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Does fire or productivity drive relative bark thickness from desert to savanna in northern Australia?

Schubert, Andrew Timothy (2014). Does fire or productivity drive relative bark thickness from desert to savanna in northern Australia?. Honours Thesis, Charles Darwin University.

Document type: Thesis
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Author Schubert, Andrew Timothy
Title Does fire or productivity drive relative bark thickness from desert to savanna in northern Australia?
Institution Charles Darwin University
Publication Date 2014
Thesis Type Honours
Abstract Bark thickness trends were examined across a large scale fire-productivity gradient from desert to dry savanna in northern Australia. Models showed overwhelming support for local fire activity as the driver of bark thickness trends along this gradient, and no support for the influence of productivity, as measured by mean annual rainfall or the mean normalised difference vegetation index (NDVI). The observed increase in bark thickness was mainly associated with the progressive loss of thin-barked species with increasing fire activity. In regions of high fire activity, thin-barked tree species were restricted to localised areas with suppressed fire regimes. An increase in bark thickness was also associated with increased fire intensity; at sites with similar fire-frequencies trees in spinifex vegetation had thicker bark than trees in non-spinifex vegetation.

Bark thickness reflected predicted allocation to fire resistant and resilience traits. Within trees, relatively thick bark (fire resistance) was strongly associated with trees that resprout epicormically after fire. In contrast, basal resprouters and reseeders, which achieve fire resilience via below-ground bud banks and fire-cued seed banks respectively, had relatively thin bark. With increasing aridity there was a general shift from epicormic resprouters to shrub and mallee species, which were all basal resprouters or reseeders with relatively thin bark. However, under arid conditions, where water resources were sufficient to support tree growth, relative bark thickness remained an important fire-resistance trait.

These overall patterns were supported by bark thickness comparisons within phylogenies. Pairwise species comparisons showed that species from high fire activity environments had consistently thicker bark than congeneric species from low fire activity environments, and comparisons within Eucalyptus demonstrated a strong association between local fire activity and bark thickness. Together these results confirm that bark thickness is an evolutionarily labile and adaptive plant functional trait driven by the plant response to prevailing fire regimes, and provide compelling evidence for thick bark as an adaptive trait for fire resistance in fire-prone habitats even under arid conditions.

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