Photosynthesis of lichens from lichen-dominated communities in the alpine/nival belt of the Alps - II: Laboratory and field measurements of CO2 exchange and water relations

CO₂ exchange and water relations of selected lichen species were investigated in the field and also in the laboratory, at a height of 3106m 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 255h of measurements. The air temperatures measured close to the ground ranged between -0.7 and 17.1°C and their daily fluctuation was lower than 10.7°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°C and the maximal CO₂ uptake was found to be at 10°C. In the field the lichens were metabolically active at air temperatures between -0.7 and 12.8°C. The light compensation points (LCP) of both lichen species ranged in the laboratory between 50 and 200μmolm⁻² s⁻¹ PPFD (0-20°C) and in the field between 22 and 56μmolm⁻² s⁻¹ PPFD (3-8°C). At 30°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 1500μmolm⁻² s⁻¹ PPFD. NP in B. atrofusca reached light saturation at low temperatures (-5 to +5°C). At higher temperatures light saturation was almost detectable. On sunny days the lichens in the field were metabolically active only for 3h 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%.