Groundwater variables and vegetation in dune slacks

Basiphilous pioneer plant communities, influenced by phreatic groundwater, are endangered in The Netherlands. In dune slacks of the Dutch Wadden Sea islands, their area has also declined, though they still occur fairly frequently. For purposes of nature conservancy it is important to know how to maintain or restore suitable habitats. The aim of this investigation is to identify variables which describe the hydrological conditions required by these dune slack communities adequately without being easily disturbed by climatological fluctuations. Data sets of groundwater table depths from 38 sites in 16 dune slacks were analysed. Plots of site-specific seepage fluxes versus groundwater table depths were inferred which served well in modelling the hydrological regimes of all sites on the basis of a given start value for table depths and subsequent precipitation surplus data. Three groundwater variables describing the plots to a large extent were derived. To infer the physical meaning of these new variables they were related to three conventional hydrological variables (mean lowest groundwater level (MLG), inundation percentage and daily fluctuation) by applying a principal component analysis. The new variables could easily be interpreted in terms of known hydrological features and two of them could describe the hydrological variation adequately. The relationships of these two site-specific groundwater variables to abiotic habitat features and vegetation composition were assessed. Together they very well indicated the presence of basiphilous pioneer vegetation. Also they related clearly to favourable abiotic conditions for this vegetation. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.