The response of photosynthetic CO # exchange to tissue water content in five spp. of Sphagnum from contrasting microhabitats (S. fuscum (Schimp.) Klinggr., S. papillosum H. Lindb., S. magellanicum Brid., S. balticum (Russ.) C. Jens. and S. cuspidatum Ehrh. ex Hoffm.) was measured in the laboratory using an infrared gas analyser technique. Experiments were designed to test recovery of net photosynthesis after periodic and long lasting desiccation.The contact between capitula and basal parts of the mosses seems to be important for survival. Isolated capitula cut off from any contact with the water table were not able to recover after complete desiccation (at 15 mC for 2-4 d). When contact with the water table is lost, e.g. during long periods of desiccation, recovery of net photosynthesis can take place but only if the water content of the capitula does not fall too far below c. 10-20 % of the water content at compensation point.There was no relationship between the ability of net photosynthesis to recover from desiccation and the wetness of the natural microhabitat. Sphagna survive dry periods by avoidance of drying out by high capillarity or dense growth form (as in S. fuscum). Key words : CO# exchange, photosynthesis, desiccation, recovery, Sphagnum. Peat mosses (Sphagnum) cover large areas in the boreal regions, especially on bogs (ombrotrophic mires). Bogs are characterized by a conspicuous microtopography, consisting of hummocks and hollows, formed by the mosses. Different species occupy different levels above the water table. Earlier studies on the interactions among hollow and hummockinhabiting Sphagnum spp. were reviewed by Rydin (1993 a). The present study aims to give an understanding of water relations and desiccation tolerance among different Sphagnum spp. in ecophysiological terms. Such knowledge is important in view of the consequences of possible future climatic changes : higher atmospheric CO # concentrations will be followed by

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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 light, 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.Barbara Schipperges, Institut fur Polurokologie. Unioersitar Kiel. Wisrhhofstr. 1-3. Gebaude 12, 0-2300 K i d . Germuny. IntroductionThe objective of this study was to analyse relationships between snow cover and productivity of lichens. The C 0 2 gas-exchange, i.e. photosynthesis and respiration respectively, is suited for the investigation of productivity as a measure of plant growth rate (net dry weight gain per unit time) (Osmond et al. 1980) and also as a measure of fitness and adaptedness of plants to the habitat conditions (Mooney 1972). Cetraria nivalis and Cetraria delisei were chosen in this investigation because they are abundant and partly dominating in Arctic ecosystems. moisture compensation maximum thallus hydration optimum thallus hydration (at which Pmax is reached) light compensation light saturation photosynthetic activc radiation maximum net photosynthetic rate temperature compensation optimum temperature (at which Pmax is reached) thallus water content.In Arctic areas there are complex vegetation patterns not only on larger topographic scales, such as along altitudinal gradients, but also on smaller scales, such as on ridges or frost structured grounds (Polunin 1945; R~n n i n g 1969; Brossard et al. 1984). The patchiness of vegetation is mainly the result of meso-or microtopographic variations, which cause uneven distribution of snow and thus differences in soil moisture, heat exchange and protection against windblast (Billings 1973 Material an...

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.