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Abstract

Decay and collapse of trees with hollows in Eastern Australian forests: impacts on arboreal marsupials.

Abstract

Decay and collapse of trees with hollows were studied in the montane ash forests of the Central Highlands of Victoria, Australia. The forests are usually monotypic and are dominated by one of 3 species: Eucalyptus regnans, Eucalyptus delegatensis, Eucalyptus nitens. The condition of >1400 trees was monitored for varying periods of time between 1983 and 1993 on 181 sites that varied considerably in stand age, tree species composition, slope, aspect, disturbance history, and a wide range of other parameters. The data analysed in this study were from: (a) 302 trees on 32 sites, each of 3 ha, measured first in 1983 and then again in 1988 and 1993; (b) 1006 trees on 132 3-ha sites surveyed both in 1988 and 1993; and (c) 399 trees, measured in 1990 and again 1993, that occurred within 49 wildlife corridors where the surrounding forest had recently been clear felled. Results revealed that >40% of the 302 trees first measured during 1983 had collapsed and fallen to the forest floor by 1993. The annual rate of collapse of trees was more rapid between 1988 and 1993 (4.8%) than between 1983 and 1988 (3.6%). Transition matrices constructed for the probability of tree collapse during each of these 5-yr sampling periods did not differ significantly (P < 0.05) between sampling periods or groups of sites. Logit regression modelling was used to estimate the likelihood of collapse of hollow-bearing trees in relation to their physical characteristics and the attributes of the sites on which they occurred. The collapse of trees with hollows was significantly (P < 0.05) influenced by a range of factors, including tree form and diameter as well as the aspect of the site on which they occurred. These parameters were different from those in wildlife corridors, where tree collapse was related to factors such as aspect and the size of the adjacent logged area. Hollow-bearing trees are a critical component of the vegetation structure in montane ash forests, providing nest and den sites for a large proportion of the vertebrate fauna that occur in these areas. The rate of tree fall exceeds recruitment of new hollow-bearing trees within montane ash forests. Predictions of the future availability of hollow-bearing trees, assuming that the underlying processes affecting the data persist, showed that very few 3-ha sites are likely to support more than five hollow trees within 50-75 yr. These projections suggest a negative effect of tree loss for populations of hollow-dependent animals such as arboreal marsupials. The results emphasize the need for careful management of montane ash forests over the next century to conserve populations of hollow-dependent fauna such as the endangered arboreal marsupial, Leadbeater's possum (Gymnobelideus leadbeateri).