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Saproxylic beetles associated with habitat features in Eucalyptus obliqua trees in the wet eucalypt forests of southern Tasmania

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Harrison, KS (2008) Saproxylic beetles associated with habitat features in Eucalyptus obliqua trees in the wet eucalypt forests of southern Tasmania. PhD thesis, University of Tasmania.

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Abstract

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 (>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.

Item Type: Thesis (PhD)
Copyright Information:

Copyright 2007 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (PhD)--University of Tasmania, 2008. Includes bibliographical references

Date Deposited: 09 Dec 2014 00:13
Last Modified: 08 Nov 2016 02:05
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