Alluvial grasslands in south-eastern New Zealand: vegetation patterns, long-term and post-pastoral change

Grasslands in alluvial systems are studied in 12 valleys and intermontane b asins in south-eastern New Zealand. Vegetation patterns and their environme ntal determinants are described overall, and in five different valley group s ("clusters"). Data from grazing-exclosures are used to examine effects of the cessation of grazing on vegetation, and long-term plots and management -induced contrasts at fencelines are used to examine grazing effects. Clima te (minimum temperature of the coldest month and rainfall) and landscape po sition are the determinants of the primary vegetation gradient. A secondary vegetation gradient represents vegetation modification (i.e., the degree o f exotic species dominance) in alluvial systems. Exclosures indicate post-p astoral decreases in exotic species dominance, in terms of abundance, in tw o alluvial sites, and increased exotic dominance in three alluvial sites. I n terms of species numbers, a decrease in exotic proportion is indicated at one site, and an increase is indicated at four sites. Differences between the exclosed and continually grazed vegetation in native species richness, and in the amount and proportion of native biomass, are negatively related to the biomass of exotic species present in the ungrazed vegetation, and ar e not related to the environmental characteristics of the sites. Historic d ata are insufficient to identify general trends in grazed alluvial grasslan ds. In three sites for which data are available, the dominance of exotic sp ecies had increased, and the proportional contributions of all native speci es, and of native grasses, had decreased with time. In terms of species num ber, however, proportions of native species had increased at two sites, and decreased at one site. Widespread and well established exotic grasses appe ar to have reached their potential environmental range in the alluvial syst ems studied, but the more recent invader, Hieracium pilosella, may not yet occupy its potential range. The practicability of post-pastoral conservatio n of native vegetation in alluvial systems might be indicated by the identi fication of modification thresholds. However, threshold identification requ ires data from a greater number of long-term plots than presently exists in southern alluvial grasslands.