THE SAND DUNE VEGETATION OF CHRYSTALLS BEACH, SOUTHERN NEW-ZEALAND, WITH PARTICULAR REFERENCE TO THE CUSHION COMMUNITY

The sand dune system at Chrystalls Beach, New Zealand comprises front dune, dune hollow and rear dune areas. On the front dunes are two comm unities: one on the seaward face of the dunes, and another on the land ward face. Most of the dune hollow is occupied by a series of grasslan d communities, but there are areas of a distinctive cushion community, still largely native in its species composition. It is suggested that the cushion community is maintained against invasion by a switch, pos sibly involving sand grain size, soil organic content, water supply, w ind and magnesium:calcium ratio. The coarse sandy/pebbly substrate, wi th its low organic content, causes water stress. The water stress perm its only low vegetation, which exacerbates the low soil organic conten t. The low vegetation results in high wind, again favouring cushion sp ecies. The water regime has also allowed a buildup of Mg:Ca ratio, esp ecially in the lower layers of the soil.