ation biology, analysing abiotic or biotic factors limiting planterformance (Menges, 2008; Godefroid et al., 2011). However,ecognising that new urban woodlands need more than trees tochieve their amenity and ecosystem functions and services, someesearch on the introduction of herbaceous layers into recentlylanted urban woodlands was initiated during the 1980s, partic-larly in the UK (Francis et al., 1992; Cohn and Packham, 1993),ut also in Denmark (Bundgaard Andersen and Mikkelsen, 1983)nd Sweden (Hammer, 1991). These studies have shown that aumber of vernal geophytes and woodland edge species can estab-

ish successfully after simple hand-sowing (Hammer, 1991; Francisnd Morton, 2001). In comparison, shade-tolerant summer-greenpecies have been found to be more problematic to establish fromeed, due to poor germination and inability to tolerate competitionrom weeds during slow seedling establishment and growth. Con-equently, a higher rate of establishment from planting comparedith sowing has previously been reported (e.g. Hammer, 1991;

rancis and Morton, 2001; Petersen and Philipp, 2001; Mottl et al.,006; Menges, 2008; Godefroid et al., 2011).

Planting forest herbs may thus provide a faster and more reliablestablishment than sowing, but is also more expensive. However,everal of the characteristic shade-tolerant herbs in European tem-erate forests possess the ability of rapid vegetative spread byhizomes or stolons (Brunet and von Oheimb, 1998). By provid-ng favourable growth conditions, already a limited number oflanted individuals may therefore be sufficient to establish a flow-ring carpet of understory herbs and thereby rapidly increase theecreational value of a new urban forest.

The use of compost is an old and still widely used practice tomprove growing conditions in urban settings where soil condi-ions are unfavourable for plant growth (Saebø and Ferrini, 2006).ompost addition as site preparation prior to planting as well asompost addition in existing planting improves the physical andhemical structure of the soil and increases the water-holdingapacity and cation exchange capacity (Saebø and Ferrini, 2006;ldfield et al., 2014; Palmer and Davies, 2014). While most stud-

es demonstrated positive effects of compost treatment on soil

onditions, effects on tree seedling survival and growth are oftenbsent (Larchevêque et al., 2008; Palmer and Davies, 2014). To ournowledge, no previous study has examined the effects of com-

an Greening 17 (2016) 158–165 159

post treatment under controlled conditions on the performance ofplanted understory herbs in urban woodlands.

The objective of this experiment was to analyse the initial effectof compost addition and weed control on the survival, floweringand abundance of three shade-tolerant, clonal herb species intro-duced as plug plants in an 11-year old Quercus petraea (Matt.)Leibl. stand on post-agricultural land. The species used, Galiumodoratum (L.) Scop. (sweet woodruff), Lamiastrum galeobdolon (L.)Ehrend. and Polatschek (yellow archangel) and Stellaria holosteaL. (greater stitchwort), are characteristic carpet-forming ancientwoodland indicator herbs in temperate broadleaf forests of north-west Europe. The hypotheses tested in the study were that: (1)compost addition improves the initial survival, growth and flow-ering of the three planted clonal woodland herbs and (2) weedcontrol further increases early growth of these woodland speciesby reducing potential competitors.

2. Materials and methods

2.1. Experimental site

The study was carried out in the Landscape Laboratory locatedadjacent to the campus of the Swedish University of AgriculturalSciences (SLU) in Alnarp (55.39N; 13.04E) at the fringe of MalmöCity, southern Sweden. The Landscape Laboratory was successivelyestablished on former arable land between 1983 and 1998 as a full-scale trial, teaching and demonstration area on urban woodlandand parks (Gustavsson, 2002; Nielsen, 2011). The original soil atthe Landscape Laboratory site is fertile loamy clay with low organicmatter content.

The experiment was performed during 2009 and 2010 in ahomogeneous Q. petraea stand established on flat ground (<0.5 mtopographical variation within the stand) with whips planted with1.5 × 1 m spacing in spring 1998 (equal to app. 6600 trees perhectare), i.e. 11 and 12 years after tree planting. The stand bor-ders open fields, except to the south where it borders a stand withQuercus robur (Fig. 1). No shrub species were planted along thestand perimeter, resulting in abrupt edges where especially thepredominately western northwestern winds from the sea (1 kmaway) penetrate under the canopy. A Quercus stand was chosenbecause this genus is among those most frequently used in urbanafforestation in countries in north-west Europe, such as Denmarkand Sweden, where it is also one of the most common broadleaftrees in commercially managed forests (Anon, 2009; Jensen andSkovsgaard, 2009). Mechanical weeding was done in the initialthree years after stand establishment. As an effect of this and subse-quent canopy closure, herb layer vegetation prior to establishmentof the experiment was generally sparse (ca. 3% cover, unpublisheddata), and was mainly composed of the native forbs Chenopodiumspp., Plantago lanceolata L., and Taraxacum spp., and the nativegrasses Dactylis glomerata L., Elytrigia repens L., and Poa nemoralisL. Most of the ground was covered by a thin layer of Quercus leaflitter.

The stand was thinned in winter 2009. While the thinning wasdone just prior to establishment of the experiment it was notprimarily motivated by the experiment but rather because theQuercus’ crown depth was beginning to reach less than the desired50% of total tree height. Following usual management practice inyoung oak plantations, 33% of the stems were removed (includingthe few spontaneously established understory shrubs), resulting in

a post-thinning tree density equal to app. 4400 trees per hectarewith uniform spacing and no understorey. To avoid soil damage,thinning and extraction of all stems and branches from the standwas done manually.

160 G. Richnau et al. / Urban Forestry & Urban Greening 17 (2016) 158–165

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ig. 1. Map over the field experiment with block number and treatments. Each treapen grassland to the west, north and east (dotted raster) and to the south it borde

.2. Experimental design

The experiment was initiated in April 2009 and monitored untilhe end of September 2010. As compost addition may increase therowth of competitive weedy species, the compost treatment wasombined with a weeding treatment in a fully randomized factorialesign and replicated in eight blocks (Fig. 1). Each plot measured

m × 6 m and included five rows of Quercus trees, with a tree rowpacing of 1.5 m. In half the plots, a 10 cm thick layer of gardenaste compost obtained from a municipal recycling station wasanually applied in April 2009 on the soil surface without mix-

ng or removing the litter layer, while the other plots were left asntreated controls (Fig. 2a). The compost used was certified accord-

ng to Swedish certification standard SPCR 152.G. odoratum, L. galeobdolon and S. holostea were chosen as test

pecies because they are able to form large carpets by vegeta-ive growth and flower abundantly in spring and early summer.ence, rapid establishment of these species would considerably

mprove the recreational and ecological value of young urbanoodlands. G. odoratum is a wintergreen hemicryptophyte, spread-

ng vegetatively by thin rhizomes. L. galeobdolon is an evergreenhamaephyte, spreading vegetatively above ground by stolons.. holostea is another evergreen chamaephyte, spreading vegeta-ively by thin rhizomes or laying stems with adventitious roots.

hile these differences in leaf duration and means of vegetativepread may also result in different growth rates of the three stud-ed species, all are able to gradually colonise post-arable woodlandsdjacent to source populations (Brunet and von Oheimb, 1998;runet et al., 2012). However, they rarely colonise isolated wood-

ands in fragmented landscapes (Brunet, 2007; Brunet et al., 2011),hy planting or sowing is generally necessary for introduction toost urban woodlands.

t plot has a size of 6 × 6 m. The Quercus petraea plantation (white) is surrounded by Quercus robur L. stand, also planted in 1998.

An even layer of compost was added manually in early April2009, using shovels, wheelbarrows and rakes. Plug plants (plugvolume 93 cm3) of all three herb species were obtained from acommercial nursery specialising in wild plants and planted intothe compost and the untreated mineral soil, respectively in mid-April 2009 using a hand-shovel (Fig. 2b). In each of the four stripesbetween the tree rows in a treatment plot, six plants were planted at1 m spacing, with the three species alternating in as well as betweenthe planting rows. Each plot thus contained 8 plants of each species,giving a total of 24 plants per plot and 768 plants (3 species × 8individuals per plot × 32 experimental plots) in the experiment. Allplants were irrigated at the time of planting, but did not receive anyother care. After establishment, the experimental area was fencedto reduce herbivorous mammals, especially rabbits.

Competing vegetation remained poorly developed in 2009. Weexpected a more vigorous growth of competing weeds in thecompost treatment in 2010 as a combined result of seed influx,favourable growing conditions and the elevated light conditionsdue the recent thinning of the stand. In the second growing season(2010), competing vegetation within 50 cm of living transplantswas therefore removed in one randomly chosen compost plot andone control plot per block. This weeding process was conductedin the beginning of May and again in mid-June. Later during thegrowing season, further weeding was found to be unnecessary.

2.3. Measurements of plant performance

The number of flowering individuals was counted in mid-June

during both years. To determine the aboveground area occupied byeach plant, the area was approximated to an equivalent rectangleand the length of the sides was measured with a ruler at the end ofthe growing season in early September 2009 and 2010, when also

G. Richnau et al. / Urban Forestry & Urb

Fig. 2. Photos from the experiment: (a) Layout of the compost and control plotsbefore planting; (b) situation in September 2009; (c) situation in June 2010; (d)situation in June 2012.

an Greening 17 (2016) 158–165 161

dead plants were recorded. For L. galeobdolon, it was occasionallydifficult to separate different individuals from each other in 2010due to their rapid stolon growth. Therefore in these cases, the treerow between two neighboring individuals was used to separateindividuals for further area calculation.

2.4. Data analysis

The effects of compost treatment (2009 and 2010) and weeding(2010) on species cover, flowering and survival were analysed usinggeneral linear models (GLM sensu Olsson, 2002) with experimentalblock as a random factor. Data for each species were tested in sep-arate GLMs using the mean values of all living plants per treatmentplot. The analyses showed significant (P < 0.05) results of composttreatment and block, while weeding did not influence the mea-sured variables mean cover and survival (flowering not included asit was measured just prior to the second weeding). Therefore, thefinal results presented here are restricted to GLMs including inter-actions between compost treatment and block, while interactionswith weeding are excluded. The data was analysed in SPSS 19.0.0.

To provide background information for interpretation of thetreatment effects, precipitation data were assessed and soil mois-ture and nutrient supply measured as described in the following.

2.5. Soil chemistry

In each of the eight blocks, a sample of the topsoil (0–5 cm depth)was taken in each compost plot and in the mineral soil of the controlplots at the end of the experiment in September 2010 (2 × 16 sam-ples), and pooled to one bulk sample per treatment before analysis.Pooling of the samples was motivated because the compost wasfrom one uniform batch, and the experimental site has been aneven-level agricultural field for a very long time, and was plantedwith trees in a uniform manner. Therefore in both cases large vari-ation in soil chemistry is highly unlikely.

Soil pH (H2O-extraction), total element concentrations (ana-lysed by ICPAES, ICPMS or element analyser, depending onelement) and readily exchangeable element concentrations (Spur-way extraction) were determined for fresh compost and the twobulk samples at the Plant Ecology Laboratory, Dept. of Biology, LundUniversity.

Analyses of the fresh compost in 2009 and of the topsoil in thetreatments at the end of the experiment in 2010 showed that soilpH and total Mg and K content were rather similar in the compostand the mineral soil of the control plots, while total content of C,N, Ca, P and S was much higher in the compost and total content ofFe was much higher in the mineral soil, which is at least partlyexplained by the much higher content of organic matter in thecompost (Table 1, note that values are based on dry weight). Inter-mediate measurements of element concentrations in the compostplots in 2010 indicated that some mixing of compost and underly-ing mineral soil had occurred during the two growing seasons. Theresults of the Spurway extraction showed that plant-available con-centrations of all macronutrients (Ca, Fe, K, Mg, P) were several-foldhigher in the compost plots than in the mineral soil of the controlplots (Table 1).

2.6. Rainfall and soil moisture

Rainfall during the growing season (April–September) was270 mm in 2009, which represented close to average long-termamounts, and 390 mm in 2010 (SMHI, 2014). Rainfall in April was

below 50% of normal amounts in both years, while May and Junehad average or slightly more than average rainfall. July was drierthan normal in both years. This drought continued into Septemberin 2009, while August was very moist in 2010 (SMHI, 2014).

162 G. Richnau et al. / Urban Forestry & Urban Greening 17 (2016) 158–165

Table 1pH (H2O) and total macroelement concentrations (mg g−1 dry weight) in fresh compost prior to treatment in 2009 and in topsoil (0–5 cm) in the compost plots and inthe control plots in September 2010. Readily plant-available macroelement concentrations (�g g−1 dw) determined by Spurway extraction of topsoil samples collected inSeptember 2010 are also shown.

Fresh compost 2009 Compost treatment 2010 Control treatment 2010 Compost treatment 2010 Control treatment 2010Total Total Total Spurway Spurway

pH 7.6 7.5 7.2C 163.0 121.0 24.0N 11.0 11.0 3.0Ca 23.4 18.2 5.7 7991 1566Fe 7.4 9.6 16.1 14 2K 5.9 3.2 4.1 671 37

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Mg 2.7 2.5 2.9

P 2.0 1.4 0.5

S 1.9 1.1 0.3

Soil water potential was measured at 8 cm depth in the compostr untreated mineral soil, respectively during both growing sea-ons with a stationary gypsum block (5201 Soil Moisture Blocks,oil Moisture Equipment Corp., CA, USA) in four different com-ost plots and four different control plots. The plots were selectedo that the eight gypsum blocks were distributed in all the eightlocks. Measurements were made from 1 May until 1 October,ith approximately one measurement every two weeks; 15 and

0 measuring occasions in 2009 and 2010 respectively.The repeated measure of Soil water potential was analysed using

mixed linear model with the gypsum block as random effect andime and treatment including their interactions as fixed. The modelas run with and without distance of the block from the western

dge as a covariate. This was done using the Proc Mixed proceduren SAS 9.4 with REML as estimation method and an Autoregressive

oving Average covariance structure.Including distance of the block from the western edge as a

ovariate resulted in a lower Bayesian Information Criteria (BIC)n both years and higher F-values in 2009 for all variables espe-ially the compost treatment, indicating a lower soil water potentialn the blocks closest to the western edge. For the 15 time pointsn 2009 there was both a strong compost treatment (F 1,5 = 28.43;

= 0.0031) and time effect (F 14,80 = 22.37; p < 0.0001) as well as annteraction effect between them (F 14,80 = 3.12, p = 0.0007). Despitehese differences, soil moisture content remained relatively high0.0 to −0.2 bar of matric suction) during most of the 2009 sea-on in both the compost and control treatments. However, inugust/September a strongly negative soil water potential (−1.5 to2.6 bar of matric suction) developed in the control soil, while theffect in the compost plots was weaker (−0.3 bar of matric suction).or the 10 time points in 2010 there was no treatment (F1,5 = 0.35;

= 0.5795) or interaction effect (F 10,59 = 0.60; p = 0.8096) but only aime effect (F10,59 = 32.54; p < 0.0001). Again the soil moisture con-ent remained relatively high (0.0 to −0.2 bar of matric suction)uring most of season, except a modest decrease in water potential

n both compost (−0.7 bar of matric suction) and control treatment−0.8 bar of matric suction) during early August.

. Results

.1. Plant cover

All three species grew much better in the compost plots thann the controls, with plant cover in both autumn 2009 and 2010eing several-fold higher (Table 2) and with the largest treatment

ffect in 2010 on L. galeobdolon, followed by S. holostea and G. odor-tum (Table 3). Plant cover differed, however, also significantlymong blocks in G. odoratum and L. galeobdolon, including a signif-cant interaction with the compost treatment in the latter species

626 66576 53

(Table 3). Further examination showed that plant cover of thesespecies was lower in the westernmost blocks (1, 2, and 5, Fig. 1).

3.2. Flowering

The percentage of flowering plants was significantly higher inthe compost plots than in the controls in all three species in 2010(Table 2), with the largest treatment effect in S. holostea, followedby L. galeobdolon and G. odoratum (Table 3). In 2010, most of thetransplants of all species flowered in the compost plots (Fig. 2c).

3.3. Plant survival

Only two of 256 G. odoratum transplants died during the studyperiod, both in the control treatment. The survival rate for L. gale-obdolon was also high in both the control and compost plots. OnlyS. holostea had a significantly lower survival rate in the control thanin the compost plots (Tables 2 and 3). Most of this mortality wasrecorded already six weeks after planting in June 2009, with verylow additional mortality thereafter.

4. Discussion

The results demonstrate that compost has a great potential toenhance the initial establishment, growth and flowering of thethree clonal shade-tolerant woodland herbs tested, G. odoratum,L. galeobdolon and S. holostea, which confirms our first hypoth-esis. Especially the evergreen L. galeobdolon was able to expandfaster by means of stolon growth than the other two species whichpredominantly spread by rhizomes.

The original mineral soil at the experimental site is calcare-ous and rich in nutrients. This suggests that the positive effectof compost on the three woodland herbs is not generally causedby unsuitable soil chemistry in the control treatment. However,caution is needed when making inferences on the impact of thesoil chemistry on the observed treatment effect, because poten-tial variability in soil chemistry was not analysed between theblocks. Dispite this methodological limitation, we believe that themarkedly higher plant cover and rate of flowering in the composttreatments is mainly related to the more favourable conditions forgrowth and expansion of these woodland herbs by rhizomes andstolons provided by the loose soil structure of the compost com-pared to the relatively compact mineral soil. Many woodland herbs,including the tested species are morphologically adapted to grow inwoodland soils with persistent litter. They have shallow but exten-sive root systems, mainly exploiting the humus-rich top soil andthe lower litter layer of woodland soils. Earlier experimental stud-

ies have also shown that the performance of these species is poorwhen introduced on bare mineral soil, but can be improved by addi-tion of leaf litter (Sydes and Grime, 1981; Hammer, 1991; Cohn andPackham, 1993). In the present study, compost addition allowed

G. Richnau et al. / Urban Forestry & Urban Greening 17 (2016) 158–165 163

Table 2Plant cover (dm2), flowering (% individuals), and survival (% individuals) of transplants. Mean values (bold face) and 95% confidence intervals are given.

Species Soil treatment Cover Sept. 2009 Cover Sept. 2010 Flowering 2010 Survival Sept. 2010

G. odoratum Compost 9.4 (5.6–13.2) 45.9 (40.2–51.7) 99.2 (93.6–104.8) 100 (98.4–101.6)Control 3.3 (2.2–4.4) 20.5 (14.7–26.2) 87.6 (82.0–93.2) 98.4 (96.8–100.1)

L. galeobdolon Compost 15.6 (13.7–17.4) 215.2 (199.6–230.9) 88.4 (75.9–101.0) 95.3 (90.1–100.5)Control 1.6 (−0.3–3.4) 17.7 (2.1–33.4) 43.5 (31.0–56.1) 92.2 (87.0–97.4)

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uch a growth pattern to develop better than in the control treat-ent. In combination with a high availability of macronutrients andater, the compost soil seems to be a near optimal growth medium.

Desiccation has often been mentioned as a major factor in mor-ality in plant introduction experiments in various habitats (Cohnnd Packham, 1993; Godefroid et al., 2011). In the present study, thedded compost maintained higher soil moisture throughout muchf the two growing seasons, including a dry period in late summer009, when the plants in the untreated control plots may have suf-ered drought stress. The ability of the compost to buffer againstesiccation is likely to be part of the explanation for the much

ncreased growth in the compost plots compared with the con-rols. However, it is less likely to explain the high initial mortalityn S. holostea in the control treatment during late April–May 2009,

hen the soil water potential did not differ much between the com-ost and control plots and the mortality of the two other speciesas not affected. A more likely explanation is lower vigour of the S.

olostea plug plants, which may have required the more favourableoil structure and higher availability of all macronutrients providedn the fresh compost for their initial survival.

Increased dessication also explains the lower plant growth of G.doratum and L. galeobdolon in the three block closest to the west-rn edge of the stand. Thus, the trend of lower soil water potentialn these blocks is likely related to an increased evaporation fromhe soil and transpiration from the plants caused by the predom-nantly western winds penetrating the open edges. We concludehat forest edges should be kept dense and closed, preferably with

fringe of shrubs, when woodland herbs are planted in exposedtands.

The method of compost addition may be even more importantor initial plant establishment on dry sandy or gravel soils with aower water holding capacity than the loamy clay soil studied here.he observed capacity of compost to prevent desiccation could alsoelp introduced woodland herbs to establish on such soils until aature humus layer has developed that buffers variations in soil

oisture.

The compost applied had a high content of plant availableitrogen and phosphorous which may favour ruderal and habitat

able 3esults of statistically significant (P < 0.05) GLMs for mean plant cover, flowering and survi1.15), Block (7.15), Compost × Block (7.15), Weeding (1.15).

Compost Block

F P F P

Mean coverG. odoratum 44.39 <0.001 3.39 0L. galeobdolon 361.98 <0.001 22.02 <S. holostea 130.16 <0.001 2.31 0

FloweringG. odoratum 7.33 0.016 0.75 0L. galeobdolon 28.81 <0.001 2.60 0S. holostea 73.32 <0.001 1.96 0

SurvivalS. holostea <0.001 2.43 0

.9 (41.6–52.1) 95.4 (85.4–105.4) 96.9 (87.2–106.6) (2.5–13.0) 41.6 (31.6–51.5) 52.3 (42.6–62.1)

generalist plant species over typical woodland herbs (Verheyen andHermy, 2004; Baeten et al., 2009a,b). However, unlike our expec-tations, a competing vegetation of other vascular plant speciesdeveloped poorly in the compost plots during both years. The gen-erally low abundance of competing species may be the main reasonwhy the hypothesis that weeding treatment would further increaseperformance of the planted herbs was not confirmed. It seems thatthe tree canopy of the Quercus stand was sufficiently dense toinhibit vigorous growth of light-demanding ruderal or generalistspecies. Through light to moderate canopy thinning, as done priorto planting the herbs in this study, forest managers can regulatethe competitive outcome on the woodland floor by maintaininglight levels which are too low for undesirable tall herbs such asUrtica dioica, while still allowing growth of shade-tolerant wood-land herbs (De Keersmaeker et al., 2004; Thomaes et al., 2014). Inthis regard Quercus species native to northern Europe are highlysuitable, because their closed canopy suppresses weeds and tallherbs, but allows sufficient light for establishment and growth ofshade-tolerant woodland herbs to reach the woodland floor (Brunetet al., 2012).

However, on post-agricultural soils more prone to acidificationthan in our study, plantation of oak often causes strong acidifi-cation already within 20–30 years, which may prevent long-termsurvival of acid intolerant woodland herbs (Thomaes et al., 2012).For example, studies have shown that G. odoratum and L. galeob-dolon do not grow on soils with a pH below 4.2 (measured directlyin undiluted soil solution), while S. holostea is somewhat more acid-tolerant (Falkengren-Grerup and Tyler 1993a,b). Mixing Quercuswith tree species with easily decomposable litter, such as Fraxi-nus excelsior L., Tilia spp., Acer spp. or Populus spp., can maintainfavourable soil conditions for such herb species (Thomaes et al.,2013, 2014). Admixture of Tilia spp. and Acer spp. which createdense canopies should be preferred, because higher light availabil-ity under Fraxinus spp. or Populus spp. may favour the developmentof a dense understory of undesirable tall herbs with negative effects

on smaller woodland herbs (Baeten et al., 2009a,b).

Plant introductions that seem successful during the first fewyears, such as that reported here, may turn out to be failures in

val in 2010. N.a. means not analysed. Df were as follows for the full model: Compost

Compost × Block Weeding

F P F P

.022 0.72 0.657 0.93 0.3500.001 14.93 <0.001 2.90 0.109.097 1.77 0.182 0.18 0.677

.636 0.36 0.915 n.a. n.a.

.054 0.80 0.603 n.a. n.a.

.135 2.82 0.051 n.a. n.a.

.071 2.19 0.096 1.19 0.293

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64 G. Richnau et al. / Urban Forestry

he long term (Petersen and Philipp, 2001; Godefroid et al., 2011;rayton and Primack, 2000, 2012). However, the rates of initial sur-ival and flowering observed in our study are considerably higherhan the average rates reported in the review by Godefroid et al.2011). Also, the vigorous clonal growth in the compost plots duringhe second year of the present study indicates successful estab-ishment. In summer 2012, four growing seasons after planting, allhree herb species formed large carpets, especially in the compostlots (Fig. 2d). This pattern prevailed through to 2015 (personalbservations). While this suggests that compost addition is ableo bridge the time span until a stable humus/litter layer is devel-ped in young woodland plantations, future monitoring of thexperiment is necessary to determine the long-term establishmentf viable populations, including further expansion into adjacentoodlands by clonal expansion or establishment from dispersed

eeds (Godefroid et al., 2011).

. Conclusion

To conlude, this study showed that combining planting withpplication of compost is an effective method for rapid establish-ent of profusely flowering carpets of herbaceous layers in youngoodlands. The specific method used here, loose tipping of greenaste compost, is easy to apply and should be further tested for

ther understory species. Even where this method is not affordablever large areas, it could be used along paths and other frequentlysed areas to create recreationally attractive environments in the

arge number of urban woodlands established in north-west Europend elsewhere in recent decades. The method may also be appli-able in other woodland restoration projects as an effective wayo establish well-dispersed patches from which typical woodlanderb species can establish and eventually merge into larger stands.

cknowledgements

We dedicate this paper to the memory of our dear friendnd colleague Gustav Richnau, the article’s lead author, who lefts suddenly and tragically while the results of this study werenalysed and prepared for publication. This study was fundedy the Swedish Research Council FORMAS (Bio-Architectural andynamic Approaches for Urban Plantations, 2007-456). We arerateful for the practical help by Erik Svensson, Cecilia Öxell, Henrikjöman, Anna Monrad Jensen and Gustav Johnsson.

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