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-zeland ine in the Harper-Avoca catchment, Canterbury. These were remeasured a t 5 or 10 year intervals between 1965 and 1990. Change was widespread. It was characterised by invasions by exotic species, declines in nati ve species (including F. novae-zelandine), and a trend towards vegetat ion dominated by the flatweeds Hieracium lepidulum and H. pilosella, a nd the grass Agrostis capillaris. The effects of different histories o f sheep-grazing were examined on the transects and on 174 quadrats est ablished in 1988. Although prolonged grazing generally promoted declin e in native species and invasion by exotic species, including H. lepid ulum, these trends also developed on sites protected from grazing for 22 or 35 years. There was no evidence that the rate or extent of invas ion by Hieracium pilosella was enhanced by continued sheep grazing, or that removal of grazing prevented invasion. Two common hypotheses see k to explain Hieracium success as either a symptom of ecosystem deplet ion or an example of an aggressive invader. However, neither hypothesi s alone adequately accounts for the observed patterns of Hieracium inv asion. Such single-factor explanations fail to account for interaction s between the many mechanisms that affect plant populations at differe nt 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.