FIELD-MEASUREMENTS OF PHOTOSYNTHESIS OF UMBILICARIOUS LICHENS IN WINTER

The photosynthetic performance of Lasallia pustulata and Umbilicaria s podochroa was measured both in the field on granitic rock in southern Norway and in the laboratory under controlled conditions of light and temperature. In the field thallus temperatures varied between - 2 and + 5 degrees C during the daylight period in January 1994. In situ wate r contents were between 50 and 400% d.wt. in L. pustulata and between 100 and 500% d.wt. in U. spodochroa. The lichens were active during th e whole period of investigation. Photosynthetic rates reached 13.03 mu mol CO2 g Chl(-1) s(-1) in L. pustulata and 5.56 mu mol CO2 g Chl(-1) s(-1) in U. spodochroa. Ice formation on the thallus surface did not impede CO2 exchange. Light was mainly the limiting factor, as could be confirmed by the laboratory experiments. In general, habitat conditio ns never provided optimum photosynthetic rates but photosynthetic carb on balance was positive during 4 of the 5 days investigated. The colde st day was photosynthetically almost as efficient as the warmest day d uring this period.