&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 D 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 m2 s1), 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:
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. glutinosa isindigenous 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 (Dring-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: email@example.comTel.: +431-880-4041; Fax: +431-880-4083L. KappenBotanical Institute, Kiel University, Olshausenstrasse 40,D-24098 Kiel, Germany &/fn-block:
Trees (1999) 14:2838 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 andKppers 1979; Zhang et al. 1987; Gowing et al. 1993).
To test whether the regulation of the leaf water rela-tions of A. glutinosa is 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 ( D 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
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 06N and 10 15E, 29 m NN).The climate istemperate, humid and subatlantic (mean annual temperature8.1C, mean annual rainfall 697 mm). Typical windspeeds are inthe order of 3 m s1.
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 1823 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 810 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.
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 l1) 15.6 31.2Water potential (MPa) 0.2 0.4Leaf gas exchange was measured at the different water potentialsusing the stationary gas exchange measuring system. The mea-surements were made under natural climatic conditions.
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 Me- 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 25days, 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-
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.
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:
Gertebau, 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.
Transpiration and photosynthesis rates were calculated from themeasured data according to Kppers (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=(GmaxGmin) +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
D W on leaf-air vapour pressure dif-ference ( D 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).
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 911; Fig. 2). Photo-synthetic Photon Flux Density (PPFD) reached a maxi-mum of about 1400 m mol m2 s1 and air temperaturesvaried between 15 and 25C through the day. Maximaldaily leaf-air vapour pressure difference ( D W) increasedover th...