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

Abstract

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.