Original article

Water extraction by tree fine roots in the forestfloor of a temperate Fagus-Quercus forest

Christoph Leuschner

Plant Ecology, FB 19, University of Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany

(Received 15 January 1997; accepted 19 June 1997)

Abstract - Water retention and water turnover were investigated in the forest floor of a temperatemixed Fagus-Quercus forest on poor soil in NW Germany. By field and laboratory measurementsthe aim was to quantify the water extraction by those tree fine roots that concentrate in the super-ficial organic layers. The 8-10.5-cm-thick organic profiles stored up to 45 mm of water underQuercus trees but significantly smaller amounts under Fagus (and even less under Pinus trees ina nearby stand). The water retention capacity (i.e. the difference between saturating water con-tent after wetting and water content prior to wetting) and the resulting percolation rate out ofthe forest floor were measured by infiltration experiments in relation to their dependence on theinitial water content of the humus material. The water retention characteristics of the humusmaterial differed from the sandy mineral soil material by i) a much higher maximum water con-tent (porosity), ii) a higher storage capacity for water in the plant-available water potential range,and iii) a marked temporal variability of the water retention capacity. A one-dimensional waterflux model for the forest floor of this stand has been developed. According to the model results,the forest floor contributed 27 % (in summer 1991) or 14 % (in summer 1992) to the stand soilwater reserves, and 37 % (summer 1991) or 28 % (summer 1992) to the water consumption of thisstand. Water was turned over in the forest floor twice as fast as in the underlying mineral soil; how-ever, fine roots in the mineral soil apparently extract more water per standing crop of root biomassand, thus, are thought to operate more economically with respect to the carbon cost of wateruptake. (© Inra/Elsevier, Paris.)

Fagus sylvatica / fine roots / forest floor / deciduous forest / water content / water extraction

Résumé - Extraction de l’eau par les racines fines dans les horizons superficiels du sol d’uneforêt tempérée de chênes et de hêtres. La capacité de rétention et les flux d’eau ont été analysésdans les horizons superficiels organiques du sol d’une forêt mélangée de chênes et de hêtres, surun site pauvre du nord-ouest de l’Allemagne. L’objectif de ce travail était de quantifier l’extrac-tion de l’eau dans le sol par les fines racines des horizons superficiels riches en matière orga-nique. La capacité de stockage en eau de la tranche superficielle de 8 à 10,5 cm d’épaisseur attei-

* Correspondence and reprintsTel: (49) 5618044364; fax: (49) 5618044115; e-mail: leuschne@hrz.uni-kassel.de

gnait 45 mm d’eau sous les chênes, mais était significativement plus faible sous les hêtres, etencore plus faible sous une pinède proche. La capacité de rétention en eau (calculée par la diffé-rence d’humidité entre la capacité de saturation avant et après humectation), ainsi que le taux depercolation sous l’horizon organique ont été mesurés par infiltration expérimentale, et mis enrelation avec la teneur en eau initiale de l’humus. Les caractéristiques de rétention en eau del’humus montrent des différences par rapport à un sol minéral de type sableux par a) une teneur eneau maximale très supérieure, liée à la porosité, b) une plus grande capacité de stockage de l’eaudans la gamme des potentiels hydriques utilisables par les arbres, et c) une forte variabilité temporellede la capacité de rétention. Un modèle monodimentionnel de transfert d’eau dans les horizonsde surface a été développé pour le peuplement étudié. Selon les simulations, la contribution de lacouche organique assurait 27 % (en été 1991), ou 14 % (en été 1992) de la réserve en eau totale dusol, et 37 % (été 1991), ou 28 %( été 1992) de la consommation en eau du peuplement. Le renou-vellement de l’eau dans la tranche superficielle était deux fois plus rapide que dans les horizons miné-raux sous-jacents. Toutefois, le taux d’extraction d’eau par les racines fines était plus important parunité de biomasse racinaire dans les horizons minéraux ; de ce fait, ces racines ont montré unfonctionnement plus économique en terme de coût en carbone. (© Inra/Elsevier, Paris.)

Fagus sylvatica / racines fines / litière / forêt feuillue / teneur en eau / extraction d’eau

1. INTRODUCTION

Forest ecosystems on nutrient-pooracidic soils are characterized by thickorganic layers at the forest floor whichplay a key role in the nutrient cycles ofthese systems [6, 13]. For various tem-perate and tropical forests on poor sub-strates, the organic profile has been iden-tified as the main source of nutrient supplythat contains high densities of tree fineroots [12, 17, 19]. Much less attention hasbeen paid to the moisture regime of theorganic profile although much of the bio-logical activity in the forest floor dependson the moisture status of this medium [23,25]. Furthermore, water infiltrating intothe soil first passes through this upper-most horizon where it meets a high densityof tree fine roots, mycorrhizal hyphae andmicroorganisms [3]. Thus, a rapid uptakeof water by superficial roots in the forestfloor could represent a crucial advantagefor plants that compete for water [21].

Research in forest floor hydrology hasbeen conducted predominantly by foresterswho were interested in erosion control orwished to predict the threat by ground firesas a function of the forest floor water con-

tent (e.g. [2, 4, 8, 16]). Organic materialat various stages of decomposition repre-sents a unique medium that retains andalso conducts water in a rather differentmanner when compared to the mineral soilmatrix [9, 16]. Hydrologists concernedwith the soil-vegetation-atmosphere trans-fer of water (SVAT) only recently paidattention to the fact that the water flux in

many forest ecosystems on poor soils can-not be described accurately as long as theorganic profile is ignored in the models ortreated in analogy to the mineral soil [20].

This study investigates availability andturnover of water in the forest floor of adeciduous two-species (Fagus-Quercus)forest stand in NW Germany in its rela-tion to tree fine root distribution. The main

questions were:

1) Does the forest floor significantlycontribute to the root water uptake of thetrees?

2) Is the type of litter (or the treespecies) an influential factor in the forestfloor hydrology?

3) What relation exists between fineroot abundance and water extraction inforest floor and mineral soil profile?

The study is part of a comparative anal-ysis of the water and nutrient cycles inthree forest and heathland stands that rep-resent early, mid and late stages of a sec-ondary succession (cf. [12, 18]). Otherresearch activities concentrated on thewater flux in the mineral soil, the over-storey evapotranspiration (Leuschner, inprep.), and the distribution and turnoverof fine roots ([3]; Hertel, in prep.).

2. MATERIALS AND METHODS

2.1. Study site

The investigations were carried out from1991 to 1993 in an old-growth mixed Fagussylvatica L.-Quercus petraea Matt. (Liebl.)forest on poor sandy soil in the diluvial low-lands of NW Germany (site OB5). The stand islocated west of Unterlüss in the southeastern

part of the Lüneburger Heide (52°45’ N, 10°30’E) in level terrain and stocks on fluvio-glacialsandy deposits (predominantly medium-grainedsand) of the penultimate (Saale) Ice Age witha low silicate content and a high soil acidity[pH values (in 1 M KCl) of the topsoil:2.6-2.8]. The ground water table is far bey-ound the rooting horizon. The soil type is aspodo-dystric cambisol; the 8-10.5-cm-deepforest floor is built by a three-layered (L, F, Hhorizons) Mor-type organic profile (mainlyHemimors and Hemihumimors according tothe classification of Klinka et al. [10]; cf. [11]).The profile is significantly thicker in the directvicinity of oak stems than at beech stems(Leuschner, unpubl.). Ninety percent of thestems are beeches (age: 90-110 years), 10 %are oaks (180-200 years). A herbaceous layeris lacking. The climate is of a temperate sub-oceanic type (annual precipitation ca 730 mm,mean air temperature 8.0 °C).

For comparison, several analyses were alsoconducted in a 30-year-old 12-m highpine-birch (Pinus sylvestris L., Betula pen-dula Roth) stand in the vicinity (site BP3, withpine dominance). On similar geological sub-strate, an iron-humus podzol with a 8-9 cmthick Mor profile (Hemimors, Hemihumimorsand Xeromors) is present here.

2.2. Hydrological measurements

The basic method to monitor the water con-tent of the forest floor &thetas; was a sequential cor-ing technique with gravimetric determination ofthe water content in the OF and OH layers. Rep-resentative plots with predominant oaks orbeeches (or pine at site BP3) were separatelysampled. From May 1991 until December1992, eight samples each per tree species weretaken weekly (in summer) or 2-4 weekly (inwinter) with a 5-cm-diameter root corer sys-tematically at a distance of 40-200 cm from astem. By simultaneous measurement of theprofile depth in undisturbed samples, the watercontent data could be expressed as volume per-cent (vol. %) or fractional water content (cm3cm-3) and also in terms of water storage (inmm per profile). The spatial variability of &thetas; inthe forest floor is characterized by an annualmean coefficient of variance of the moisture

samples of 14.2, 15.8 and 23.4 % at the beech,oak and pine sites, respectively. The water con-tent of the mineral soil profile was monitoredfortnightly by TDR technique and by gravi-metric determination until a depth of 70 cm.

Water retention curves (i.e. the relationshipbetween soil water matric potential &Psi;m andvolumetric water content &thetas;) were measured at’undisturbed’ samples of 250 cm3 volume fromthe organic OFH layers by desorption withhanging water columns in the laboratory. Fivesamples each from oak and beech (site OB5)and pine (site BP3) humus were analysed. Forcomparison, sandy material of the uppermostAh horizon was also investigated. Water held atmatric potentials < -1.5 MPa was termed ’non-

root-extractable’, water held between -100 hPaand -1.5 MPa was considered as ’plant-avail-able’. The water content directly after a satu-rating infiltration is taken as the ’saturatedwater content’ &thetas;s of the humus material. Thisis lower than the maximum water content &thetas;max(= porosity) of the organic material with all airspace filled with water.

Laboratory infiltration experiments wereconducted to establish relationships betweenrainfall amount, water retention of the humusmaterial (wetting curves) and resulting percola-tion loss out of the forest floor. Undisturbedforest floor sods of 17 x 37 cm size (sampledunder beech) were treated with 0.5-30 mm ofartificial rain. The sod weight was determined5 min after application and the retained andthe percolated water were expressed as a func-

tion of rainfall and initial humus water con-tent. This procedure was repeated with sodsof varying moisture content (10-31.5 mm ini-tial water storage). Each treatment was con-ducted with five replicates that were averaged.

In order to quantify the water turnover ofthe organic profile it was attempted to mea-sure the relevant water fluxes directly in thefield with appropriate techniques and todescribe the water flux with a one-dimensionalmodel (forest floor water flux model) in tem-poral resolution of one day. Details on the fluxmeasurements and the model will be publishedelsewhere (Leuschner, in prep.). Here, only ashort overview on the methods and the basic

philosophy of the model are presented. Waterinput to the forest floor is generated by canopythroughfall (TF) and, locally, by stemflow (SF).The model considers only throughfall and, thus,is applicable only to stem distances > 1 m. Out-

put terms are the percolation out of the organicprofile into the mineral topsoil (seepage, SP),evaporation from the litter surface (EV), fluxinto/out of the storage in the profile (ST) andwater uptake by fine roots in the densily rootedorganic profile (UP). Capillary rise from themineral soil is neglected. To estimate EV, thePenman-Monteith equation was applied to theforest floor in a semi-empirical approach withnet radiation, air and surface temperature, and

air humidity recorded continuously. The sur-face conductance gco is known to be fairly wellrelated to the square root of the number of dayssince rainfall [5] and was estimated from gravi-metric water loss determinations of humus nets

being exposed in situ at the forest floor. Theaerodynamic conductance for water vapourtransfer above the forest floor gav was approx-imated from wind speed measurements abovethe canopy.

The model uses a mass balance approachand is based on empirically established rela-tionships between rainfall amount, water reten-tion of the humus material (wetting curves)and resulting percolation loss (see above). Itrequires daily throughfall and stand microcli-matological data as well as the humus mois-ture content at a weekly interval as input data.After solving the water balance equation, theresulting term is taken as the water uptake byroots in the organic profile (UPorg):

Table I gives an overview of the methodsused to measure the fluxes directly; the empir-ical results served to validate the model.

In order to assess the relative contribution ofroot water uptake from a) the organic profileand b) the mineral soil, the results from the

forest floor water flux model were related to

energy balance (Bowen ratio) measurementson a tower above the forest canopy. Wholestand evapotranspiration rates (ET) werederived from 30-min means of temperatureand air humidity gradients above the canopyin the summer periods of 1991 and 1992(Leuschner, unpublished data). On dry days,the calculated root water uptake rate in theorganic profile (UPorg) was subtracted togetherwith the litter evaporation rate (EV) from ET toestimate the water extraction by roots locatedin the mineral soil profile (UPmin) and to assessthe relative contribution of the forest floor to thestand water uptake

2.3. Fine root analysis

Tree finest root biomass (diameter < 1 mm)and the number of fine root tips were countedin 100 cm3 samples (ten replicates per hori-zon) taken in July/August 1993 in various hori-zons of the forest floor and the underlying min-eral soil down to 60 cm deep. Samplingprocedure and separation of biomass and necro-mass are described in detail in [3].

3. RESULTS

3.1. Hydrologic characteristicsof ectorganic material

The water storage in the forest floordepends on I ) the water retention curveof the humus material, 2) the water con-ductivity of the material, and 3) the profiledepth. The water content-soil water matricpotential relationship (water retentioncurve) as determined in the laboratory bydesorption gave a maximum water con-tent &thetas;max (= porosity) of about 90 vol. %for ectorganic material in the OF and OHlayers of the study site. This is twice ashigh as for the quartzitic, medium-grainedsand that underlies the forest floor (fig-ure 1). More important, the organic mate-rial retained two to four times more waterin the plant-available matric potentialrange (-100 hPa to -1.5 MPa) than thesand. These properties favour root wateruptake especially in the lower moredecomposed layers of the organic profile

and render the humus a suitable mediumfor root growth.

The water retention curve of humusmaterial differs markedly between thethree litter types (tree species) investi-gated: while humus derived from eitherbeech or oak debris showed nearly iden-tical desorption characteristics, gave pinehumus retention curves that were mark-

edly shifted to lower water contents in thephysiologically important potential range(figure 1). The amount of plant-availablewater, therefore, was by 20 vol. % lowerfor pine humus than for oak or beechhumus (table II). In contrast, humus of allthree species retained much water in thenon-root-extractable range (water <

-1.5 MPa) with no significant differencesbetween beech, oak and pine.

Infiltration experiments with undis-turbed forest floor sods gave empiricalrelationships between the amount of rain-fall and the resulting seepage loss to themineral soil (figure 2: lower part). Theserelationships are influenced by 1) the wet-ting characteristics of the humus material,i.e. the tendency of the matrix to absorba part of the infiltrating water (figure 2:upper part) and 2) the conductivity of the

organic profile. Both properties arestrongly dependent on the initial watercontent of the humus material. Quadraticequations were used to describe the waterabsorption following infiltration (wettingcharacteristics). They allow the calcula-tion of the saturating water content &thetas;s (i.e.the water content immediately after a sat-urating infiltration) and the water reten-tion capacity &thetas;r (i.e. the difference betweensaturating water content &thetas;s and initialwater content) under various water con-tents for the forest floor of the study site(table III).

For the beech forest floor, &thetas;s is smaller

by a factor of three for initially dry humus(10 mm water content in figure 2: curveno. 1, upper part) than for wet humus(31.5 mm content, curve no. 4). On theother hand, dry material (curve no.1, lowerpart) has a five times higher water reten-tion capacity and, as a result, releases lessseepage water to the mineral soil than wet-ter material. The saturating rainfall(throughfall) amount that is needed toreach &thetas;s is much higher, however, for dryhumus than for initially wet humus(table III). Thus, large seasonal fluctua-tions of the humus water content result in

considerable temporal variations in both &thetas;sand &thetas;r and, consequently, in the amountof water that percolates to the mineral soilunder a given infiltration rate.

3.2. Humus moisture status

The 8-10.5-cm-thick Mor profiles atthe study site contain considerable waterreserves not only during wet seasons butalso during periods of summer drought.While winter values peaked at 50 vol. %

under oak trees, summer values rangedbetween 25 and 40 vol. % in wet periodsand reached minima of 18 % in periodsof drought (figure 3). Organic profilesunder beech (with minima at 10 vol. %)were somewhat drier than those under

neighbouring oaks in the same stand. Forcomparison, pine humus, which consistsmainly of the hydrophobic Pinus needles,reached summer minima < 5 vol. % (fig-ure 3). As a consequence of these differ-ences among the tree species, the averagewater storage in the organic profiles was

more than three times larger under oakthan under pine during summer (table IV).Maximum storage peaked at 45 mm underoak and beech in winter but reached only27 mm under pine.

3.3. Water turnoverin the organic profile

Figures 4 and 5 give the results of thewater balance calculations for the forest

floor at the study site for the summermonths (May-September) in 1991 and1992. Based on daily canopy throughfalldata, the forest floor water flux model gavedaily rates of the water balance equationcomponents, which are depicted asmonthly averages in the graphs.When comparing the canopy through-

fall and the seepage rates, it becomes evi-dent that, during summer, only wet monthssuch as June 1991 and August 1992 yielda significant percolation through theorganic profile and lead to an infiltrationinto the mineral soil. During the 1991 and

1992 summers, only 60 % of the through-fall events resulted in a seepage out of the

organic profile (Leuschner, unpublisheddata) and, more important, only 56 %(1991) and 37 % (1992) of the through-fall amount reached the mineral soil (seetable V).

The model calculated remarkably con-stant water uptake rates of 0.5 mm d-1 forthe tree roots in the organic profile dur-ing the summers in 1991 and 1992. Valuespeaked at 0.8 and 1.0 mm d-1 in the wetmonths August 1991 and August 1992(figures 4 and 5). Even in the dry July

1991 a high root uptake rate was calcu-lated for the organic profile, which is con-sistent with the data on water reserves inthe forest floor in this time (figure 6). Overthe period May to September, nearly halfof the water that infiltrated into the organicprofile was extracted by the tree roots inthis horizon. Given the small volume ofthe organic profile with a mean water stor-age during summer between 12.5 mm (forplots under beeches in 1992, see table IV)and 32.2 mm (for plots under oaks in1991), root water uptake (88 and 89 mm inthe summers 1991 and 1992, respectively)was very high. This indicates a rapid waterturnover in the forest floor.

Litter evaporation as estimated fromboth energy balance calculations at theforest floor and gravimetric water lossdetermination showed maximum rates of0.2 mm d-1 during the vegetation periodand of 0.3 mm d-1 in the leafless season

(e.g. April 1992).

4. DISCUSSION

Organic profiles can significantly con-tribute to the water supply of trees if a)the profile is thick enough to function as awater reservoir, b) litter decompositionhas resulted in the forming of conspiciousOF and OH humus layers with good water

retention properties, and c) the type of lit-ter supplied favours the storage of con-siderable amounts of water. These condi-tions are met in deciduous temperateforests on poor soils, which are charac-terized by an accumulation of ectorganicmatter in the range of 25 to 30 kg C in theforest floor [24]. In old-growth deciduousforests on intensively podzolized soilssuch as the studied oak-beech stand, evenhigher ectorganic carbon reserves in therange 35-50 kg C have been measured[11]. These conditions are decisive if theorganic profile is to play an important rolein the water supply of forests.

4.1. Different hydrologiccharacteristics of mineral soiland forest floor

When compared to the sandy mineralsoil, ectorganic OF and OH material of theoak-beech forest differs in its hydrologicproperties in a three-fold manner:

1) The ’maximum water content’ &thetas;max(porosity) is more than twice as highowing to the very large pore volume andgives the forest floor an exceptionally highwater storage capacity; it decreases, how-ever, with proceeding litter decomposi-tion downward in the profile.

2) The ’saturated water content’ &thetas;s is

highly variable over time: it can increaseby more than 50 % when humus materialchanges from a low to a high materialwater content. Apparently, an increasinghumus moisture content alters the texture,the surface properties and also the volumeof the organic material with the conse-quence that basically wet material has amuch larger saturated water content &thetas;sthan drier material. Thus, ectorganic mate-rial shows markedly different hydrologic

properties over dry and wet periods of aseason; this variability contrasts sharplywith the much more stable hydrologicproperties of the mineral soil material.

3) The ’water flow’ through the organicprofile (i.e. the percolation rate) is char-acterized by i) a high spatial and temporalheterogeneity (cf. [20]) with laminar flowbeing the exception, and ii) a strong depen-dence on the material water content andthe hydrophobic surface properties of theorganic debris. What makes an analysis

of water flow even more difficult is thefact that water potential measurements inthe organic material are more problematicthan in the mineral soil, which limits theapplication of Darcy’s equation [9]. Someresearchers have tried to solve this prob-lem by placing the tensiometers in theunderlying mineral soil and refer to them(e.g. [20]). A more direct approach is theestablishment of empirical relationshipsbetween rainfall amount, humus watercontent and resulting precolation rate byinfiltration experiments as has been per-formed in this study. However, this pro-cedure can introduce some artefacts and

may not be suitable for a general forestfloor water flux model since organic pro-files with different texture and thicknessare expected to behave differently. Fur-thermore, the experimental results fromthe laboratory require validation by fieldmeasurements as was achieved in this

study by monitoring the water flow at themineral soil/forest floor interface (seeMethods, and Thamm and Widmoser[22]).

An important result of our infiltrationexperiments was the finding that, duringsummer, about half of the canopy through-

fall is turned over in the organic profilevia evaporation or root uptake and doesnot reach the mineral soil. Thus, duringsummer, a relatively dry forest floor moreor less isolates the mineral soil profilelower down from the rainfall events. Thisis important for assessing the hydrologicalrole of the forest floor in this stand, butalso must have consequences for waterflow models in forest ecosystems which,with very few exceptions, ignore the for-est floor.

4.2. Relative importanceof the organic profilein the stand water balance

How important is the forest floor in theLüneburger Heide oak-beech forest forthe water demand of the trees? Figure 6contrasts the ’water storage’ in the organicprofile with the water reserves in theunderlying mineral soil profile. Duringthe summer months of 1991 and 1992, theforest floor contributed on average 27 %

(in the moderately dry summer 1991) and14 % (in the dry summer 1992) to the totalsoil water reserves (down to 70 cm deep,table VI).

The organic profile plays an even moreimportant role when its contribution to thestand ’water uptake’ is considered: accord-ing to the calculations of the forest floorwater flux model, about 37 % of the watertranspired by the stand from May toSeptember 1991 must have been taken upby roots in the forest floor while theremaining 63 % originated from the min-eral soil (table VI). For the summer in1992, a forest floor contribution of 28 %was calculated. Thus, in this forest stand,the organic profile of only 8 to 10.5 cmdeep represents an important source ofwater for consumption by the trees. This islinked to a rapid water turnover in theorganic profile which apparently is muchhigher than in the mineral soil (table VI)and is supported by i) the very high fineroot density, and ii) the favourable mois-ture status in the forest floor (see also tableIV). For stands with a thinner organic pro-file and/or with less favourable waterretention characteristics (such as manyconifer forests), only a small or even anegligible contribution of the forest floorto the root water uptake was found: for aDouglas fir stand in the Netherlands witha 5-cm-thick forest floor of poorly decom-posed needles, Schaap [20] calculated thatonly 2.2 % of the total root water uptakewas derived from the forest floor.

4.3. Water uptakeand root distribution

Superficial rooting is a typical attributeof trees on nutrient-poor acidic soils [ 14,15]. Intensive studies on the fine root sys-tem of this stand ([3]; Hertel, unpublisheddata) showed that roughly 45 % of thestand total of the finest root biomass

(diameter < I mm) is concentrated in theorganic profile (table VII). The density offinest roots (expressed in mg biomass per100 cm3), therefore, is three to four timeshigher in the organic OF and OH horizonsthan in any mineral horizon [ 12]. Even

more striking is the fact that more than90 % of the living root tips of the totalprofile occurred in the organic horizons.

When the root distribution patterns arecontrasted with the water extration ratesas calculated for the summer (May toSeptember) 1992, the following three con-clusions on the functionality in wateruptake of the tree root system can bedrawn:

1) From the mineral soil to the organicprofile, the soil-volume-related waterextraction rate (in cm3 water per cm3 vol-

ume) increases in parallel with the den-sity of finest roots. One could concludethat the more rapid turnover of the waterreserves in the organic profile (seetable VI) is mainly a result of the higherfinest root density here (cf. [1]). However,alternative explanations are also possible:i) a better water availability in the forestfloor (i.e. a larger soil-root potential gra-dient) could allow a higher specific wateruptake rate of these roots; ii) a higherdegree of branching and more fine roottips, as is typical for the organic profileroot system, result in a higher specific sur-face of the forest floor finest roots, whichcould enable a higher water influx per rootmass.

2) Since the concentration of fine roottips (and ectomycorrhizas, ECM) is 90times higher in the organic profile than inthe nutrient-poor mineral soil whereas thevolume-related water extraction rateincreases by a factor of three only, it is tobe concluded that both tips and ECM con-tribute only marginally to the uptake ofsoil water in this stand. The key functionof these organs is to be seen in the con-text of nutrient absorption [7].

3) Although we do not have informa-tion on the life span and the maintenancecosts of finest roots in this stand, one canassume that, in the context of water uptakealone, the finest roots in the mineral soilshould operate more economically thanthose in the organic profile: the amount

of water taken up in the summer 1992 perbiomass of finest roots was more thantwice as high in the mineral soil than inthe organic profile. This has to be con-trasted with the higher soil-volume-relatedwater uptake which, in theory, should leadto a smaller extension of the root systemand, thus, to reduced carbon costs of wateracquisition.

The forest floor plays an important rolein the hydrology of this forest not onlythrough its contribution to the stand waterdemand: the comparably high humuswater content is a basic requirement for ahigh microorganism activity and decom-position rate [23]. More important, theintensive nutrient uptake, which takesplace in the forest floor, is also dependenton a favourable humus moisture status.

ACKNOWLEDGEMENTS

This research was supported by grants fromthe German Federal Ministry for Education,Science, Research and Technology (BMBF:project no. P.6.3.8., Stabilitätsbedingungenvon Waldökosystemen, ForschungszentrumWaldökosysteme, Universität Göttingen) andfrom the Commission of the European Com-munities (contract no. EV4V-0148-C(BA)).Much of the field work was conducted by GabyGörlitz, Andrea Dageförde, Dietrich Herteland Katharina Backes which is gratefullyacknowledged.

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