FACTORS PREDISPOSING FORESTS TO CANOPY COLLAPSE IN THE SOUTHERN RUAHINE RANGE, NEW-ZEALAND

The introduced brushtail possum Trichosurus vulpecula Kerr is recognis ed as the primary agent of defoliation and stand-level dieback in New Zealand broadleaved forests (except Nothofagus). The distribution and magnitude of canopy collapse of forest in 33 500 ha of the southern Ru ahine Range, New Zealand was mapped from 1995 polychrome aerial photog raphs. Relationships between canopy collapse and forest type, altitude , aspect and slope were analysed using generalised additive models. Ca nopy composition was the strongest factor predicting the extent of col lapse and modification: broadleaved-conifer forest is most affected, w ith 68-87% of the area of six such forest types replaced by scrub-low- forest and tree-fernland; and surprisingly, three Nothofagus-dominated types have up to 28% of their former area now in shrubland or tussock grassland. The susceptibility to collapse of Nothofagus forest was po sitively correlated with distance to non-Nothofagus forest types. Suba lpine scrub, which is dominated by species not favoured by possums, ha s increased in area by 32%, replacing former upper montane forest, Phy siographic factors were much less important in multiple regressions. H owever, in general terms, forests on steeper slopes, in the upper mont ane-subalpine zone, and on warm westerly and northerly aspects were mo re susceptible to collapse and modification than elsewhere. It is post ulated that possums were the primary agent responsible for collapse of non-Nothofagus broadleaved forest, and that red deer Cervus elaphus a nd goats Capra hircus were responsible for inhibiting canopy replaceme nt by eliminating regeneration in forest understoreys. Secondary effec ts, such as outbreaks of defoliating insects and mechanical damage fro m wind also contributed to canopy, collapse and were possibly triggere d by possums opening up the canopy. Collapse of Nothofagus canopies po ssibly results from a breakdown in the recruitment phase of stand turn over, by the smothering effects of deer-induced shrubs and tussock gra sses on seedlings and saplings. (C) 1997 Elsevier Science Ltd.