Fire or water – which limits tree biomass in Australian savannas?

Fire or water – which limits tree biomass in Australian savannas?

Thu, 24/07/2014 - 10:00 to 11:00

About the Presenter

Dr Brett Murphy 
Research Fellow
Quantitative and Applied Ecology Group
School of Botany, University of Melbourne 

Brett Murphy's research focuses on the role of fire in shaping and maintaining the northern Australian biota, and how fire can be best managed to for biodiversity conservation. Brett is currently studying: (i) the impact of fire regimes on northern Australia's declining small mammals; (ii) recruitment dynamics of savanna trees, and whether functional traits can predict fire responses; (iii) the effectiveness of prescribed burning as a management tool in northern Australian savannas, aimed at decreasing annual fire extent and increasing the abundance of long unburnt habitat; (iv) the environmental controls of fire regimes throughout Australia and the likely impacts of global environmental change.


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Processes allowing co-existence of trees and grasses in tropical savannas have long intrigued ecologists. Early theories focussed on climatic controls, but a conceptual model has emerged suggesting that savanna trees are subject to a fire-mediated recruitment bottleneck, with frequent fires preventing recruitment of saplings into the tree layer and maintaining biomass well below the climate-determined carrying capacity.

I propose that this conceptual model has been overemphasised in Australian tropical savannas, where tree abundance is primarily limited by water availability, not fire. The dominant trees, eucalypts, have a remarkable capacity to grow through the ‘fire trap’ to reach fire-resistant sizes. This fire-tolerance makes eucalypts relatively unresponsive to reductions in fire frequency and intensity. Other trees in these savannas are typically more fire-sensitive and respond positively to reductions in fire frequency and intensity.

There are suggestions that savanna fire management could lead to increases in woody biomass, but I contend that if tree biomass is strongly limited by water availability, then potential increases in tree biomass may be relatively limited, at least for the dominant eucalypt component. There is potential to increase the biomass of the more fire-sensitive non-eucalypt, but the carrying capacity of non-eucalypts in these eucalypt-dominated systems remains poorly understood.

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