Wardle - Environment and N.Z. vegetation

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Home < Publications < Our Journals < New Zealand Journal of Botany < 1985

New Zealand Journal of Botany abstracts

Environmental influences on the vegetation- of New Zealand

P. WARDLE

Botany Division, DSIR
Private Bag, Christchurch, New Zealand

Abstract This review develops three themes, the first of which assesses how far the environment is reflected in the form and functioning of native plants. It is shown that the mild New Zealand win- ters favour evergreen trees with limited cold-tol- erance, as well as life-forms such as cushion plants and tussock grasses. Shrubs and juvenile trees, with small leaves and slender, divaricating branchlets, also characteristic of New Zealand, probably resulted from the combined selection pressures of browsing, drought, and wind on a floristic pool deficient in the kinds of plants that resist these pressures in other parts of the world. Slow growth of some native trees adapts them to infertile soils. The tendency of many species to flower at three- yearly or longer intervals, is also discussed. Next, the adjustment of native vegetation to the existing environment is considered. For the South Island, J. T. Holloway developed an hypothesis of recent climatic change, based on anomalous dis- tribution patterns in the beeches, disappearance of forest from rain-shadow districts during the last 1 000 years, and widespread failure of native co- nifers to regenerate. Reassessment of this hypoth- esis suggests that the beech distribution patterns were initiated not later than the end of the last gla- ciation, that fire is a sufficient explanation of the forest destruction, and that cessation of browsing by moas may have allowed fast-growing palatable species to suppress native conifer seedlings. The recovery of native vegetation after numbers of feral herbivores were reduced is contrasted with the continuing inroads through agriculture and the spread of introduced plants. The short tussock grasslands, especially, consist largely of introduced species, and bear little resemblance to grasslands existing at the time of European colonisation, which in turn mostly originated after burning of forest. The final theme considers whether the isolation of New Zealand has led to a flora unable to ade- quately fill the habitats available, so that there is room for invasion by better-adapted adventive species. The forest, despite de<" mation of its flora during glacial periods, resists invasion by adven- tives, except in fringes and remnants. In the sub- alpine and alpine zones, a rich flora of shrubs and herbs has evolved, but there is a niche for hardy introduced pines such as Pinus contorta. The rain- shadow regions have evoked much less speciation than the high mountains, and the open landscapes are highly vulnerable to invasion by introduced plants, including deciduous and evergreen trees. Gravel flood plains now support an almost totally adventive vegetation. The review concludes by presenting options for native vegetation, which range from management for protection, through laissez-faire, to exploitation and replacement by introduced species deemed to be useful.

Keywords environment; growth forms; adap- tation; growth rates; flowering; regeneration; biotic changes; moas; land clearance; introduced species; management options

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