Studies on lichen-dominated systems. XXII. The environmental control of nitrogenase activity in Stereocaulon paschale in spruce–lichen woodland

Abstract

Stereocaulon paschale (L.) Hoffm. collected from spruce–lichen woodland (Northwest Territories) at different times of the year was assayed for nitrogenase activity (acetylene reduction) under factorial combinations of photosynthetically active radiation (PAR), thallus temperature, and moisture. Marked seasonal changes in C2H2-reducing capacity were recorded, potential activity being minimal in samples from beneath snow beds in March and maximal in summer collections. Physiological–environmental relationships established in the laboratory were largely consistent with diurnal and seasonal variations in nitrogenase activity in S. paschale mats recorded in situ. When thallus water content was not limiting (i.e., values > 120% of thallus dry weight), diurnal variation in nitrogenase activity was correlated with changes in PAR and thallus temperature or, under isothermal conditions, with changes in PAR alone when C2H2 reduction continued throughout dark periods at ca. 40% of daylight rates. However, while nitrogenase activity in S. paschale at any point in time was dependent upon concurrent environmental conditions, it also was strongly influenced by pretreatment: both winter frost and thermal stress during summer droughts impeded recovery of nitrogenase activity following the onset of ameliorative conditions. It is suggested that a large fraction of the total annual N assimilation by S. paschale in drier, continental subarctic habitats may be achieved in summer during a few comparatively short episodes of suitable moisture conditions.