DUNE SLACK VEGETATION IN SOUTHERN NEW-ZEALAND

A range of slack vegetation in southern New Zealand was described by d etailed sampling of four dune slacks, contrasting in topographic situa tion and in vegetation. Comparison is made with a slack previously sam pled on Stewart Island. The five slacks differed markedly in the plant communities present. One slack, where there was considerable peat acc umulation, was dominated by the megaherb Phormium tenax and the restia d Leptocarpus similis. In another, the peat was deep and had apparentl y accumulated over a long period; the vegetation contained the trunked sedge Carex secta and was similar in species composition to carr vege tation described from the region. These two slacks were predominantly native in species composition, though the European Erica lusitanica wa s a component at the second. The other two slack sites were on substra te largely comprising sand; the vegetation was shorter, and included a mixture of native and exotic species. Many of the exotic species foun d in these sites have been recorded in European slacks. Communities we re defined by Cluster analysis. With rare exceptions, each plant commu nity was specific to one or another of the five sites. Even the few co mmunity/site overlaps were in the vegetation of the surrounding dune a reas, not of the slack itself. The communities within a site generally formed discrete zones, related to small differences in elevation. The environment was characterised in terms of elevation, water table leve l, and soil salinity (chloridity), organic content, pH, physical textu re and fertility (assessed by bioassay). There were varying amounts of organic matter accumulation, but the mineral part of the substrate wa s almost pure sand throughout, except that at one site a stream had br ought in silt. In three of the sites, the mineral base was below high tide level. The water table fluctuated through the year, with the same pattern as described for Northern Hemisphere slacks. Chloridity was l ow, but varied through the year. Some features of the yearly variation could be related to weather events. It is concluded that slacks in th e area show considerable variation in vegetation, much of which can be correlated with peat accumulation.