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.

Green aboveground shoots of the moss Polytrichum alpinum Hedw., growing on the tundra at Point Barrow, Alaska, were exposed to 14CO2 in the field. Autoradiography and subsequent counting revealed preferential translocation of the labeled compounds into new belowground shoots arising from the extensive underground stem system. Within 24 h all stem systems extracted had been labeled throughout, a maximum distance of 7.5 cm being covered in this time, indicating minimum rates of 3.1 mm h−1.Increases in the number of new shoots aboveground throughout the season revealed the extent and importance of this translocation. Innate markers of seasonal growth enabled ages of the shoots at two sites to be determined and also the ages of shoots at production of gametangia in a third site where fruiting was common. The relative photosynthetic rates of the successive seasons' growth were also determined, and showed that by the 3rd year the photosynthetic capacity had dropped to about 40% of that in the youngest tissue on the same shoot.The patterns of growth and translocation of photosynthate in Polytrichum alpinum are discussed in relation to survival on the tunda.

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