Photosynthetic productivity of the epilithic lichen Lecanora muralis: long-term field monitoring of CO2 exchange and its physiological interpretation: II. Diel and seasonal patterns of net photosynthesis and respiration

CO2 exchange of the epilithic, green algal lichen Lecanora muralis was monitored continuously for 15 months by means of an automatic cuvette in its natural habitat in the Botanical Garden, Würzburg (Bavaria, Germany). Simultaneously weather data and chlorophyll fluorescence were recorded. The periods of metabolic activity for the lichen were restricted because of the low water holding capacity of its thin thallus and high desiccation rates. The total activity was only 35.6% of the year made up of net photosynthetic CO2 uptake, 16.7%, and respiratory CO2 release, 18.9%. This was substantially less than found for gelatinous or foliose lichens under similar conditions. The active time was longest in October (57.7%) and shortest in June (17.2%). Lecanora muralis suffered from strong suprasaturation depression of net photosynthesis so that its carbon gain was substantially limited after heavy wetting. Averaged over the total year, net photosynthesis (NP) of the lichen was heavily depressed through suprasaturation for 38.5%; of that time when photosynthesis was possible. NP took place under snow and at subzero thallus temperatures (between -4 and -5°C). Water status was the most important factor that determined the diurnal patterns of NP. Diel time courses of photosynthetic and respiratory performance could be divided in several response types such as lichen dry all day, lichen moistened by dew or frost, rain during night and subsequent drying during the day, rain during the day with or without suprasaturation depression, and lichen below snow. The seasonal distribution of these response types was analysed. Optimal NP was only possible over a very small range of WC (water content) that rarely occurred in combination with optimal light and temperature conditions. Thus, maximal rates of NP were seldom attained and then lasted only for a short time, sometimes no longer than a few minutes. The interplay between weather conditions and physiological responses of the lichen is discussed whilst diel, seasonal and annual carbon gain and carbon balance of L. muralis will be analysed in a further communication.