Modelling the ecohydrological plasticity in soil hydraulic properties of Sphagnum mosses

Sphagnum mosses are a keystone peatland species whose ecohydrology governs carbon sequestration processes in many peatlands. Globally, there are similar to 380 Sphagnum species that occupy a wide range of ecohydrological niches (microforms) based on their ability to grow at or above the water table, broadly grouped by hummock (furthest from water table), lawn, and hollow (closest to water table) microforms. The further from the water table a given species can grow is controlled by the ability to effectively retain and transmit water to the capitula (growing surface) during dry periods. However, Sphagnum species can have a relatively plastic ecohydrological niche, often occupying different niches (microforms) in different environments. We used numerical modelling parameterized by previous field and laboratory studies to compare the hydrological function between Sphagnum hummock, lawn, and hollow microforms. We determined (a) how two different organizations of a hummock of Sphagnum fuscum and (b) a lawn or hollow of S. magellanicum (S. divinum/S. medium) or S. rubellum differed between two different overarching climates (sub-humid boreal and humid temperate). The hydrological function, expressed as the cumulative water fluxes, was similar between species and ecohydrological microform (water table position) when water was plentiful, despite differences in soil hydraulic properties of the same species, but began to diverge during a prolonged simulated dry period (30-day drought). These results suggest a single species of Sphagnum moss can exhibit a wide range of soil hydraulic properties (i.e., sphagnum morphology) but have essentially the same consequential hydrology. Only the S. fuscum from the sub-humid climate was hydrologically stressed enough to show differences in the simulated evaporation rates. This study highlights the need for more physical research to determine the sensitivity of Sphagnum spp.'s soil hydraulic properties to overarching hydroclimatic factors so that we can more effectively incorporate these processes into large-scale numerical modelling efforts.