KEY ENVIRONMENTAL VARIABLES DETERMINING THE OCCURRENCE AND LIFE-SPAN OF BASIPHILOUS DUNE SLACK VEGETATION

Environmental processes controlling the occurrence of basiphilous pion eer vegetation were identified in seven representative dune slacks on the Dutch Wadden Sea Islands. The variation in vegetation and relation s with soil and groundwater composition were established first. Cluste r analysis of the vegetation in the dune slacks resulted in six subtyp es of basiphilous pioneer vegetation and three types representing olde r succession stages. Canonical correspondence analysis suggested that moisture and pH are the main habitat factors in the dataset and that h abitats of older succession stages, compared to basiphilous pioneer st ages, are either acidified or eutrophicated. A principal component ana lysis 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, co nsidered key variables in describing the environmental processes deter mining suitable habitat conditions for basiphilous pioneer vegetation. To be able to distinguish between a conditioning role of tl-le hydrol ogical regime (c.q. exfiltration of mineral rich groundwater) and a co nditioning role of (former) geomorphological processes (c.q. the prese nce of CaCO3 minerals), the soil CaCO3 content was added as key enviro nmental variable. In primary dune slacks on the islands of Schiermonni koog 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 vegetat ion varies considerably with differences in local circumstances, espec ially in strength of prevailing pH-buffering mechanisms. These, in tur n, are determined by differences in environmental conditions on a land scape scale (hydrology, geomorphological history).