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Fire-mediated alternative stable states in the vegetation communities of Southwest Tasmania

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Wood, SW (2011) Fire-mediated alternative stable states in the vegetation communities of Southwest Tasmania. PhD thesis, University of Tasmania.

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Abstract

The World Heritage listed landscapes of southwest Tasmania are an ideal model system for
investigating the role of fire in shaping the distribution of fire-sensitive and fire-adapted
vegetation communities. Two models of vegetation dynamics have been put forward to
explain the mosaic of rainforest, sclerophyll and treeless moorland vegetation in this region:
the alternative stable states model of Jackson (1968) and the sharpening switch model of
Mount (1979). Drawing on the concepts in these models, this aims of this thesis were to (a)
contribute new evidence on the decadal, century and millennial scale dynamics of southwest
Tasmanian vegetation communities and (b) investigate the role of interactions between fire,
vegetation, soil and the physical environment in determining observed vegetation dynamics.
The application of geospatial statistics to mapped distributions of rainforest vegetation and
the spatial pattern of four very large fires confirmed that the distribution of fire-sensitive
rainforest is related to topographic fire refugia. Topography clearly plays an important role in
mediating the feedbacks between fire and vegetation in southwest Tasmania. This study also
provided the first integrated data on the relative flammability of the major vegetation
communities at the landscape scale.
Repeat image analysis of aerial photographs (1948, 1988) and satellite imagery (2010)
revealed that forest and non-forest vegetation communities have been largely stable over the
last sixty-two years. Decadal scale fluctuations near forest boundaries were related to
recovery from stand replacing fire and the very slow encroachment of trees into moorland.
Analyses of soil samples collected across stable forest boundaries suggested that firevegetation-
soil feedbacks may contribute to the maintenance of vegetation communities over
time. Evidence from radiocarbon dating showed that interactions involving soils are also
influenced by topography and its effect on drainage. This study also showed that stable carbon isotopes in organic soil profiles cannot be used to examine millennial scale vegetation
boundary shifts in southwest Tasmania.
Dendro-ecology studies revealed that an even-aged cohort of Eucalyptus regnans and
Phyllocladus aspleniifolius established after a stand-replacing fire in 1490-1510AD.
Therefore, overstorey E. regnans trees can live for well in excess of 500 years. Current
models of forest dynamics for southwest Tasmania clearly underestimate the time frame
required for century scale transitions from eucalypt forest to rainforest in productive systems.
Whilst there is some evidence to support the alternative stable states model for southwest
Tasmanian vegetation communities, this support must remain equivocal. Vegetation patterns
in southwest Tasmania appear to be predominantly stable at a range of temporal scales and
interactions between fire, vegetation, soil fertility and topography are likely to contribute to
the resilience of vegetation communities. Transitions between vegetation types can occur,
although the time frames for transitions appear to be underestimated by current models.
Further investigations into feedback mechanisms, the role of the physical environment and
state-transition dynamics are required. The implementation of a well designed vegetation
monitoring system is an important step toward our understanding and management of the
relationships between fire and vegetation in southwest Tasmania under current and future
climates.

Item Type: Thesis (PhD)
Keywords: alternative stable states, fire, rainforest, Eucalyptus reynans, repeat arial photography, Tasmania
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Date Deposited: 14 Dec 2011 00:30
Last Modified: 11 Mar 2016 05:53
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