Contrasting Water Relations of Photosynthesis for Two Sphagnum Mosses

Sphagnum fallax and S. nemoreum differ in their vertical distribution above the water table on hummocks in Bloomingdale Bog (Adironkack Mountains, New York), and both exhibit decreasing water content with increasing height above the water table. We tested the prediction that S. nemoreum, a species better developed on hummock tops, would photosynthesize at greater rates when relatively dry than would S. fallax, which is more abundant in hollows and on hummock bases. Net photosynthesis declined sharply at tissue water contents (fresh—/dry—mass ratio) below 7.2 for S. fallax and 8.9 for S. nemoreum collected in spring. These values fell to 5.1 and 7.4, respectively, for plants collected in late summer. Thus both species exhibit adaptive seasonal acclimation, as reflected by the maintenance of relatively high net photosynthetic rates to lower water contents after a dry summer. Neither species showed significant response differences when collected from different heights above the water table. In direct contradiction to our hypothesis, Sphagnum fallax fixed carbon at greater rates than S. nemoreum at low water contents for both spring and summer collections. This physiological contrast appears inconsistent with observed field distribution patterns for these species.