Upland tundra in the foothills of the Brooks Range, Alaska: Diurnal CO2 exchange patterns of characteristic lichen species

Abstract

Summary CO2 exchange, water content, and microclimate conditions were observed for seven characteristic lichen species in their natural habitat within upland tundra communities of northern Alaska. Diurnal courses of lichen gas exchange response were recorded over five-day periods during the arctic summer and fall (from July to early September 1988 and 1989). Water availability is the environmental factor of foremost importance in determining rates of primary production. Water sources were rain, fog, and dew fall, as well as high air humidity, which alone could reactivate most of the green algal species after desiccation. Despite high variability in environmental conditions, certain patterns in the diurnal course of thallus hydration occurred repeatedly, so that five different weather types were defined within which gas exchange performance was predictable. Even short periods of favourable hydration were used by lichens for positive net photosynthesis (NP). There was no indication of adverse consequences of “resaturation respiration”. Even after a dry period of 3 days, sudden rehydration resulted in carbon gain without delay. For short periods of time, the combinations of water content, temperature and light imposed on the lichens enables high rates of NP. Individual species differed greatly in their maximal NP rate, which correlated with chlorophyll and nitrogen content. At favourable times in the field, observed NP rates approached the maximum capacity found in laboratory experiments at natural ambient CO2. Often with sufficient hydration, carbon gain was limited by light. CO2 exchange became negative even during daylight hours due to the effects of fog and clouds with light below compensation levels. Reduced but still positive rates of NP were observed with snow and with frozen lichens. In order to draw general conclusions about activity over the summer season, the time was calculated for a characteristic set of sampling days during which thalli were inactivated due to dehydration (no CO2 exchange measurable), during which they photosynthetically fixed CO2, and during which CO2 was released. The thalli were inactivated on the average 42.5 % of the time. Species-specific differences with respect to the total period of dehydration were surprisingly small (from 39.0% forStereocaulon alpinum to 45.5 % for Cetraria cucullata). Thus, growth-form specific morphology and anatomy of the samples, which were exposed side-by-side at the same site did not result in large differences in active phases. On the other hand, differences in physiological traits between species result in varying division of active phases with respect to positive and negative NP. For example, Dactylina arctica photosynthesized 1.36 times longer than Peltigera malacea. Other traits tend to offset the negative effect of long periods with respiratory CO2 release. As a result, the cyanobacterial lichen Peltigera malacea with the shortest total period of positive NP was, nevertheless, the most productive species due to its high photosynthetic capacity. Our field observations strengthen the viewpoint that studies of lichen physiological differentiation are essential for understanding species autecology and that approaches based on interpretation of morphological attributes may sometimes be exaggerated in their importance.