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.
Increased dieback in Westland rata (Metrosideros umbellata)-kamahi (Weinmannia racemosa) forests has been linked to the build-up of populations of the Australian brush-tailed possum (Trichosurus vulpecula). Within these forests young even-aged stands are observed to be more resilient to dieback than older stands. The effect of possum browsing on individual rata trees was related to the level of defoliation. Trees which had been not or only lightly browsed maintained intact canopies.
High deer numbers in northern Fiordland in the 1960s significantly changed forest understorey composition. The density of woody plants in the understorey was reduced in some areas by as much as 50%, and preferred plants became less abundant than those seldom eaten. However, the impact of deer and wapiti varied between forest types.
A significant recovery of food plants preferred by introduced deer (Cervus elaphus) occurred between 1969 and 1984 on 57 permanent plots in the alpine grasslands of northern Fiordland. During this period the deer population was reduced markedly by hunters operating from helicopters.
Forest types of the Wapiti, Doon and Glaisnock catchments, ranked in order of proportion of preferred food species for deer, paralleled a gradient of landform stability. Seral forests and low altitude silver beech forests were preferred deer habitat because they contained the largest proportions of highly preferred species. They often occurred on unstable landforms such as debris cones, colluvial sideslopes, and terraces with recent and compound soils, assumed to be of high nutrient status.