A study of plant succession at Lake Manapouri

Abstract

This study was undertaken to assess the ability of native plants to revegetate areas disturbed during the development of the Lake Manapouri Power Scheme. Description of the main trends in plant succession was complimented by observations on the growth and development of several prominent species.

Correlations of age, and above ground dry matter, with the basal diameter of wineberry, manuka, mountain beech and kamahi seedlings-were obtained, enabling prediction of these factors. The growth rates of wineberry and manuka were shown to be superior- to that of mountain beech. The growth responses of species to nitrolime and sulphur superphosphate application were recorded in the field, and in soils from the Lake Manapouri area used in glasshouse experiments. Wineberry seedling growth was improved by the application of nitrolime, as was that of silver beech. Mountain beech seedlings also responded positively to nitrolime, as well as to sulphur superphosphate 'initially. The growth of tree tutu is likely to be markedly affected by soil pH; the yellow-brown loam soil with a pH of 6.0 allowed superior growth compared to that in the podzolised yellow-brown earth having a pH of 4.4. At the lower pH, both seedling survival and nodulation were also reduced. An overall germination rate of 16.7% was obtained for tree tutu seed, though fertiliser application reduced this proportion. The application of sulphur superphosphate improved the growth of tree tutu seedlings, while nitrolime appeared not to suppress root nodulation.

An ordination of 70 successional communities was completed using a modified Bray and Curtis technique. This was less informative than hoped, most likely because of the inadequacy of the field data in the event of wide site variation. Other unsatisfactory features were the inclusion of many species present in only a few stands, and the employment of a faulty endstand selection technique. However, by an ordering of the stands along the primary axis, the ordination- confirmed the presence of a successional continuum from pioneer plants to mature mountain beech-podocarp forests. A slope-particle size complex was also identified as influencing- community development. This is most likely to be through the determination of drainage, root space and aeration, and highlights the reciprocal relations of manuka and wineberry. The latter species is most prominent on sites with excellent drainage and root space, on steep rubble cones. Though not outstandingly successful, the ordination was shown to be useful in studying species relationships and their importance throughout phases of succession.

The successional communities studied were subjectively placed in broad categories according to basic site classifications. Manuka is the dominant seral species on alluvial delta sites, progressing to tall podocarp and beech-podocarp forest within 100 years of lowering of water levels. Manuka is also important in beech sand and gravel successions. With an improvement in conditions, such as the lowering of storm water levels, mountain beech can then establish. Stream fans have a ready source of available tree seeds, and readily develop towards the surrounding mature forest type, though manuka may be an important early component. In secondary successions where the previous- forest- has-been felled, the path towards the surrounding mature forest type is aided by the presence of forest seedlings, a good seed source, and an initial forest floor cover. Following fire, the secondary, succession passes through a manuka phase before seeds of the surrounding forest establish. The extremely variable nature of the steep debris avalanches characteristic of much of the area, prevented the determination of distinct patterns of development apart from the location of wineberry dominant communities in areas of good drainage and root aeration.

The vigorous and diverse plant successions on sites of natural disturbance at Lake Manapouri, indicates that most man-made-surfaces will return to native forest through herb and shrub phases within a century. The purposeful introduction of N-fixing tutu, and in places other species, would do much to provide an ecologically valid improvement in the rate and appearance of plant succession on most of the disturbed ground not naturally revegetating satisfactorily.