Supporting dune management by quantitative estimation of evapotranspiration

Research was conducted in the nature reserve De Westhoek (B) in order to estimate the hydrological impact of shrub removal in favour of the recolonisation and development of herbaceous vegetation types in the dune slacks. Dune slacks are one of the most rare ecotopes in Europe. Therefore, the evapotranspiration of herbaceous and shrub vegetation types was estimated based on experimentally obtained data and modelling. Analysis of the experimentally obtained stomatal resistance values revealed that there was no difference in the minimal stomatal resistance values (in absence of any stress) between herbs and shrubs. Stomatal resistance was modelled based as a function of climatic variables. Maximal rooting depth was similar in both vegetation types, and was maximal 60cm. For both vegetation types more than 60% of the roots were found in the upper 15cm. The mean leaf-area-index (LAI) of herbaceous and shrub vegetations is respectively 2.11±0.34 and 3.27±0.20 m².m-². Evapotranspiration of both vegetation types was modelled with a multi-layer dynamic vegetation model FORUG and seasonal evapotranspiration amounted roughly 200 and 550mm for the herbaceous and shrub vegetation types respectively. Although these estimates can be somewhat refined, from these results it can be concluded that shrub removal, and the replacement of this vegetation type by a herbaceous vegetation type, will not result in a lowering of the groundwater table. This knowledge can help managing hydrologically disturbed dune ecosystems.