UPLAND TUNDRA IN THE FOOTHILLS OF THE BROOKS RANGE, ALASKA - INFLUENCE OF LIGHT, WATER-CONTENT AND TEMPERATURE ON CO2 EXCHANGE OF CHARACTERISTIC LICHEN SPECIES

In a previous publication we described diel courses of CO2 exchange an d microclimate conditions for characteristic Lichens in their natural habitat within upland tundra communities of northern Alaska. The influ ence of individual environmental factors on net photosynthesis (NP) of Cetraria cucullata, Dactylina arctica, Masonhalea richardsonii, Pelti gera aphthosa, Peltigera malacea, Stereocaulon alpinum, and Thamnolia vermicularis was analyzed in the present study. CO2 exchange measureme nts were conducted in the laboratory, and clear response characteristi cs with respect to light, water content (WC), temperature, and externa l CO2 concentration were established under controlled conditions. In a ddition, dependencies of NP on these factors were extracted from field data. These measurements show a high scatter in data points, however, they represent the range of actual performance of the lichens under n atural conditions. In general both, field and laboratory data sets, ag ree well with respect to absolute rates of photosynthetic capacity as well as response characteristics. The combined information from both s ources enable us to identify and describe those physiological features which are relevant for photosynthetic production of the lichens at th is tundra site. There were large differences in maximal rates of NP at tained under natural ambient CO2 which were expressed more strongly un der conditions of CO2 saturation. Photosynthetic capacity of the cyano bacterial P. malacea is ten times higher than that of the green algal M. richardsonii. In the field, actual photosynthesis often seemed to b e depressed due to photoinhibition. Photosynthetic carbon gain occurre d even with thallus temperatures of -10 degrees C, while the temperatu re optimum of NP was between 11 and 22 degrees C. Most of the species responded to supra-optimal degrees of WC with a pronounced depression in NP. Elevated ambient CO2 concentration prevented this decrease in N P, indicating that it was caused by increased resistance of the thallu s to CO2 diffusion. Depression of NP at high thallus WC regularly occu rred under natural conditions, impairing primary production. Response characteristic of the lichens to experimental increase in ambient CO2 is highly dependent on thallus hydration. At optimal WC some species a re already saturated by natural ambient CO2 at least at lower light in tensities. Possible future increase in natural ambient CO2 concentrati on will impact lichen NP in particular when the thalli are highly wate r saturated.