Modelling Sphagnum moisture stress in response to projected 21st-century climate change

Sphagnum moss is an important genus of plants responsible for large stores of soil organic carbon associated with wet habitats, such as northern peatlands. Northern peatlands, which contain globally significant quantities of legacy carbon, may be vulnerable to enhanced summer moisture deficits due to climate change. We adapted HYDRUS-1D and a semi-arid soil-moisture model to investigate the role of microtopographic position and depth dependence of peat hydraulic properties on Sphagnum moisture-stress response to current and projected climate conditions in an idealized northern Michigan peatland. Water table (WT) level was shown to have a strong control on pore-water pressure () and thus on Sphagnum moisture stress. As a result of the close correspondence between surface peat hydraulic properties for hummocks and hollows used to parameterize our model, the microtopographic position was shown to have a greater impact on Sphagnum moisture stress. Model behaviour suggests that, while maintains equilibrium-profile values relative to the WT level for relatively shallow values, surface becomes nonlinearly related to the WT level below a value of approximately -0.4 m, thus, greatly increasing the likelihood of desiccation under future climate scenarios, where growing-season soil-moisture deficits are projected to increase. The simulated range of instantaneous and cumulative moisture stress for hollows under future climate conditions closely corresponds to the contemporary range exhibited by hummocks. Therefore, in order to assess the competitive advantage of various Sphagnum species to future climate conditions, we argue that more data are needed to better inform a physiological psi-based moisture-stress threshold, the evolution of the stress response to increasing levels of desiccation and its subsequent recovery dynamics. Copyright (c) 2015 John Wiley & Sons, Ltd.