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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 basins in south‐eastern New Zealand Vegetation patterns and their environmental determinants are described overall, and in five different valley groups ("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 fencehnes are used to examine grazing effects Climate (minimum temperature of the coldest month and rainfall) and landscape position are the determinants of the primary vegetation gradient A secondary vegetation gradient represents vegetation modification (i e, the degree of exotic species dominance) in alluvial systems Exclosures indicate post‐pastoral decreases in exotic species dominance, in terms of abundance, in two alluvial sites, and increased exotic dominance in three alluvial sites In 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 btomass, are negatively related to the biomass of exotic species present in the ungrazed vegetation, and are not related to the environmental characteristics of the sites Historic data are insufficient to identify general trends in grazed alluvial grasslands In three sites for which data are available, the dominance of exotic species had increased, and the proportional contributions of all native species, and of native grasses, had decreased with time In terms of species number, however, proportions of native species had increased at two sites, and decreased at one site Widespread and well established exotic grasses appear to have reached their potential environmental range in the alluvial systems studied, but the more recent invader, Hieracium pilosella, may not yet occupy its potential range The practicability of post‐pastoral conservation of native vegetation in alluvial systems might be indicated by the identification of modification thresholds However, threshold identification requires data from a greater number of long‐term plots than presently exists in southern alluvial grasslands