PHOTOSYNTHETIC PRIMARY PRODUCTION OF THE LICHEN CLADONIA-PORTENTOSA IN A DUNE HABITAT AT THE ISLAND BALTRUM IN THE NORTH-SEA .1. FIELD-MEASUREMENTS OF MICROCLIMATE, WATER RELATIONS AND CO2-EXCHANGE

In its natural habitat of stabilized dunes on the island Baltrum (North Sea), microclimatic conditions for Cladonia portentosa were recorded (air and thallus temperature, rel. humidity, photosynthetically active radiation). In parallel, water content of the lichen (WG, % of dry weight) was determined. With respect to lichen hydration, the 56 diurnal courses of measurements, which were spread over several seasons, could be grouped into 5 different types: (1) WG continuously low; (2) nocturnal moistening (e.g. by dew fall) and high WG at sunrise; (3) days with fog and high WG throughout; (4) days with rain and long lasting high WG; (5) lichens covered by snow. In addition, during 24 days in the summer and in the winter, diurnal courses of CO2 exchange of the lichen were followed together with the WG and the microclimatic conditions. These days could be separated according to with the above types. This provides an overview for the performance in production ecology of the lichen. The range of water content of the lichen was measured from 7% to > 500%. Cladonia portentosa was able to absorb water vapor quickly from the atmosphere, however, dew, mist, and rain were the main sources for hydration. At a WG between 150 and 180% and under sufficient light, the lichen reached its optimum of net photosynthesis (NP), higher degrees of hydration resulted in a slight depression. The moisture compensation point (FKP) of NP was lower in summer (around 20% WG) than in winter (25-35% WG), due to temperature conditions. Also under field conditions, the lichen maintained photosynthetic activity at temperatures below 0-degrees-C. With optimal conditions (120-200% WG, 17-19-degrees-C, 500-900-mu-E m-2 s-1 PAR) maximal rates of NP around 20 nMol CO2 g-1 DW s-1 were attained.