vegetation change

Natural grasslands are among the most threatened biomes on Earth. They are under pressure from land cover change including afforestation, farming intensification, invasive species, altered fire regimes, and soil amendments, all of which impact native biodiversity and ecosystem functioning. In Aotearoa New Zealand, tussock-dominated native grasslands expanded due to increased fire activity during waves of human settlement. These areas have subsequently been maintained as modified grasslands by agricultural pastoral land management practices and effects of introduced feral mammals.

We studied vegetation change on 142 permanently marked transects spread throughout tussock grasslands of Otago and Canterbury, in areas subject to both pastoral and conservation management. The transects were established between 1982 and 1986 and remeasured between 1993 and 1999, providing a record of vegetation change at each site over an interval varying from 10 to 15 years. Each transect consisted of 50 quadrats, each 0.25m(2), in which the presence of all vascular plant species had been recorded.

Vegetation changes were investigated on 27 transects in agriculturally unimproved short tussock grasslands dominated by Festuca novae-zelandine in the Harper-Avoca catchment, Canterbury. These were remeasured at 5 or 10 year intervals between 1965 and 1990. Change was widespread. It was characterised by invasions by exotic species, declines in native species (including F. novae-zelandine), and a trend towards vegetation dominated by the flatweeds Hieracium lepidulum and H. pilosella, and the grass Agrostis capillaris.

Polynesian settlement of New Zealand (c. 1000 yr B.P.) led directly to the extinction or reduction of much of the vertebrate fauna, destruction of half of the lowland and montane forests, and widespread soil erosion. The climate and natural vegetation changed over the same time but had negligible effects on the fauna compared with the impact of settlement. The most severe modification occurred between 750 and 500 years ago, when a rapidly increasing human population, over-exploited animal populations and used fire to clear the land.

Species abundance, species richness, and ground cover were measured over 10 years on nine paired grazed and exclosure plots in short-tussock grassland in the early stages of invasion by Hieracium species. With and without grazing, H. pilosella and H. caespitosum increased markedly and H. lepidulum increased locally. In contrast, 50% of all other common species and species groups, and total, native, and exotic species richness declined significantly.