Der CO2- und H2O-Gasaustausch von Pflanzen in der Krautschicht eines Kalkbuchenwaldes in Abhängigkeit von Standortsfaktoren. II. Einfluß von Temperatur und Luftfeuchte

Abstract

Summary In the years 1982, 1983 and in spring 1984 the CO2 gas exchange of two spring geophytes (Allium ursinum, Arum maculatum), of a wintergreen herb (Asarum europaeum), of a summergreen herb (Mercurialis perennis) and of two summergreen grasses (Hordelymus europaeus, Melica uniflora) was studied in the understorey layer of a beech forest on limestone. The significance of leaf temperature (T1) and of air humidity for CO2 gas exchange (Pn), for leaf conductance (LF), for transpiration (Tr) and for the transpiration coefficient (Tr/Pn) during the season were analyzed. Air humidity was expressed as the vapor concentration gradient between the leaf and the surrounding air (ΔW). Especially in spring and in autumn the plants attained high photosynthetic rates at cool temperatures. In summer the temperature demand was a little bit higher. In the course of the year the optimal temperature range for photosynthesis was between 8 and 22 °C. The light saturation point of photosynthesis and maximum photosynthetic rate were equally influenced by changing temperature. The maximal apparent quantum yield increased with increasing temperature at a low temperature level (under 15 °C). At temperatures over 15 °C the apparent quantum yield decreased with increasing warming. In general, photosynthesis was little or not at all influenced by ΔW. Only A. ursinum showed a depression of Pnmax as well as of LF and of Tr with increasing ΔW. LF of the other plant species decreased also with increasing ΔW. This decrease was of different intensity. In spring, the young leaves of A. europaeum, in summer also the leaves of A. europaeum and of the two grasses regulated the stomata very well. A. maculatum in spring and M. perennis in summer scarcely closed the stomata with increasing ΔW. That is why especially in the dry summer 1982 but also in summer 1983 M. perennis had the lowest leaf water potentials of all the investigated plants.