Year

2013

Degree Name

Doctor of Philosophy

Department

School of Biological Sciences

Abstract

Fire is a key process that governs the structure and function of terrestrial ecosystems. Changes in fire regimes have the potential to greatly affect biodiversity and ecosystem functioning. Fire regimes differ between ecosystems due to variation in fuel dynamics (i.e. vegetation growth and moisture content), fire weather and ignitions. Patterns of growth, fuel accumulation and flammability in response to resource availability will differ fundamentally between fuel systems dominated by grasses compared to woody plants. As such, the effect of fuel dynamics on fire will beinfluenced byfactors that determine the relative abundance of grass and shrub cover.

This study aimed to determine the processes controlling fuel dynamics and fire regimes along moisture and soil fertility gradients of the Mediterranean-climate region of south eastern Australia; a transitional region between grass and shrub-dominated fuel systems. The relative influence of climate and soils on controlling grass and shrub cover and resultant fire probabilities was examined. Effects of competitive interactions as well as elevated CO2 in interaction with climate and soils on relative grass and shrub cover were experimentally determined, in order to separate the confounded factors that occur in the field (i.e. in a southward direction temperature decreases, rainfall increases, soil fertility decreases and shrub cover increases).

Climate and soils, rather than competition, were the over-arching determinants of the relative cover of grasses and shrubs across the Mediterranean climate region of south eastern Australia. The cover of the perennial hummock grass, Triodia scariosa and woody shrubs changed along the rainfall and temperature gradients in accordance with predictions given the transitional position of the region; T. scariosa cover was relatively lower in the south under cold, wet conditions than in the north under hot, dry conditions while the opposite occurred for shrub cover. While shrub cover was higher on the less fertile soils, soils did not influence the cover of T. scariosa in the field under a given level of rainfall. By contrast, experimental analysis revealed temperature was likely to be a more important determinant of the growth and abundance of T. scariosa, than the distribution of soils.

Climate and soils also influenced fire probabilities, butthe nature of these influences largely differed between grass and shrub-dominated vegetation types. Fire frequency increased with rainfall, which supports predictions based on the general relationships between productivity and fire. However, fire frequency also increased with soil fertility in grass-dominated communities, while the opposite trend was apparent in shrub-dominated communities. In addition, fire probability was more dependent on fuel-age on low fertility soils in shrub-dominated communities at high rainfall. This demonstrates that the effects of soil fertility and moisture on fire regimes do not simply align. Such effects are driven by complex responses of key species to these differing influences and their ultimate outcome in terms of community composition (i.e. the relative cover of shrubs and grasses). Conceptual models of fire as a function of varying productivity need to account for this complexity.

There is strong potential for the nature of fire regimes to be altered in the future via climate change mediated shifts in fuel types and flammability across the Mediterranean region of south eastern Australia. Changes in temperature, moisture and CO2, in association with the distribution of soils have the potential to drive changes in the relative mix of grasses and shrubs. The results suggest that the less fertile soils that dominate the south of the region may promote an increase in the cover of T. scariosa and a decrease in the cover of the woody shrub,Leptospermumcoriaceum(and possibly other sclerophyllous shrubs), under warmer, drier, high CO2 conditions projected to occur in the future. This may ultimately lead to a change in community states from shrub to grass-dominated fuel systems, with corresponding effects on fire regimes (i.e. a decline in the frequency and fuel-age dependency of fire).

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