Blattwasserzustand und Wasserumsatz von vier Buchenwäldern entlang eines Niederschlagsgradienten in Mitteldeutschland

Abstract

1. The aim of the study was to analyze the plasticity of common beech (Fagus sylvatica L.) in water use and the regulation of water balance in adaptation to forest sites with widely differing precipitation regimes. The climatic site conditions of the four old-growth beech stands studied varied from a sub continental climate with 520 mm annual precipitation at one of the driest beech forest sites in central Europe to a sub oceanic montane climate with 1040 mm annual precipitation, thus covering a wide range of the climatic spectrum of central European beech forest sites. 2. During three growing seasons, the precipitation regime of the investigated beech forests was assessed, together with microclimatic site conditions (global radiation, air temperature, water vapour pressure deficit (VPD) of air) and soil water status (matric potential, volumetric water content). 3. Daily time courses of stomatal leaf conductance were measured by porometry. In parallel to this and pre-dawn, leaf water potentials were measured by a pressure chamber. An analysis of pressure volume relations revealed characteristics of leaf water status (osmotic potentials, relative water content). Stand transpiration was monitored by xylem sap flow measuring systems. Stand conductance could be derived from stand transpiration rates in relation to microclimatic parameters. Increment measuring tapes, litter buckets and leaf area measuring systems were used to investigate wood increment, leaf area index (LAI) and morphological characteristics of leaves. 4. During this study the local regimes of global radiation and VPD were remarkably similar at all the beech forest sites investigated. However, the precipitation regimes differed distinctly and monthly precipitation sometimes showed considerable deviations from 30-year means. 5. All studied beech stands except the montane beech forest with highest precipitation showed seasonal features of drought-stress. These were most pronounced at the site with the lowest 30-year mean precipitation and at a site in the Lüneburger Heide with unusually low rainfall during the growing seasons studied. Those two sites have sandy soils and continuously showed the lowest storage of extractable soil water. 6. As a result of an extreme drought period in mid summer 1995, in two beech stands pre-dawn values of leaf water potential dropped to - 1.0 MPa and - 1.15 MPa respectively, indicating a severe deficit of extractable soil water. During the vegetation periods in 1996 and 1997, under drought stress pre-dawn values of leaf water potential dropped to - 0.7 MPa and midday leaf water potential reached - 2.6 MPa at the site with the lowest 30-year mean precipitation. At the other stands no pre-dawn values lower than - 0.5 MPa were observed and daily minimum values of leaf water potential hardly dropped below a treshold value of - 2.0 MPa. Furthermore, the stand with the highest rainfall showed the highest values of midday leaf conductance (124 - 311 mmol m-2 s-1), whereas the two stands with low soil water storage showed particularly low values of midday leaf conductance (28 - 183 mmol m-2 s-1). There was no proof of an adaptation to drought stress by active osmotic adjustment. 7. Annual stand transpiration of the beech forests studied varied between 216 mm at the site with the lowest 30-year mean precipitation and 303 mm at the montane site with the highest amount of rainfall. No correlation could be detected between annual stand transpiration and structural parameters of the stands like stem density, basal area, fine root biomass or LAI. Daily stand transpiration was mainly determined by global radiation and VPD but was only slightly related to soil moisture. 8. Restriction of leaf- and stand-conductance under soil water depletion and dropping leaf water potentials emerged clearly as the most important regulatory mechanism of water balance in the studied beech stands. 9. The beech stands with the driest soils showed a seasonal reduction of wood increment under late summer drought. However, a comparison of total annual wood increment between the beech forests examined gave no indication of a remarkable reduction of carbon gain and vitality by drought stress even at one of the driest beech forest sites in central Europe.