The Influence of Light Intensity and Temperature on the Field Carbon Dioxide Exchange of Dicranum fuscescens in the Subarctic

Measurements of diurnal net photosynthetic rates were conducted on fully hydrated samples (ca. 300% water content on a dry weight basis) of Dicranum fuscescens (Turn.) under field light and temperature conditions at Schefferville, Quebec, Canada (latitude 55°N). A highland semitundra and lowland lichen woodland site were chosen for study, and measurements were made periodically throughout the summer of 1974. Photosynthetic rates of samples within temperature-controlled assimilation chambers were determined using an open-flow infrared gas analysis system. Water content of field-collected plants was monitored during the days of diurnal measurements in July and August. Maximum rates of net photosynthesis were found to be between 8 and 9 mg CO2· g dry wt-1· day-1. The pattern of net CO2exchange and field water content of 4 days during the summer was analyzed and the seasonal course of net photosynthesis was determined. Temperature accounted for most daily variations in net photosynthesis of the hydrated samples while variations in both temperature and light intensity accounted for hourly variability. There appeared to be no marked seasonal senescence and concomitant decrease in net photosynthesis. Maximum realized rates of net photosynthesis for water saturated samples in the field were similar to potential rates determined in previous laboratory analyses.