IMPACT OF INVADING BRUSHTAIL POSSUM POPULATIONS ON MIXED BEECH-BROADLEAVED FORESTS, SOUTH WESTLAND, NEW-ZEALAND

The impact of browsing by introduced brushtail possums on mixed beech - broadleaved forests in South Westland was estimated from the amount of conspicuous canopy dieback present in 1989-1990. Aerial and ground- based reconnaissance in all catchments indicated most canopies (84%) w ere intact. The remaining 16% of canopies were affected by conspicuous dieback, principally of southern rata (Metrosideros umbellata) and/or fuchsia (Fuchsia excorticata). Major dieback nuclei were located in t he three areas with the longest history of browsing by possums, which had spread from three known liberation centres. At each dieback nucleu s, the amount of dieback reflected the abundance of possum-preferred c anopy species. Because South Westland forests contain lower proportion s of such species, they are less susceptible to dieback than the conif er-broadleaved forests of central Westland. However, the present low a mounts of dieback in South Westland mainly reflect low overall possum densities and a short period of occupation. The occurrence of key poss um-preferred species indicates that about one-third of the forests cou ld develop conspicuous canopy dieback if possum numbers increase and 4 4-94% are susceptible to canopy and/or understorey depletion. By 1992, the few areas selected for sustained possum control effort in Westlan d under-represented the range of forest composition. However, recently boosted funding for possum control has provided the opportunity to pr otect representative forest tracts before the onset of widespread ecos ystem depletion in South Westland.