Saproxylic beetles associated with habitat features in Eucalyptus obliqua trees in the wet eucalypt forests of southern Tasmania
Creators: name: Harrison, KS
Publisher: name: University Of Tasmania
Year: 2023

description: Old, larger-diameter eucalypt trees are a common structural feature of mature, unharvested wet forests in temperate Australia. In Tasmanian production forest, such trees will become rarer since there is insufficient time under the current prescribed rotation length of 80-100 years for replacements to regrow following clearfelling. Drawing on insights from overseas studies, one question that arises is what effect this anticipated change in age structure may have on tree-dependent biodiversity. In Tasmania, the relationships between tree age and saproxylic organisms (dead wood dependent) are insufficiently understood to allow explicit consideration of either their role as 'ecosystem engineers' (e.g. in hollow formation) or their conservation requirements in production forest landscapes. This thesis seeks to redress this deficiency. It documents two main studies that together represent the first attempt to quantify the saproxylic beetle fauna of living Eucalyptus obliqua trees - the dominant wet eucalypt species in Tasmania. The first study investigated the saproxylic beetle fauna of young (69 years old), medium-aged (105 years old) and old (&gt;150 years old) E. obliqua trees. Beetle associations with different aged trees and different wood fractions (stem, branches and bark) were explored by emergence trapping of wood fractions from six trees in each of the three age classes. The habitat within the stems of trees from each of the three age-classes was investigated by destructive sampling of wood billets. Wood within the stem was classified into eleven rotten wood types. Trees in the oldest age-class were found to support a richer and more distinctive saproxylic beetle fauna compared to those in the two younger age-classes. They had higher overall species richness per tree and per surface area sampled; higher richness of obligate saproxylic beetle species; and a different assemblage composition of obligate saproxylic beetle species compared to trees in the younger age-classes. By comparison, trees in the two younger age-classes shared similar species richness and composition. Stem-dwellers accounted for much of the distinctiveness of the saproxylic beetle fauna of trees in the oldest age-class. This may have been due to the more complex array of habitats present in the stems of these old trees compared to stems of trees in the two younger age-classes. Trees in the oldest age-class harboured more rotten wood types and a larger proportion of decayed stem compared to the two younger age-classes. A strong correlation was found between the assemblage composition of obligately saproxylic beetles and the assemblage composition of rotten wood types present within a tree. Specific associations with particular rotten wood types were found for several beetle species. Eucalyptus obliqua wet forests are naturally fire-prone and likely to harbour beetle species which are adapted to this disturbance, and which may count as 'ecosystem engineers' if they then facilitate the creation of more complex habitat features (such as tree hollows) that are utilised by various species including arboreal mammals and birds. Thus the second study was a manipulative experiment examining the role of saproxylic beetles in habitat formation in trees, and their association with fire-wounds and mechanical wounds. Beetle associations were assessed by capturing (a) beetles attracted to newly created mechanical wounds and fire-wounds (sticky trapping), and (b) beetles that colonised these same wounds (emergence trapping a year later), on 45 trees (fifteen trees from each treatment: wound, burn, and control). Preferences for burnt over unburnt recently exposed sapwood were identified in seven saproxylic beetle species. Several further species with the potential to act as 'ecosystem engineers' in ageing E. obliqua trees were identified through comparing findings from the two studies. On the basis of the findings from these studies, management recommendations are made that, if implemented, would demonstrate a more pre-emptive approach to saproxylic biodiversity conservation in production forests. In general, efforts should be made to ensure that long-term structural complexity is enhanced beyond levels which will eventuate under the current silvicultural practices. This could be achieved by introducing variability in harvesting intensity and mature tree retention levels, in rotation length and in coupe size. Additionally, where the forest landscape is currently dominated by younger managed stands, 'ecological thinning' could be considered as a means of accelerating the development of structurally more complex forest. At the landscape level, management planning should consider maintaining sufficient spatial connectivity and temporal continuity of a range of tree age-classes to ensure the maintenance of the dependent fauna.