To assess the effect of possum browse on plant growth, an index of the amount of foliage on about 50 trees of Fuchsia excorticata and the number of trees that died or were completely defoliated was measured at five sites in South Westland over 5 years. This index was compared to possum density indices taken at each site each year. At one site, possums were reduced from a high density about 6 months before the final measurement. The degree of defoliation of fuchsia was significantly related to the density of possums at each site.
Brushtail possums began colonising a rata/kamahi forest in the Taramakau catchment, Westland, about 1950 and by 1973 had caused widespread conspicuous canopy defoliation. They were poisoned in one block of this forest in 1970, at about the time they reached peak density, and again in 1974. In an adjacent block they were poisoned in 1974 only. A survey of forest canopy condition in 1985 showed that, in the block poisoned at peak density, 21% of the basal area of palatable trees had died compared with 47% in the block where poisoning was deferred for 4 years.