The Effect of Control of Brushtail Possums (Trichosurus vulpecula) on Condition of a Southern Rata Kamahi (Metrosideros umbellata-Weinmannia racemosa) Forest Canopy in Westland, New Zealand

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. This suggested that control at or before populations attain peak density is critically important for limiting canopy mortality. After the 1974 poisoning, possum numbers in both blocks recovered to within 20% of their pre-control level in 10 years, indicating that control should be carried out at about decade intervals. Defoliation indices and species condition patterns showed that canopy dieback was least evident on relatively unpalatable trees. Of the palatable trees, kamahi was in best overall condition with least canopy dieback, followed by cedar, southern rata, then Hall's totara.