The ecology and habitat requirements of saproxylic beetles native to Tasmanian wet eucalypt forests : potential impacts of commercial forestry practices

Large diameter Eucalyptus obliqua decaying logs are characteristic features of wet eucalypt forests in Tasmania. In production coupes, however, rotation lengths of around 80 years will eventually lead to their elimination. This thesis investigates the role of these features as habitat for saproxylic beetles, and thus whether their retention is warranted to maintain biodiversity. Large diameter (> 1 OOcm) logs derived from commercially over-mature trees were compared with small diameter (30-60cm) logs derived from trees of an age approaching commercial maturity, in two forest types: mature, unlogged forest; and 20-30 year logged forest that had regenerated after clearfelling. Two field studies were conducted, in which a highly species rich fauna of 360 saproxylic beetle species (representing 54 families) are first recorded. The first, destructive sampling study investigated whether small diameter logs follow similar decomposition processes to large diameter logs, and so support similar rot types and beetle assemblages. Eleven rot types were differentiated, each associated with a particular region within the log. Small diameter logs had a relatively high incidence of white rot towards their outer edges, probably originating from fungal colonisation after treefall. Large diameter logs had a higher incidence of brown rot towards their cores, probably originating from internal decay already present in older, living eucalypt trees prior to treefall. Some of the beetle species characteristic of this brown rot are possibly poor dispersers and may be of particular conservation concern in production forests. The second, log emergence trapping study examined the extent to which beetle assemblages differ between small and large diameter logs, and whether they respond in the same way to forest successional processes induced by stand level disturbance. Distinct suites of species were associated with large diameter logs irrespective of forest type, yet there were no apparent small log specialists. Assemblages differed significantly between the mature and logging regenerated forests; there was also significant variation among sites that could not be attributed to forest type. Small diameter logs in the logging regenerated forest lacked some apparent mature forest specialists that were present in large diameter logs in the same forest type. This research indicates that large diameter logs have unique habitat qualities for saproxylic beetles, and they are important in providing continuity of habitat for the re-establishment of certain species following stand level disturbances, whether induced by logging or by wildfire. A precautionary and multi-scaled approach towards dead wood management is advocated, with particular consideration of the temporal scale at which the dynamics of the forests operate. In line with current conservation management strategies employed and explored by Tasmanian forestry, retention of some trees during harvesting to improve stand structure complexity and future dead wood supply is strongly recommended as one means of mitigating potential negative impacts. The planning of tree retention should aim to provide sufficient oldgrowth features, and sufficient quantity and continuity of dead wood types, throughout successive forest regeneration cycles for conservation of dead wood dependent biota. At the landscape scale, managing the production forest matrix as a habitat mosaic through diversifying silvicultural regimes is also recommended.