Mires and Peat (Apr 2007)
The sensitivity of Sphagnum to surface layer conditions in a re-wetted bog: a simulation study of water stress
Abstract
The behaviour of the water table in re-wetted bogs varies widely between different locations so that recolonising Sphagnum is vulnerable to water stress, especially when the water table is drawn down in summer. It is important to understand how physical site conditions influence the occurrence of water stress so that adequate management measures may be applied. In the work reported here, the respective roles of the hydrophysical properties of the uppermost peat layer and micro-scale site conditions are investigated using a Soil-Water-Atmosphere-Plant (SWAP) model, which simulates water table fluctuations and soil moisture conditions. The variables are: (a) cover and thickness of the Sphagnum layer, (b) microtopography (presence of open water), (c) hydrophysical properties of the uppermost soil layer and (d) rate of downward seepage. Data for the model are derived from field observations, from published literature, and from laboratory determinations of moisture characteristic curves and saturated and unsaturated hydraulic conductivity (k–h–Θ relationships) for peat. The simulation indicates that microtopography and the thickness of the moss layer are the dominant factors affecting groundwater behaviour and the risk of water stress. Sphagnum layers a few centimetres thick should be relatively well supplied with water from the underlying peat but as the Sphagnum carpet thickens, water movement through the unsaturated zone to the growing capitula will become increasingly difficult. Sphagnum layers appear to be most vulnerable to water stress when they are 5–15 cm thick. Beyond this thickness, water stored within the Sphagnum layer itself begins to offset the decline in the flux from below, and thus to reduce the dependence of the water supply to the stem tips on the maintenance of hydraulic continuity with the water table. The results obtained using the model underline the close interdependence between Sphagnum development and the accompanying changes in soil hydrophysics during the re-wetting phases of bog restoration projects.
