Vegetation change over 25 years in a New Zealand short-tussock grassland: Effects of sheep grazing and exotic invasions

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. The effects of different histories of sheep-grazing were examined on the transects and on 174 quadrats established in 1988. Although prolonged grazing generally promoted decline in native species and invasion by exotic species, including H. lepidulum, these trends also developed on sites protected from grazing for 22 or 35 years. There was no evidence that the rate or extent of invasion by Hieracium pilosella was enhanced by continued sheep grazing, or that removal of grazing prevented invasion. Two common hypotheses seek to explain Hieracium success as either a symptom of ecosystem depletion or an example of an aggressive invader. However, neither hypothesis alone adequately accounts for the observed patterns of Hieracium invasion. Such single-factor explanations fail to account for interactions between the many mechanisms that affect plant populations at different spatial and temporal scales. Similarly, single-factor prescriptions for preventing or controlling Hieracium invasion, such as the removal of grazing, may not provide widespread success.