Der Wasserumsatz von Pflanzen in der Krautschicht eines Kalkbuchenwaldes
In the understorey layer of a beech forest on limestone the H2O gas exchange of the summergreen herb Mercurialis perennis was continuously studied in 1982, in 1983 the transpiration (Tr) of six plant species was alternatingly analysed: two spring geophytes, Allium ursinum and Arum maculatum, one wintergreen herb, Asarum europaeum, one summergreen herb, M. perennis, and two grasses, Hordelymus europaeus and Melica uniflora. In addition to the gas exchange the seasonal course of some climatic factors (photosynthetical active radiation (PAR), air temperature (Ta) saturation deficit of the air (SD)) and of the water potential of the leaves (θL) were measured. Applying multiple regression analyses an attempt was made to explain the variability of the daily H2O losses of the six plant species during their vegetation period referring to these exogeneous and endogeneous variables. Additionally the time was taken into account as an independent variable for this analyses. By using the independent variables, which were continuously recorded, together with the leaf water potential, the time and the leaf conductance for water vapour (LF), the complete seasonal course of the daily H2O losses of all six plant species have been calculated. The photosynthetic active radiation in the herbaceous layer was quite different during the summers 1982 and 1983. As the PAR was one of the most important independent variable, the calculations for the total water losses in the six plant species were done independently, first with the daily PAR values of 1983, subsequently with the daily PAR values of 1982. A. maculatum shows the highest leaf related Tr-rates of the six plant species. The high of the values of M. perennis in 1983 and of the two grasses are comparable to those of A. maculatum. The Tr-rates of A. europaeum and A. ursinum were relatively low as well as the values of M. perennis during the summer 1982. There was a relatively small influence of the different light climates on the H2O gas exchange of the plant species during the vegetation periods of 1982 and 1983. The higher PAR in 1983 enhanced the water loss of the species in the herb layer between 60 % (A. europaeum) and 200 % (M. uniflora). It is possible to calculate the H2O losses per ground area by multiplying the leaf related H2O losses of the six plant species with their leaf area index (LAI) in the research plot. Referring to ground area, M. perennis or A. ursinum achieved the highest H2O losses because of their high LAIs. The waterloss per ground area or the grasses was very low as well as that of A. maculatum. Using the water balances of these sixplant species the waterbalance of the herb layer was calculated. The plants transpired between 20 mm (1982) and 40 mm (1983). This water loss via transpiration means ca. 10 % of the total yearly evaporation rate of this forest ecosystem.