Journal of the Royal Society of New Zealand abstracts
Alluvial grasslands in south-eastern New Zealand: vegetation
patterns, long-term and post-pastoral change
Susan Walker*, William G. Lee+
*Botany Department, University of Otago, P.O. Box 56, Dunedin, New Zealand,
email: walkers@landcare.cri.nz
+Landcare Research, Private Bag 1930, Dunedin, New
Zealand
Grasslands in alluvial systems are studied in 12 valleys and intermontane
basins in south-eastern New Zealand. Vegetation patterns and their
environmental determinants are described overall, and in five different valley
groups ("clusters"). Data from grazing-exclosures are used to examine effects
of the cessation of grazing on vegetation, and long-term plots and
management-induced contrasts at fencelines are used to examine grazing effects.
Climate (minimum temperature of the coldest month and rainfall) and landscape
position are the determinants of the primary vegetation gradient. A secondary
vegetation gradient represents vegetation modification (i.e., the degree of
exotic species dominance) in alluvial systems. Exclosures indicate
post-pastoral decreases in exotic species dominance, in terms of abundance, in
two alluvial sites, and increased exotic dominance in three alluvial sites. In
terms of species numbers, a decrease in exotic proportion is indicated at one
site, and an increase is indicated at four sites. Differences between the
exclosed and continually grazed vegetation in native species richness, and in
the amount and proportion of native biomass, are negatively related to the
biomass of exotic species present in the ungrazed vegetation, and are not
related to the environmental characteristics of the sites. Historic data are
insufficient to identify general trends in grazed alluvial grasslands. In three
sites for which data are available, the dominance of exotic species had
increased, and the proportional contributions of all native species, and of
native grasses, had decreased with time. In terms of species number, however,
proportions of native species had increased at two sites, and decreased at one
site. Widespread and well established exotic grasses appear to have reached
their potential environmental range in the alluvial systems studied, but the
more recent invader,
Hieracium pilosella, may not yet occupy its
potential range. The practicability of post-pastoral conservation of native
vegetation in alluvial systems might be indicated by the identification of
modification thresholds. However, threshold identification requires data from a
greater number of long-term plots than presently exists in southern alluvial
grasslands.
Keywords New Zealand grassland; invasion; alluvium;
vegetation pattern; vegetation change; grazing management; exclosure
(c) Journal of The Royal Society of New Zealand,
Volume 30, Number 1, March 2000, pp 72-103
PDF file of entire paper: medium quality (1996K); (scanned from paper original: notes about this process)
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