An ecosystem model approach to sustainable pastoral use in the South Island high country, New Zealand

Abstract

A spatial and dynamic evaluation of the vegetation of The Grampians station, Mackenzie Country, New Zealand was carried out to interpret changes in grassland composition and the increase in Hieracium from an ecosystem perspective.

Based on a TWINSPAN classification of 203 species from the Investigated 175 sites, 12 different communities were identified. These communities represent a series of stages from unmodified tall tussock grasslands through to highly modified short tussock grasslands and weed communities. Each stage is recognised by changes in floristic composition and physiognomy. Such stages are defined ecologically. The diversity of native species is shown to be closely associated with tall tussock grasslands (Chionochloa spp.).

Two hundred and fifty one species and nine environmental variables from the 175 sites were also interrelated in a direct unimodal ordination (canonical correspondence analysis). Ordination results are interpreted with respect to changes in community structure and composition. Variations in species distribution were highly correlated with an altitude and soil depth gradient. Degenerative changes in grassland composition and physiognomy are diagnostic of successional processes in grassland ecosystems. Most modification has occurred in the montane zone. Unimproved short tussock grasslands are now closely associated with the dominance of weed species; notably Hieracium. The increase in Hieracium is seen as a further stage in a grassland succession after the conversion of tall tussock to short tussock grasslands. A distinct trend between the decrease in inter-tussock species diversity and density and the increase in Hieracium is shown.

A successional trend towards post-Hieracium communities discussed in earlier papers are now evident In this ecosystem. The decrease in the abundance of Hieracium and the increase in the amount of bare ground is interpreted as relating to increasing degradation. The role of Anthoxanthum odoratum and Rumex acetosella in revegetating severely depleted sites is discussed.

The implications of these findings for pastoral management purposes are discussed.