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The vegetation of an area of the Upper Clutha basin, New Zealand, with a ’semi-arid’ climate, was sampled with 95 quadrats in a nested randomised design. All types of vegetation were sampled, from near-natural to managed pastures. Twenty four environmental factors were measured in each quadrat.

Five ’formations’ are described, and 14 ’communities’ recognised within them, although there are few constant or faithful species. Such weak structure, and relatively weak correlation with the environment, are partly attributed to non-equilibrium. All formations, and many of the communities, are scattered over the area. The most important environmental factors in determining the vegetation are latitude, elevation, soil fertility (especially sulphate) and water.

All sites contain exotic species; some contain only exotics. Correlation between the sizes of the native and exotic guilds gave no evidence that natives and exotics were competing for niches. The lowest proportion of exotics is at higher elevations and on steeper slopes. Analyses showed a strong gradient from near-natural vegetation to managed pastures, but with no discontinuity. Agricultural communities, especially lucerne fields, include species typical of unmanaged sites in similar conditions.