&p.1:Abstract The tree species black alder [Alnus glutinosa(L.) Gaertn.] typically inhabits wet sites in central Eu-rope but is also successful on well drained soils. To testthe physiological adjustment of the species in situ, con-ductances, transpiration rates and water potentials (Scho-lander pressure chamber) of black alder leaves were in-vestigated at two neighbouring sites with different waterregimes: alder trees at an occasionally water logged alderforest and alder shrubs in a nearby, much drier hedgerow.Additional experiments with alder cuttings in nutrientculture showed that leaf conductances and gas exchangewere both strongly influenced by the substrate water po-tential. In situ however, there was little spatial variabilitywithin the different parts of a crown and we found thatphysiological regulation at leaf level was hardly influ-enced by different site water regimes or different treesizes. Diurnal courses of leaf water relations as well astheir regulation at the leaf level (e.g. the hyperbolic rela-tionship between conductances and ∆W) were strikinglysimilar at both sites. Leaf water potential in black alderwas shown to be a consequence of immediate transpira-tion rates, which were high in comparison to other treespecies (up to 4 mmol H2O m–2 s–1), rather than the wa-ter potentials being a factor that influenced conductanceand, therefore, transpiration. The always high leaf con-ductances and consequent high transpiration rates are in-terpreted as a strategy to maximise productivity throughlow stomatal limitation at sites where water supply isusually not limited. However, at the same time this be-haviour restricts black alder to sites where at least thedeep-going roots can exploit water.

&kwd:Key words Alnus glutinosa· Black alder · Conductance ·Transpiration · Water potential&bdy:

Introduction

In natural temperate forests different indigenous treesspecies preferably grow under specific site conditions.They occupy different ecological niches. However, as therealisation of the niche depends on competitiveness aswell as on the specific response to the environmentalconditions, the physiological optimum demands of a treespecies might be different from the conditions of its usu-al habitat.

Black alder [Alnus glutinosa(L.) Gaertn.] is known asa tree species that inhabits wet sites in Europe and adja-cent regions. In various varieties and forms A. glutinosaisindigenous to deciduous, temperate forests over a large ar-ea extending over northern Africa, the Near-East and westSiberia (Schmidt 1996) but nowhere it is the dominant treespecies at mesic sites. In forests black alder is always re-stricted to moderately or extremely wet habitats (e.g. El-lenberg 1996). Although, in the past, swamp forests ofblack alder covered large tracts of the lowlands of north-ern Europe these areas have become greatly diminished bydraining so that, nowadays, black alder trees occur only asmore or less monospecific stands on the banks of riversand lakes (Döring-Mederake 1991). However as a fastgrowing tree species with the capacity of N2-fixation (e.g.Akkermans and van Dijk 1981; Huss-Danell and Wheeler1987), black alder seems highly suited for biomass pro-duction and could be used for agroforestry. Althoughblack alder was reported to be sensitive to drought (Hegi1935; McVean 1956; Hennessey et al. 1985; Hennesseyand Lorenzi 1988), it grows well when used for amenityplantings on drier sites such as embankments along rail-way tracks and in hedgerows.

From observations of alders growing well at driersites the question arose, whether and how far black alderactually depends on the wet conditions of its usual habi-tat? Despite the widely accepted importance of A. gluti-

C. Eschenbach (✉)Ecological Research Center, Kiel University,Schauenburgerstrasse 112, D-24118 Kiel, Germanye-mail: christia@pz-oekosys.uni-kiel.deTel.: +431-880-4041; Fax: +431-880-4083

L. KappenBotanical Institute, Kiel University, Olshausenstrasse 40,D-24098 Kiel, Germany&/fn-block:

Trees (1999) 14:28–38 © Springer-Verlag 1999

O R I G I N A L A RT I C L E

&roles:Christiane Eschenbach · Ludger Kappen

Leaf water relations of black alder [Alnus glutinosa (L.) Gaertn.]growing at neighbouring sites with different water regimes

&misc:Received: 10 September 1998 / Accepted: 12 January 1999

nosa, only a few investigations on water relations ofblack alder have been conducted, and these were carriedout with seedlings in the laboratory and cover only a fewaspects (Seiler 1985; Hennessey and Lorenzi 1988).

In order to investigate the question in situ, we com-pared performances of black alder at two neighbouringsites exposed to an identical mesoclimate, but with sub-stantially different soil water regimes: the first a natural,occasionally waterlogged stand of mature alders and, thesecond, a drier, hedgerow composed of alder shrubs.

In the current study we focused on the regulation ofthe water relations at the leaf level. Within the interrela-tions between plant water relations and primary produc-tion occurring at various levels of the higher plants orga-nisation and over different time scales, the regulation ofstomatal conductance is of crucial importance. Limita-tion of transpirational water loss, caused by decreasedstomatal opening is always accompanied by decreasedCO2-assimilation. Therefore the regulation of stomatalconductance may contribute to a specific adaptation tothe natural wet environment of black alder. Stomatalconductance is known to be influenced by microclimaticdriving variables and, as well, in some species directlyby leaf water potential (Jarvis 1976; Raschke 1979;Nonami et al. 1990). There is also evidence that stomatareact to information about the soil water status mediatedby metabolic signals from the roots (Schulze andKüppers 1979; Zhang et al. 1987; Gowing et al. 1993).

To test whether the regulation of the leaf water rela-tions of A. glutinosais specifically adapted to the wetenvironment, seasonal courses of leaf water potentialsand diurnal courses of leaf water potentials, conductanceand transpiration were investigated. The dependency ofstomatal conductance on irradiance and leaf-air waterpressure difference (∆W) was also analyzed at the twosites. To assist in the interpretation of the in situ data, ad-ditional experiments were carried out with potted aldercuttings to test the influence on leaf conductance ofvarying substrate water potential.

Materials and methods

Field site

The investigations were conducted at the study site of the “Eco-system Research in the Bornhoeved Lakes Region” project. TheBornhoeved Lakes Region is located in northern Germany (Schles-wig-Holstein, 54° 06’N and 10° 15’E, 29 m NN).The climate istemperate, humid and subatlantic (mean annual temperature8.1°C, mean annual rainfall 697 mm). Typical windspeeds are inthe order of 3 m s–1.

The water relations of black alder were investigated concur-rently on trees in a natural alder forest and, about 500 m distant,on alder shrubs in a planted hedgerow (Fig. 1). The alder forest, atLake Belau, was about 18–23 m high, about 60 years old and wastyped as an Alnetum glutinosae(Schrautzer et al. 1991). The standforms a 30 m wide belt on occasionally water logged histosols de-veloped from decomposed alder peat (Schleuss 1992). The season-al course of the leaf area index (LAI) of the alder trees followedan optimum type curve, maximum LAI values (4.8) were reachedin August (Eschenbach and Kappen 1996).

The hedgerow was typical of the many that form importantlandscape elements in northern Germany. They have been con-structed with various indigenous woody species, including alder,since the eighteenth century and are cut down to allow regenera-tion every 8–10 years. The investigated shoots of the alder shrubsin the research hedgerow were about 8 years old and 4 m inheight.

A 18 m scaffold tower was set up in the alder forest stand anda 5 m scaffolding at the hedgerow. These scaffoldings allowed ac-cess to the crowns of four alder trees in the forest and one aldershrub in the hedgerow.

Cuttings

Investigations were made with 3-year-old cuttings of black alderinitially grown in a tree nursery. At the beginning of the 1991growing season they were potted in nutrient culture (Kamann andKappen 1996) and were then grown for about 15 weeks under nat-ural climatic conditions at the hedgerow site where they showedgood biomass increases.

During the gas exchange measurements water potential of thenutrient solution was varied by addition of suitable amounts ofD(–) mannitol. Mannitol has osmotic effects and, in previous in-vestigations, was found not to affect the general well-being of thealder cuttings. The osmotic potentials were:

Mannitol (g l–1) 15.6 31.2Water potential (MPa) –0.2 –0.4

Leaf gas exchange was measured at the different water potentialsusing the stationary gas exchange measuring system. The mea-surements were made under natural climatic conditions.

Gas exchange

The H2O and CO2 exchange of the leaves and the ambient micro-climatic conditions were investigated in situ with a stationary gasexchange measuring system (H. Walz Meß- und Regeltechnik,Germany) operating in the differential mode under steady-stateconditions. During the growing season (April-November) gas ex-change of leaves of the sun-lit periphery of the crown and of theshaded inner part were measured simultaneously in two air-tightPlexiglas leaf chambers (4 l volume each). Over a period of 2–5days, each leaf measured was included in the leaf chamber and da-ta were continuously recorded at 6-min intervals. The leaf cham-bers are equipped with a quantum flux sensor, leaf and air temper-ature probes and a humidity sensor. Leaves were measured at am-bient CO2 concentrations. Air temperatures, relative humidities,and irradiances in the chamber followed natural microclimaticconditions or were set to steady state conditions. In some cases ar-tificial light was used (Bega Strahler 9351, Osram HQJ-T250W/D; EVN, Kiel, Germany). Subsequent to the gas exchangemeasurements the respective leaves were harvested, and the areaof the fresh leaves was determined by means of a leaf area meter(LI-3050 A/4 LI-COR, Lincoln, Neb., USA).

The continuous measurements with the stationary gas ex-change measuring system were taken 1991 in the hedgerow and1992 in the alder forest. To obtain diurnal courses of transpirationrates and conductances of leaves from both sites at the same time aH2O porometer (steady-state-Porometer Li-1600, LI-COR, Lin-coln, Neb., USA) was used at the hedgerow for 6 days during the1992 growing season. The investigated leaves were situated on thesun-exposed southern side (S-side), the northern side (N-side) andin the shaded centre of the hedge canopy. The readings were takenat 30-min intervals throughout the day.

Remarks on the interpretation of the gas exchange data

The interpretation of gas exchange data obtained for individualleaves by cuvette measurements is subject to general qualifica-tions. Because of boundary layer effects and temperature and hu-

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midity issues, conductances and transpiration calculations in thecuvette may be different from the actual rates of a leaf in ambientconditions (e.g. Meinzer et al. 1993). Our conductance data ob-tained by means of the H2O porometer and the stationary gas ex-change system may be assumed to represent mainly the stomatalconductances of the alder leaves. First, the conductances obtainedby H2O porometer are corrected by subtraction of a previously cal-culated boundary layer resistance. Second, the boundary layer re-sistance within the leaf chamber of the stationary gas exchangesystem was minimized by good ventilation. This might reflect thealways relatively windy situation in the field site. The cuticularconductances are assumed to be negligible.

Proceeding on the assumption that the leaf conductances ob-tained with the different methods at both sites are comparable, thecalculations of transpiration rates may still be misleading. The ac-

tual in situ leaf transpirations depend on the respective ambientconditions of temperature, irradiance, air saturation deficits andboundary layer effects. While leaves are kept in the porometer on-ly for a short time (about 2 min), long-term measurements (up to5 days) were carried out with the stationary gas exchange system.Therefore the chamber was climatized according to the ambientconditions.

Thus, the measured transpiration rates are assumed to be reli-able enough for interpretation. Nevertheless, the results should on-ly be interpreted at the leaf level. While stomatal conductance andtranspiration rates for the two types may be the same, whole treetranspiration and the partitioning of hydraulic conductances mightbe different.

Water potential

Parallel to the measurements of leaves’ gas exchange diurnalcourses of leaf water potential were measured on 6 days during the1992 growing season. Leaf water potentials were determined atboth sites by means of two Scholander pressure chambers (Roth-

Fig. 1 Map of the field site. The investigations were carried out ina natural alder forest on the shore of Lake Belau and a nearbyhedgerow (both marked by an arrow) &/fig.c:

Gerätebau, Baiersdorf, Germany; Scholander et al. 1965; Ritchieand Hinkley 1975). At 30-min intervals leaves were investigated inparallel from three positions in the canopy of the alder forest (2,10, 17 m height) and in the hedgerow (N-side, S-side, middle).Three replicates from each crown position were taken at a time.

Calculations

Transpiration and photosynthesis rates were calculated from themeasured data according to Küppers (1978) and Ball et al. (1987).The gas exchange rates are expressed per unit of projected leaf ar-ea (the area of one surface). Water use efficiency (WUE) describesthe proportion of water used for the assimilation of an unit carbon.

The dependency of stomatal conductance (GI) on irradiance(I), given by a saturation type curve, was mathematically analyzedby an exponential equation according to Webb et al. (1974; Eq. 1:

GI=(Gmax–Gmin) · +Gmin where Gmax=maxi-

mum light dependent stomatal conductance, Gmin= minimum lightdependent stomatal conductance, s3 = light dependent stoma-

tal opening rate). A hyperbolic function (Eq. 2:

where s1, s2= empirical coefficients) was found most satisfactoryto describe the dependency of G∆W on leaf-air vapour pressure dif-ference (∆W).

The parameters Gmax, Gmin and the empirical coefficients s1,s2, s3 were fitted to the measured data sets by an optimization pro-cedure according to Marquardt (1963).

Results

Cuttings experiments

Diurnal courses of leaf gas exchange at varying sub-strate water potential

The weather in the middle of July 1991 was clear andsunny and the microclimatic conditions were similar onall three experimental days (July 9–11; Fig. 2). Photo-synthetic Photon Flux Density (PPFD) reached a maxi-mum of about 1400µmol m–2 s–1 and air temperaturesvaried between 15 and 25°C through the day. Maximaldaily leaf-air vapour pressure difference (∆W) increasedover the measuring period from about 12 to 18 mmolmol–1. Because of the relative constancy of the microcli-mate the differences found in plant behaviour could beattributed almost entirely to changes in water potential ofthe nutrient solution (substrate). Leaf conductance roseto a maximum in the morning and decreased later in theday. With decreasing substrate water potential maximalleaf conductance fell and the rate of decline during theday also increased. Similarly, maximal transpiration ratedeclined despite the increased ∆W and, at noon, was re-duced from 3.3 (9 July, 0 MPa) to 2.0 (10 July,–0.2 MPa) and to 1.0 mmol H2O m–2 s–1 at –0.4 MPa (11July). Net photosynthetic rates were even more stronglydepressed by the decreasing substrate water potentialprobably because CO2 exchange, unlike transpiration,was not assisted by the increasing ∆W over the 3 day pe-riod. Maximal values for net photosynthesis in the morn-ing were 12.5, 10 and 7µmol m–2 s–1, respectively, onthe successive days. A prominent midday depression de-

31

veloped at water potentials below 0 MPa so that at noonon 11 July, with –0.4 MPa substrate water potential andhigh evaporative demand (∆W=17 mmol mol–1), net pho-tosynthesis was zero. Despite this, net photosynthesis re-covered later in the day.

Field studies

Weather conditions and soil water status

The climatic characteristics of the year 1991 were closeto the long-term means, but in early summer the weatherwas unusually cool and wet. The 1992 growing seasonwas characterized by relatively sunny and dry weather.In particular, during early summer the temperatures andthe sunshine were unusually high, while there was lowprecipitation (only 2 mm rainfall from mid-May untilmid-July, Fig. 3a-c). During the 1992 growing season thesoil water regimes at the two sites differed in a typicaland expected manner (Fig. 3d,e). The forest at the lakes

1 3− − ⋅−

FH IKexpmax min

s IG G

GVPD s s2W= +1 ∆

Fig. 2 Diurnal courses of net photosynthesis (A, µmol m–2 s–1),transpiration (E, mmol m–2 s–1), leaf conductance (g, mmol m–2

s–1) and water use efficiency (WUE, µmol mmol–1) of leaves of Al-nus glutinosacuttings at different substrate water potentials (9 Ju-ly at 0 MPa, 10 July at –0.2 MPa and 11 July at –0.4 MPa). Themicroclimatic parameters are irradiance (PPFD, µmol m–2 s–1), airand leaf temperature (T, °C) and leaf-air vapour pressure differ-ence (∆W, mmol mol–1) &/fig.c:

32

Fig. 3 Climatic conditions dur-ing the growing seasons 1991and 1992 (a, b, c) and soil wa-ter regime at the natural alderforest on the shore of LakeBelau (Wötzel, unpublished da-ta) and a crop field close to thehedgerow (1992, d, e; Bornhöft1993). Values of mean annualtemperature as well as of thesums of precipitation and sun-shine are given within the pan-els&/fig.c:

shore was influenced by the water level of the lake beingwaterlogged in winter but with the water table 20–50 cmbelow soil surface in summer and soil water tensionsthen fluctuated around –30 hPa. Whilst tensiometricmeasurements in the soil under the hedgerow were notmade, soil water tensions for the neighbouring crop fieldare available and were strongly influenced by precipita-tion. During drought water tension decreased to valueslower than –1000 hPa at soil depths of 30 cm and 60 cm.At 160 cm depth however, variation was small and soilwater tension did not drop below –60 hPa.

Diurnal courses of leaf water potentials during thegrowing season

On all days, except October 21, the leaf water potentialsof the wet alder forest and the “dry” hedgerow followedthe same diurnal trend with higher values in the earlymorning and late evening and lowest values at noon(Fig. 4). On October 21 the weather was overcast, wetand chilly and the water potential was about –0.3 MPathroughout the day. Water potentials of the leaves at thedifferent positions of the hedgerow (N-side, S-side, mid-dle) were very similar but, in the canopy of the alder for-est in July and August, the leaves at the top of the treeshad markedly lower values. The diurnal minimum of theleaf water potential of the dry hedgerow alders declined

over the whole season from May (minimum of day–1.3 MPa) to September (–2.2 MPa) whilst, in contrast,diurnal minimum of the forest trees, after an initial de-cline from May to July, remained constant at about–1.7 MPa during August and September. Superimposedover this general tendency were the very low values ofthe upper leaves in July (minimum –2.3 MPa). Minimalwater potentials of the leaves from the inner parts of thecrown were always between –1.1 to –1.5 MPa.

Neither site showed any seasonal trend for the “pre-dawn” water potential and, sometimes, the values wereless negative in the supposedly “dry” hedgerow than inthe alder forest.

Exemplary diurnal courses of conductance, transpirationand leaf water potential

August 18 was a typical, sunny summer day, the irradi-ance increased to 1200µmol m–2 s–1 and fluctuated dueto clouds. Air temperatures were 12°C at sunrise andreached a maximum of 20°C whilst leaf temperaturesdiffered from these by less than 1 K (data not shown).Leaf-air vapour pressure difference showed almost bell-shaped courses with low values in the morning and in theevening (about 2 mmol mol–1) and a maximum of12 mmol mol–1 at noon. The black alder trees in the for-est and the shrubs in the hedgerow had nearly identical

33

climatic conditions during the day and, whilst the perip-heral sun-lit leaves had much higher PPFD, there werefew differences in temperature and ∆W (Fig. 5).

The black alder trees in the forest and the shrubs in thehedgerow were also strikingly similar in both their diur-nal courses and their absolute values for leaf conductanceand leaf water potential. Leaf conductances were highestin the morning (peripheral about 360 mmol m–2 s–1),when irradiance was high and ∆W low, and decreasedsteadily during the day. Conductances of the inner leaveswere lower because of the reduced irradiance (Eschen-bach et al. 1996) and, as a result, transpiration rates wereslightly less within the canopies at both sites. The diurnalcourses of leaf water potential reflected the immediatetranspiration rates but with an inverse pattern. Typically,water potentials gradually became more negative duringthe morning, decreasing from –0.3 at sunrise to –1.7 MPaat noon, before recovering again in the afternoon (Fig. 5).

Dependence of conductance on microclimatic conditions(irradiance and ∆W)

At saturating irradiance the relationship between leafconductance and ∆W was hyperbolic (Fig. 6). Stomata

were maximally open at low evaporative demands (low∆W) but leaf conductance decreased rapidly with in-creasing ∆W. The separated curve fitting of the data setsfrom both the wet alder forest (B) and from the drierhedgerow (A) revealed that the response of conductanceto ∆W was nearly identical for the trees from both sites(C).

Whilst no dependency of leaf conductance on temper-ature was found, light dependency of stomatal openingfollowed a typical saturation curve as PPFD increased(Fig. 7, exemplary shown for the alder forest in Septem-ber 1992). Both the maximal conductance and the initialslope of the response were inversely related to ∆W. At a∆W of 14 mmol mol–1 maximal conductance was around55% of that at a ∆W of 4 mmol mol–1. Stomata of leavesin the inner shaded parts of the alder canopy react morestrongly to irradiance than those of peripheral leaves(curves not shown).

Relationship between leaf water potential and either leafconductance or transpiration rate

The relationship between leaf water potential and leafconductance was linear over all measured water poten-

Fig. 4 Diurnal courses of leafwater potential (MPa) duringthe 1992 growing season. Top:leaves at the three different lev-els in the canopy of the alderforest (2, 10, and 17 m high),bottom: leaves at the three dif-ferent locations on the aldershrubs in the hedgerow (N-side, S-side, middle)&/fig.c:

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tials from close to zero to –1.75 MPa (Fig. 8; alder forestperipheral leaves: r2=0.73, inner leaves: r2=0.50; and,data not shown, hedgerow peripheral: r2=0.43; inner:r2=0.47). There was an even stronger linear relationshipbetween leaf water potential and transpiration (alder for-est: r2=0.68 peripheral, and r2=0.62 inner and, data notshown, hedgerow r2=0.91 peripheral, and r2=0.51 inner).

Throughout the growing season the dependence ofleaf water potential on transpiration water loss was linearand the relationship remained more or less unchanged(Fig. 9). The differences in intercept and slope of the re-lationships are not significant (Table 1: standard devia-tions and coefficients of variation). The dependence ofleaf water potential on transpirational water loss wasnearly identical at the dry and the wet site throughout thegrowing season (Fig. 9).

Fig. 5 Diurnal courses (18 August 1992) of black alder water re-lations in the forest (left) and in the hedgerow (right): leaf conduc-tance (g, mmol m–2 s–1), transpiration (E, mmol m–2 s–1) and leafwater potential (MPa) measured on leaves from the sun-lit periph-ery (solid line) and from the inner part of the crown (dashed line).The microclimatic parameters are irradiance (PPFD, µmol m–2 s–1)and leaf-air vapour pressure difference (∆W, mmol mol–1) &/fig.c:

Fig. 6 Dependence of leaf conductance on ∆W at saturating irra-diance in the alder shrub at the dry hedgerow (1991, A) and in thealder forest at the lake edge (1992, B). The data sets were fittedusing a hyperbolic function (Eq. 2, C) &/fig.c:

Fig. 7 Dependence of leaf conductance of peripheral leaves on ir-radiance at different ∆W (alder forest, September 1992). The datawere fitted according to a saturation function (Eq. 1)&/fig.c:

35

Discussion

The experiments with the alder cuttings potted in nutri-ent culture revealed a marked influence of substrate wa-ter potential on leaf gas exchange. Stomatal conductan-ces, transpiration rates and CO2 assimilation rates were

in the same range as measured in situ on forest andhedgerow plants. When substrate water potential was de-creased by mannitol solutions both the leaf conductancesand the gas exchange rates decreased and an increasinglymarked midday depression was induced. This outcome isin accordance with those found by other authors and sug-

Fig. 8 Relationship betweenleaf water potential and con-ductance (left) or transpirationrate (right) on 18 August 1992,for trees in the alder forest.Open symbolsare for the ex-posed, peripheral leaves andsolid symbolsfor leaves withinthe canopy&/fig.c:

Fig. 9 Relationship betweenleaf water potential and transpi-ration rate for leaves of the al-der forest (left) and of thehedgerow (right) at differentmonths during growing season,1992. Open symbolsare for theexposed, peripheral leaves andsolid symbolsfor leaves withinthe canopy. The dates of themeasurements are given withinthe panels&/fig.c:

36

gests that roots send a stress signal to the transpiringleaves. Changes in stomatal performance induced bymetabolic signals (ABA) are known to occur without al-teration of leaf water potential (Küppers et al. 1988;Zhang et al. 1987; Zhang and Davies 1989; Gowing etal. 1993; Lösch and Schulze 1994).

Interestingly, such a midday depression was neverfound in the field studies. Most impressive was the simi-larity in water relations performance not only of all thealders, irrespective as to whether they were in the forestat the lake edge or shrubs in the hedgerow, but also ofthe leaves at the different locations in the canopies. Thusneither the different water regimes nor the different treesizes had any marked influence on the water relations atleaf level. This contrasts with reports on several otherspecies where soil water conditions are assumed to influ-ence the transpiration rates (e.g. Hunt et al. 1991) andfrom Acer saccharumwhere leaf-level responses werefound to vary with tree size (Dawson 1996).

Alnus glutinosaalso had very high leaf conductancesin comparison with other tree species. On summer daysmaximal values in the morning were typically about350 mmol H2O m–2 s–1 decreasing to about 150 mmolH2O m–2 s–1 later in the day. On a few days stomatal con-ductances as high as 1000 mmol H2O m–2 s–1 were mea-sured. Typical maximal values for woody plants are re-ported to be in the range of 190–280 mmol H2O m–2 s–1

with most around 230 mmol H2O m–2 s–1 (Körner 1994).Stomatal responses to irradiance and DW were nearlyidentical at both sites. Comparison with (not shown) datafrom other growing seasons not as dry and sunny as thegrowing season 1992 confirmed that there was no changein the relationships with weather conditions. The accli-mation of the leaves to the shaded conditions in the innercanopy also contributed to the high conductances andconsequent high transpiration rates which reached2–4 mmol H2O m–2 s–1 on clear sunny days in summer.High leaf conductances and high transpiration rates maybe a consequence of the generally wet sites that the aldertrees naturally inhabit. However, there was hardly any re-duction in the water loss at the dry sites. Leaf water po-tential had a linear relationship with transpiration rate

which was more or less uniform at the both sites andover the whole growing season. Although limitation oftranspiration rate by leaf water potential has been report-ed for some species (Nonami et al. 1990; Hacke andSauter 1995) no such effect could be recognized for al-der, not even during the extraordinarily dry period in ear-ly summer or inside the crown where the lowest thresh-old values would be expected. Thus, the water potentialsof all the studied plants followed transpiration rates,rather than being controlled by them. Stomatal control ofleaf water potential, rather than vice versa, is discussedby Jones (1998) to be much more common than usuallyrecognized.

The leaves of both the mature trees of the wet forestand those of the shrubs in the drier hedgerow had dailyminimal water potentials of about –1.5 MPa. Lower val-ues, around –2.3 MPa, were found for the exposed leavesat the top of the tree canopies and at the S-side of thehedgerow. The especially different negative water poten-tials at the top of the wet forest trees in July 1992, whenevaporative demands were very high, might indicatesome xylem water transport limitations caused by thelength of the transport pathway in the trees. Any differ-ences that might be expected due to the contrasting soilwater status at the two sites (alder forest and hedgerow)turned out to be relatively small. As “predawn” valuesremained unchanged, nocturnal water replenishment wasoccurring at both sites even during the extraordinarilydry July of 1992.

Over this same period crop plants in the fields adja-cent to the hedgerow irreversibly wilted. While soil wa-ter tension in the upper soil layers of the neighbouringfield rapidly decreased to very low values, at a depth of160 cm it did not fall below –60 hPa. It seems, therefore,that the good water relations of the alder leaves were dueto the ability of the alder shrubs to exploit deep-seatedmoisture not available to other plants. Black alder usual-ly forms two well-developed physiological root types, asurface nutritional system and a deep-growing system(Schmidt-Vogt 1971; McVean 1956; Middelhoff, unpub-lished data). However, the gradual decrease of the diur-nal minimal values of leaf water potential during the

Table 1 Slopes and intercepts [with standard deviation and coeffi-cient of variation (CV%)] of the relationship between leaf waterpotential and transpiration rate for leaves of the alder forest and of

the hedgerow at different months during the growing season, 1992(see Fig. 9)&/tbl.c:&tbl.b:

Date, position Alder forest Hedgerow

Slope Intercept Slope Intercept

(SD) CV (%) (SD) CV (%) (SD) CV (%) (SD) CV (%)

May 21, per –0.27 (0.04) 15 –0.06 (0.06) 9 – – – –May 21, inn –0.13 (0.03) 27 –0.65 (0.08) 12 – – – –July 7, per – – – – –0.18 (0.08) 43 –0.81 (0.11) 14July 7, inn –0.50 (0.09) 18 –0.69 (0.10) 14 –0.25 (0.22) 86 –0.76 (0.22) 29Aug 18, per –0.35 (0.09) 26 –0.83 (0.13) 15 –0.38 (0.05) 13 –0.53 (0.09) 17Aug 18, inn –0.32 (0.10) 30 –0.73 (0.09) 13 –0.28 (0.13) 45 –0.73 (0.16) 22Sept 17, per –0.46 (0.10) 22 –0.76 (0.12) 16 –0.61 (0.11) 18 –0.68 (0.15) 21Sept 17, inn –0.82 (0.13) 16 –0.23 (0.12) 54 –0.76 (0.18) 24 –0.34 (0.17) 51

&/tbl.b:

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growing season at the hedgerow site indicated increasinglimitation in supply. If drought stress increases further,black alder cuts off water loss by shedding leaves. Thus,leaves of black alder avoid rather than tolerate drought.Betula pendulaexhibits a similar strategy (Wendler andMillard 1996).

However, as a reduction of transpiring leaf area alsoreduces CO2 assimilation capacity, the interaction be-tween plant water and carbon economy restricts black al-der to sites where water resources are available at least toits deep roots. Black alder cannot grow at really drysites. On the other hand, the outstanding success of blackalder at water logged and temporarily flooded sites,where low oxygen supply in the soil excludes other trees,is due to specific morphological and structural changesin response to flooding. These changes include the emer-gence of adventitious roots (Gill 1975), the formation ofhypertrophied lenticels at the stem base, the developmentof aerenchyma tissue and a gas transport system basedupon the physico-chemical effect of thermo-osmosis(Schröder 1989). Continuously high transpiration ratescould be beneficial to productivity not only because thehigh leaf conductances allow greater CO2 uptake, butalso because mass flow towards the roots would providea good nutrient supply even at poor sites. The high leafconductances and consequent high transpiration ratesmay represent a strategy to maximise productivitythrough low stomatal limitation at sites where water sup-ply is usually not limited. However, at the same time thisbehaviour holds black alder away from very dry sites.

&p.2:Acknowledgements This study was carried out within the frame-work of the interdisciplinary project ‘Ecosystem research in theBornhoeved lakes region’ which is funded by the Ministry of Re-search and Technology of the Federal Republic of Germany. Weare very grateful to Allan Green for valuable comments and forbrushing up the English version.

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