Role of morphological structure and layering of Sphagnum and Tomenthypnum mosses on moss productivity and evaporation rates

Goetz, J. D. and Price, J. S. 2015. Role of morphological structure and layering of Sphagnum and Tomenthypnum mosses on moss productivity and evaporation rates. Can. J. Soil Sci. 95: 109-124. Morphological structures of peatland mosses control moss water relations and the rate of water loss by drainage and evaporation, thus influencing their physiological functions. While many of these mechanisms are understood for Sphagnum mosses, there is a limited understanding of how these processes operate in Tomenthypnum nitens, a dominant brown moss species in northern rich fens. This study contrasts how different hydrophysical characteristics of Tomenthypnum and Sphagnum species affect capillary water flow that supports evaporation and productivity. Laboratory investigations indicate that volumetric water content (?), gross ecosystem productivity, and evaporation decreased with water table depth for both mosses, with Sphagnum capitula retaining 10-20% more water (? range of 0.18-0.32 cm³ cm⁻³) than Tomenthypnum (0.07-0.16 cm³ cm⁻³). Despite lower ? and a smaller fraction of pores between 66 and 661 µm to retain water within the Tomenthypnum structure (10%) compared with Sphagnum (27%), both mosses had similar fractions of water conducting pore spaces and were able to maintain capillary rise throughout the experiment. While there was a larger difference in the bulk density and porosity of the Tomenthypnum moss compared with its underlying peat than there was in the Sphagnum profile, a layer of partially decomposed moss of intermediate properties was sufficient to provide a connection between the moss and peat under low water table conditions. In trying to characterize the soil-water pressure (?) in near-surface mosses of Tomenthypnum based on measurements of vapour pressure, we found disequilibrium conditions that severely underestimated ? (i.e., very large negative pressures). It is this disequilibrium that drives evaporation and draws up capillary water to the moss surface for peatland-atmosphere carbon and water transfers.