Thalli of Stereocaulon paschale (L.) Fr. were collected at approximately monthly intervals during a 13-month period from a pine forest in northern Sweden. The capacity for nitrogenase activity (C2H2-reduction) was studied on each collection under standardized, favourable laboratory conditions. Significantly higher capacities were found in thalli collected from bare ground than in thalli collected under snow, but the capacity decreased in autumn close to the snow covering. At snow melt the increase in capacity was rapid.
To investigate the decrease in capacity during the autumn, S. paschale was exposed to simulated autumn conditions in the laboratory during 2-week periods. These simulation experiments indicated that the light regime was of importance for nitrogenase activity. Low irradiance during short days caused a marked decrease in the nitrogenase activity, especially in combination with night temperatures above freezing. Night temperatures below zero, per se, did not reduce nitrogenase activity; instead a tendency to stimulate it was found.