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CO2 exchange and water relations of selected lichen species were investigated in the field and also in the laboratory, at a height of 3106 m above sea level in the Austrian Alps, during the short snowless summer period from middle of July to the end of August. In the course of the field investigations, clear summer days were quite rare. Altogether 14 diurnal courses of CO2 exchange were measured spanning a time of 255 h of measurements.

The air temperatures measured close to the ground ranged between -0.7 and 17.1 degrees C and their daily fluctuation was lower than 10.7 degrees C. Fog was present for more than one-third of the measuring period and relative humidity (RH) exceeded 90% in almost half of the time. Temperature optimum of net photosynthesis (NP) of Xanthoria elegans and Brodoa atrofusca determined in the laboratory increased with increasing photosynthetic photon flux density (PPFD) from 1.5 to 11.3 degrees C and the maximal CO2 uptake was found to be at 10 degrees C. In the field the lichens were metabolically active at air temperatures between -0.7 and 12.8 degrees C. The light compensation points (LCP) of both lichen species ranged in the laboratory between 50 and 200 mu mol m(-2) s(-1) PPFD (0-20 degrees C) and in the field between 22 and 56 mu mol m(-2) s(-1) PPFD (3-8 degrees C). At 30 degrees C the NP of X. elegans surpassed the LCP, whereas B. atrofusca remained below the LCP. NP in X. elegans did not reach light saturation at 1500 mu mol m(-2) s(-1) PPFD. NP in B. atrofusca reached light saturation at low temperatures (-5 to + 5 degrees C). At higher temperatures light saturation was almost detectable. On sunny days the lichens in the field were metabolically active only for 3 h during the early morning. In this time they reached the maximal values or values close to their maximal CO2 uptake in situ. Under dry weather conditions the lichens dried out to a minimal water content (WC) of 5-12% which is below the moisture compensation point (MCP) of 34-25%. The optimal WC was between 90% and 120% dry weight (DW) in B. atrofusca and Umbilicaria cylindrica, in X. elegans between 140% and 180% DW. Species specific differences in water-holding capacity, desiccation intensity and in the compensation points of temperature, light and moisture are responsible for differences in metabolic activity. The lichens were active during less than half of the observation time. Total time of NP of X. elegans was 24% of the measuring period, for U. cylindrica 22% and for B. atrofusca 16%. (C) 2007 Elsevier GmbH. All rights reserved.