Comparative CO2 exchange patterns in mosses from two tundra habitats at Barrow, Alaska

The effects of variation in light intensity, temperature, and water content on rates of net photosynthesis and dark respiration have been investigated in two common tundra mosses, Polytrichum alpinum from drier habitats and Calliergon sarmentosum from wetter habitats at Barrow, Alaska. Optimum temperatures for net photosynthesis of 10–15 °C for both species and saturating light intensities (photosynthetically active radiation (PhAR), 400–700 nm) of about 0.12 cal cm−2 min−1 for P. alpinum and 0.15 cal cm−2 min−1 for C. sarmentosum correlate well with measurements of light intensity and moss tissue temperatures made over the season at the collection site. It is suggested that depressions in net photosynthetic rates around midday might be caused by supraoptimal temperatures and possibly supraoptimal light intensities. Calliergon sarmentosum, a semiaquatic species required a higher water content (about 450% dry weight) than P. alpinum (about 200%) to reach maximal rates of net photosynthesis. Mean maximal rates of net photosynthesis ranged from about 2.6 to 4.4 mg CO2 g−1 dry weight h−1 for P. alpinum and from about 1.5 to 3.0 mg CO2 g−1 dry weight h−1 for C. sarmentosum. Predictions of net annual production have been made for both species. Predicted levels of 171 g C m−2 per 50-day season for C. sarmentosum compare well with results obtained for species of similar growth form elsewhere in polar regions. For P. alpinum the predicted level of 38.5 g C m−2 per 50-day season compares with observed dry matter production at the same site of 43 g m−2 per season