Facets of the distribution of forest vegetation in New Zealand

Abstract

The altitudinal sequences of vegetation in New Zealand is divided into the following belts, based on floristics (especially the upper limits of species), physiognomy, and broad climatic relationships; warm temperature, cool temperature or montane, subalpine, lower alpine, high alpine, and nival. The boundaries are somewhat arbitrary, except for the subalpine/alpine boundary, which marks the upper limits of trees and large shrubs. The presence or absence of Nothofagus can give very different physiognomy to forest which exist under the same climatic regime, and which are otherwise floristically similar. The competitive ability and abundance of this genus, relative to other components of forest vegetation, increase along environmental gradients that lead away from the moist, mild, fertile “optimum” and in most places where Nothofagus borders beech-free vegetation, it tends to invade the latter. Absence of beech from climatically suitable areas can be related to Pleistocene glaciation, but the rate of re-invasion appears to be slow where the present climate is wet and foggy. Under a combination of high rainfall and leached soils with impervious horizons, “normal” forest is replaced by stunted vegetation, made up of four components: i.e., stunted representatives of the “normal” forest at the same altitude, species from higher altitudes, species exclusive to the habitat, and species characteristic of drier parts of New Zealnad. Another example of disturbance of the usual altitudinal sequence is seen where communities and species with subalpine affinities occupy the floors of mountain valleys. The distribution of forest species across the South Island reveals progressive impoverishment from the west coast towards the interior, and reapperance of many species along the east coast. In particular, no species that are restricted to the warm temperate belt penetrate far from the coast. This pattern reflect, in addition to rainfall, such factors as winter cold, humidity, soil fertility and past fires. The only features that suggest control by summer warmth are the altitude of the tree limit, and the distribution of those species which are most common in the North Island and in the east of the South Island.