Dune Slack Vegetation in Southern New Zealand

A range of slack vegetation in southern New Zealand was described by detailed sampling of four dune slacks, contrasting in topographic situation and in vegetation. Comparison is made with a slack previously sampled on Stewart Island. The five slacks differed markedly in the plant communities present. One slack, where there was considerable peat accumulation, was dominated by the megaherb Phormium tenax and the restiad Leptocarpus similis. In another, the peat was deep and had apparently accumulated over a long period; the vegetation contained the trunked sedge Carex secta and was similar in species composition to carr vegetation described from the region. These two slacks were predominantly native in species composition, though the European Erica lusitanica was a component at the second. The other two slack sites were on substrate largely comprising sand; the vegetation was shorter, and included a mixture of native and exotic species. Many of the exotic species found in these sites have been recorded in European slacks. Communities were defined by Cluster analysis. With rare exceptions, each plant community was specific to one or another of the five sites. Even the few community/site overlaps were in the vegetation of the surrounding dune areas, 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 level, and soil salinity (chloridity), organic content, pH, physical texture and fertility (assessed by bioassay). There were varying amounts of organic matter accumulation, but the mineral part of the substrate was almost pure sand throughout, except that at one site a stream had brought 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 low, but varied through the year. Some features of the yearly variation could be related to weather events. It is concluded that slacks in the area show considerable variation in vegetation, much of which can be correlated with peat accumulation.