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Patterns of vegetation in south-central Tasmania : a view based on plant macrofossils


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Astorga, GA 2016 , 'Patterns of vegetation in south-central Tasmania : a view based on plant macrofossils', PhD thesis, University of Tasmania.

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Accumulations of plant macrofossils in lake sediments provide useful tools for
identifying past local presence of plant species, community composition, and
vegetation dynamics. However, plant macrofossils have rarely been used in the
reconstruction of Lateglacial-Holocene vegetation changes in the Southern
Hemisphere. Furthermore, such macrofossil studies are hampered by a scarcity of
studies of taphonomic biases that may affect the final representation of plant species
in sediments.
This thesis is the first systematic study of macrofossils covering an almost continuous
stratigraphic sequence from the Lateglacial to the Holocene period (last 15,000 years)
in southern Australia. Being located in a transitional climatic and vegetation zone
near the treeline in south-central Tasmania, Lake Dobson is a lake formed in a glacial
cirque which provides an ideal system to investigate both the taphonomy of
Australian subalpine plants, and changes in vegetation and climate extending back to
the Lateglacial.
The first component of the thesis is a taphonomic study comparing recently deposited
leaf types from modern sediments of Lake Dobson with the vegetation surrounding
the Lake. This study suggests that proximity of the plant community to the
depositional site is critical for the representation of species in sediments.
Additionally, the floristic composition of plant assemblages in sediments strongly
agrees with the composition of species in vegetation near the lake. However, the
number of leaves in sediments does not predict the same abundance of the species in
the standing vegetation, mainly because of the large differences in leaf size and rate of leaf production that exist among tree and shrub species. Overall, the results suggest
that a correction factor accounting for proximity of the plant community to the site of
deposition, differential leaf size, and number of leaves per ground area of vegetation
produced by individual species can allow for better reconstruction of the original
forest community.
The second component of the thesis analysed the floristic composition of plant
macrofossils from a core drilled in the deepest part of Lake Dobson in south-central
Tasmania, Australia. This study provides evidence of the role of local glacial survival
versus the postglacial colonisation of plant species in the assembly of treeline
communities during a period of acute environmental changes: the transition of the last
glaciation to the modern Holocene environments. In particular, the presence of high
subalpine/alpine plant species in the oldest macrofossil-bearing sediments in the Lake
Dobson core strongly support the local survival —through at least the last part of the
glacial— of poorly dispersed plant species such as species of alpine conifers, and the
cool temperate tree Nothofagus cunninghamii. Additionally, this fossil record also
provides evidence of the time lag in migration of Eucalyptus around the early middle-
Holocene boundary. Thus, the current flora is best explained as having been
assembled from a mixture of species that survived the glacial locally and species that
migrated upslope after the climate amelioration in the Holocene. This study
represents the first quantitative study of macrofossils in Australia for this significant
period of environmental change.
The final chapter of the thesis explores the potential for anatomical aspects of leaf
macrofossils to be used as indices of forest structure (in particular, closed forest
versus open vegetation). In particular, it investigates the morphological and anatomical leaf variation of both fossil and contemporary leaves (i.e. canopy and litter
leaves) of the cool temperate tree species Nothofagus cunninghamii from Lake
Dobson. Overall, the results of this study suggest that allowing for the effects of leaf
size and stomatal density in comparisons of vein density is a useful tool for
reconstructing the structure of the vegetation from fossil leaves. In particular, it was
found that closed canopy leaves have lower vein densities relative to stomatal density
compared to open canopy leaves once differences in leaf size are taken into account.
The fact that the relationship is dependent on leaf size and stomatal density is
important because it means that canopy structure may be inferred from fossil leaves
even when those leaves have undergone shrinkage in the process of fossilisation.
However, the use of these parameters in the differentiation of open and closed litter it
is not straightforward, and further research is required to clarify the higher upper to
low canopy leaf representativity in the litter. This approach, however, represents an
improvement on prior methods that directly employ vein density and have no capacity
to allow for the effects of shrinkage.
The overall findings of this thesis have direct application to complement vegetation
reconstructions by means of pollen analysis in southern temperate Australia. In
particular, the study provides better understanding of taphonomic factors affecting the
fossilisation of plant remains, and important implications to understand plant
responses from a period of climate variability that is not yet well understood in the
Southern Hemisphere: The transition from the Last Glacial to the modern Holocene

Item Type: Thesis - PhD
Authors/Creators:Astorga, GA
Keywords: Plant macrofossils; plant taphonomy; leaf assemblages; subalpine sclerophyll vegetation; Nothofagus cunninghamii; leaf traits; glacial refugia; postglacial colonisation
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Copyright 2016 the author

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