High thallus water content severely limits photosynthetic carbon gain of central European epilithic lichens under natural conditions.

Experiments under controlled conditions have shown that net photosynthesis (NP) of many lichens is depressed when their thalli are highly hydrated. In this study we characterise the light and water content (WC) dependency of CO exchange for selected epilithic lichens in the laboratory and match this against samples monitored in their natural habitat by a novel, fully automatic cuvette. Laboratory measurements showed that, at a photosynthetic photon flux density (PPFD) of 1500 μmol m s, NP of the epilithic foliose lichen Xanthoria calcicola was reduced by about 85% (compared to NP at optimal water content) when the thallus was suprasaturated (maximal hydration was defined as WC after spraying, submerging and subsequent removal of adhering water droplets by shaking). Only after loss of about 80% of its maximal WC were the highest rates of NP possible. This depression was still substantial at 50 μmol m s PPFD. Responses were similar for the crustose epilithic species Lecanora muralis. CO exchange of both lichens was monitored under natural conditions by means of the cuvette built into a man-made wall-a common habitat of the species-in the Botanical Garden, Würzburg. For both species, rates of NP were low during and after heavy rain even if incident PPFD and temperature were favourable. This situation occurred frequently and could last through all daylight hours, resulting in a negative carbon balance when nocturnal rates of respiration were high. Often, after rainfall, there was a brief, high peak of NP when optimal WC was transiently attained before metabolic activity finally ceased through desiccation. Other periods with profitable rates of NP occurred after moderate moistening of the lichens by dew, fog or light rain. The lichens were found to perform identically in the field and laboratory. When the two data sets were compared it was clear that the full range of WC produced in the laboratory also occurred in nature and that the productivity of the epilithic lichens was regularly and severely limited by high WC. It is concluded that blockage of diffusive pathways for CO in the thallus through high water contents is an important ecological factor for productivity of these central European epilithic lichens.