Controls on CO 2 Exchange in Two Polytrichum Moss Species. 1. Field Studies on the Tundra near Barrow, Alaska

Abstract

Microclimatic habitat conditions and diel field CO2 flux were studied in P. alpinum and P. commune during the 1973 growing season and again in the late autumn of 1974, near Barrow, Alaska. Incident photon flux density differed among the 3 different habitats, a mesic meadow, polygon center and polygon rim, while differences in moss surface temperature and water content were small. Diel CO2 flux remained positive during June and much of July in unshaded samples of both species, while shaded samples of P. alpinum showed night respiration and reduced daytime CO2 uptake during much of the season. The proportion of the diel assimilate pool respired at night increased sharply from first sunset on August 2. No regular seasonal changes in maximum net photosynthetic rate could be detected in either species. Samples of P. alpinum from the shaded mesic meadow reached light compensation at lower photon flux densities than did the samples from the more open low polygon centers, especially at higher temperatures. P. commune showed a greater temperature sensitivity in net photosynthesis than P. alpinum. A seasonal total CO2 uptake of 700-800 mg/g is estimated for nonshaded aboveground shoots.