EFFECTS OF POSSUM BROWSING ON NORTHERN RATA, ORONGORONGO VALLEY, WELLINGTON, NEW-ZEALAND

Browse damage to northern rata (Metrosideros robusta) caused by brusht ail possums (Trichosurus vulpecula) was measured on 24 trees in the Or ongorongo Valley, near Wellington, in 1970-74. Fifteen of the same tre es were re-assessed annually for browse damage and defoliation in 1990 -94. Resurveying allowed a check on mortality since 1974 and an opport unity to assess the importance of natural fluctuations in possum densi ty and their impacts on northern rata. Since 1970, possum density has fluctuated between 6 and 12 possums ha(-1), the latter in 1990 being t he highest level since 1966. In 1990 all 21 trees surviving in 1974 we re still alive, and the 15 trees in this survey showed nil-to-light po ssum browse. By 1994, one tree had been wind damaged, 7 showed heavy b rowse damage and extensive defoliation, and 7 still showed only light browse. The extent of possum damage varied markedly between years, and from tree to tree, but overall damage levels increased progressively from 1990 to 1994 while possum density remained consistently above the long-term average. This study has reinforced Meads's (1976) findings that possums are the main cause of decline in numbers of northern rata , but does not confirm Meads's prediction of ongoing high mortality. T he high level of mortality in 1970-74 and the increasing levels of dam age seen in 1990-94 seem to result from a combination of periods of ab ove-average possum numbers and other stress factors such as drought.