Changes in the Structure and Composition of a New Zealand Lowland Forest Inhabited by Brushtail Possums

All specimens of 19 tree and 11 shrub species greater than 10 cm dbh (more than 3000 stems of trees and tree ferns) were labeled, measured, and mapped in 2.25 ha of lowland forest near Wellington, New Zealand. Their fate, growth, and additional recruitment were monitored in three surveys over 16 yr, from 1969 to 1985. During the vegetation study, movements, diet, density, and breeding success of the introduced Australian brushtail possum (Trichosurus vulpecula) were studied in the same area of forest. In the study plot the number of stems and their total basal area increased between 1969 and 1985. However, several species that are eaten by possums have suffered substantial losses of both stems and total basal area. These include Beilschmiedia tawa, Weinmannia racemosa, Metrosideros robusta, and the tree fern Cyathea medullaris. Species not eaten by possums have increased in both numbers and basal area. These include Hedycarya arborea, Cyathea smithii, Cyathea dealbata, and Laurelia novaezelandiae. During the study there has been a decline in basal area of emergent trees, an increase in basal area of canopy trees (but little increase in their numbers), and an increase in numbers and basal area of minor species and dead trees. If present trends in structure and composition of this lowland forest continue, the future forest will have a greater proportion of tree ferns and more short-lived, small-diameter species. Canopy height and species diversity are also likely to decrease. Wellington, in 1893 and 1894 (Pracy 1962). If they spread from these catchments at a rate similar to that of possums in the Fox catchment in the South Island (0.8-1.6 km/yr [Pekelharing and Reynolds 1983]), they could have become numerous in the Orongorongo Valley between 1920 and 1930. Possums were completely protected in the Wellington Acclimatization District up to 1916 so that their numbers could build up, but from 1921 they were sufficiently common in the Orongorongo Valley for the New Zealand Forest Service to trap them. In 1946-1947, after possum numbers peaked, 1024 possums were killed in just I yr from a strip of about 230 acres (93 ha) (Mason 1958). Part of that strip is now included in a Department of Scientific and Industrial Research (DSIR) area for research into forest ecology (Campbell 1984). The density of possums is still about 9-12/ha (Brockie 1982) and has apparently changed little since the early 1940s (M. G. Efford, pers. 277 1 Manuscript accepted August 1989. 2 Ecology Division, Department of Scientific and Industrial Research, P.O. Box 30379, Lower Hutt, New Zealand. POSSUMS (Trichosurus vulpecula) were introduced to New Zealand from Australia during the 1850s and thereafter were liberated extensively to establish a fur industry. They are herbivorous and have adapted to eating a wide range of plants in New Zealand, although clearly feeding on some plant species more than others (Pracy and Kean 1949, Kean and Pracy 1953, Mason 1958, Kean 1959, Fitzgerald 1976). Continued browsing defoliates and ultimately kills many of their food trees (Meads 1976, Fitzgerald 1978, Batcheler 1983), with serious consequences for the composition and structure of native forests. Possums were introduced to catchments adjacent to the Orongorongo Valley, near