Patterns of CO, gas-exchange and thallus water content in Arctic lichens along a ridge profile near Ny Alesund, Svalbard BARBARA SCHIPPERGES Schipperges, B. 1992: Patterns of COz gas-exchange and thallus water content in Arctic lichens along a ridge profile near Ny Alesund, Svalbard. Polar Research 11(2), 47-68. Two abundant and partly dominating lichen species, Cetruria nioalis and Cetraria delisei, were compared with respect to their thallus water content and their gas-exchange response to

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... ht, temperature and moisture in the field and in the laboratory. C. niuulis 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. nioalis. The chionophobous C. niualis 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 bc due to the increased internal C 0 2 diffusive resistance at high thallus water contcnt. The maximum thallus water contcnts of C. nioalis were higher than those of C. delisei. The lower drying rate, found in C. delisei in comparison with that of C. nioulis. is attributed to the more sheltered position of its habitat and to morphological characteristics.