rata-kamahi forest

Chamois (Rupicapra rupicapra) are usually considered an alpine species in New Zealand, but also occur in forests in areas such as Westland. A postal survey of commercial helicopter-based hunters indicated that chamois are present within Westland forests from timberline to sea level and are most abundant within an area of about 1600 km(2) extending from the Wanganui River in the north to the Karangarua River to the south. The diet of 40 chamois shot in spring and summer was determined by analysis of rumen contents.

The amount of conspicuous canopy dieback in all central Westland southern rata-kamahi forests east of the Alpine Fault, between 500 m altitude and treeline, was assessed and mapped from aerial photographs taken in 1984-85 and verified by aerial reconnaissance of selected areas in 1988. At least 20% of all canopy trees, predominantly southern rata (Metrosideros umbellata) and Hall's totara (Podocarpus hallii), were dead in 1984-85.

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.

An estimate of intrinsic rate of increase (r(m)) of a brushtail possum (Trichosurus vulpecula) population was calculated from the measured increase in possum numbers after a poisoning operation in Westland rata/kamahi forest. Our empirical estimate of r(m) (0.22—0.25) was lower than published estimates for this species (0.30, 0.34, 0.59). Consequently, the control operation was effective for longer than predicted by population models that used these published values.

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.