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 communities: one on the seaward face of the dunes, and another on the landward face. Most of the dune hollow is occupied by a series of grassland 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, possibly involving sand grain size, soil organic content, water supply, wind and magnesium:calcium ratio. The coarse sandy/pebbly substrate, with its low organic content, causes water stress. The water stress permits only low vegetation, which exacerbates the low soil organic content. The low vegetation results in high wind, again favouring cushion species. The water regime has also allowed a buildup of Mg:Ca ratio, especially in the lower layers of the soil.