Ecohydrology of Sphagnum moss hummocks: mechanisms of capitula water supply and simulated effects of evaporation

Abstract

ABSTRACTMaintaining sufficiently high surface (capitula) soil‐water pressures to avoid the draining of hyaline cells (desiccation) is paramount to hummock‐forming Sphagnum species' survival; however, the mechanisms of capitula water supply are poorly understood. This study investigates how the hydraulic characteristics of different Sphagnum species (Sphagnum fuscum, Sphagnum rubellum and Sphagnum magellanicum) contribute to desiccation avoidance, on the basis of numerical simulations parameterized with measured soil hydraulic characteristics for each species. Although having similar unsaturated hydraulic conductivity values, the upper 5 cm of S. magellanicum retains ~20% less moisture under tension than S. fuscum and S. rubellum; in fact, S. rubellum on average retained slightly more water than S. fuscum. Hydrus‐1D was used to simulate daytime and nighttime conditions over a 7‐day period, where daily potential evaporation was 4 mm, to explore the governing mechanisms controlling water supply to the capitula. The simulations showed that S. fuscum and S. rubellum were able to retain sufficiently high moisture content under the prevailing simulated water demand to sustain surface soil‐water pressure heads (greater than −100 cm), whereas S. magellanicum could not prevent depressurization and the concomitant desiccation of its surface layer. A similar number of the same size pores were observed in all species; however, there was lower pore connectivity in S. magellanicum leading to the desiccation of the capitula. Contrary to previous studies, the results of this study indicate that it is not only soil‐water retention but also pore connectivity that allows hummock species to thrive above the water table. Copyright © 2012 John Wiley & Sons, Ltd.