Patterns of CO2 gas-exchange and thallus water content in Arctic lichens along a ridge profile near Ny Ålesund, Svalbard

Two abundant and partly dominating lichen species, Celraria nivalis and Cetraria delisei, were compared with respect to their thallus water content and their gas-exchange response to light, temperature and moisture in the field and in the laboratory. C. nivalis had higher net photosynthetic rates than C. delisei. The differences between the species were more pronounced when photosynthesis was related to dry weight than to chlorophyll content. Light compensation and light saturation of photosynthesis increased with increasing temperature. Higher light compensation and saturation values were found in C. delisei than in C. nivalis. The chionophobous C. nivalis showed moisture compensation and optimum water content for net photosynthesis at higher thallus water contents than the chionophilous C. delisei. Depression of net photosynthesis at thallus saturation, found in both species, is thought to be due to the increased internal CO2 diffusive resistance at high thallus water content. The maximum thallus water contents of C. nivalis were higher than those of C. delisei. The lower drying rate, found in C. delisei in comparison with that of C. nivalis, is attributed to the more sheltered position of its habitat and to morphological characteristics