Key environmental variables determining the occurrence and life span of basiphilous dune slack vegetation

Environmental processes controlling the occurrence of basiphilous pioneer vegetation were identified in seven representative dune slacks on the Dutch Wadden Sea Islands. The variation in vegetation and relations with soil and groundwater composition were established first. Cluster analysis of the vegetation in the dune slacks resulted in six subtypes of basiphilous pioneer vegetation and three types representing older succession stages. Canonical correspondence analysis suggested that moisture and pH are the main habitat factors in the dataset and that habitats of older succession stages, compared to basiphilous pioneer stages, are either acidified or eutrophicated. A principal component analysis of shallow groundwater samples from all dune slacks revealed the dominant influence of inundation with sea water in the total dataset. In the freshwater dataset Ca2+ and HCO3 concentrations predominated. Cl- and Ca2+ concentrations of shallow groundwater were, therefore, considered key variables in describing the environmental processes determining suitable habitat conditions for basiphilous pioneer vegetation. To be able to distinguish between a conditioning role of tl-le hydrological regime (c.q. exfiltration of mineral rich groundwater) and a conditioning role of (former) geomorphological processes (c.q. the presence of CaCO3 minerals), the soil CaCO3 content was added as key environmental variable. In primary dune slacks on the islands of Schiermonnikoog and Texel, having comparable initial lime contents (0.5-1%), the life span of basiphilous pioneer vegetation was estimated to be 30-50 years without a mowing regime and 100-150 years under a mowing regime. In secondary dune slacks the life span of basiphilous pioneer vegetation varies considerably with differences in local circumstances, especially in strength of prevailing pH-buffering mechanisms. These, in turn, are determined by differences in environmental conditions on a landscape scale (hydrology, geomorphological history).