Carbon Balance for Two Sphagnum Mosses: Water Balance Resolves a Physiological Paradox

Abstract

Sphagnum fallax is generally found closer to the water table than S. nemoreum along the hummock—hollow gradient of an Adirondack bog. However, at low water contents S. fallax unexpectedly fixes carbon at higher rates than S. nemoreum, as we have previously reported. We here compare water and carbon balances for these species to resolve this puzzling contrast. S. nemoreum maintains a more favorable water balance by virtue of its greater water—holding capacity, lower rates of water loss from moist plants, and apparently more effective capillary conduction of water. The consequence is a significantly higher tissue water content in the field compared to adjacent patches of S. fallax for a wide range of dates and heights above the water table. We developed a simulation model, PEATMOSS, to evaluate the relative importance of contrasting physiological relationships and water balances of the two species. The model integrates physiological and field data to estimate 24—h carbon balance for these mosses in adjacent patches at two heights above the water table under different conditions. PEATMOSS estimated a greater carbon gain for S. fallax at the low site, where it is a clear dominant over S. nemoreum, and a markedly more favorable carbon balance for S. nemoreum at the hummock site, where S. nemoreum is common but S. fallax is infrequent. Morphological features of S. nemoreum which effect its more favorable water balance apparently confer an advantage over S. fallax when water availability is limited.