KENNETH SCOTT,1 SAMANTHA A. SETTERFIELD,1* MICHAEL M. DOUGLAS,1,2
CATHERINE L. PARR,4 JON SCHATZ3 AND ALAN N. ANDERSEN3
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Abstract
The structure of tropical savanna ecosystems is influenced by fire frequency and intensity. There is particular interest in the extent to which long-term fire exclusion can result in a shift from savanna to forest vegetation that is not easily reversed by the reintroduction of fire.This study examined changes in the structure and composition of a long-unburnt site within the northern Australian savannas following an extended period of active fire exclusion (>20 years), and the effect of the reintroducing fire through experimental fire regimes, including fires in the early and late dry season at a range of frequencies. After the long period of fire exclusion, the vegetation community was characterized by a well-developed midstorey and canopy layer, low grass cover, substantially higher densities of woody sprouts and saplings than frequently burnt savanna. The community composition included a high proportion of rainforest-affiliated species. Three years of experimental fires had no detectable effect on the overall composition of grass layer and woody plants but had an effect on woody vegetation structure. Continued fire exclusion further increased the density of woody stems, particularly in the midstorey (2.0–4.99 m), whereas
moderate-intensity fires (>800 kW m-1) significantly reduced the density of midstorey stems.The reintroduction of
higher moderate intensity fire events resulted in the vegetation in some compartments reverting to the open savanna
structure typical of frequently burnt sites. Such rapid reversibility suggests that in general, the woody thickening
resulting from long-term fire exclusion did not represent a biome shift to a non-savanna state. However, there was
a small proportion of the site that could not sustain the fires applied to them because grass cover was very low and
patchy and therefore appeared to have crossed an ecological threshold towards closed forest. Key words: biome shift, fire, fire behaviour, resprout, savanna, tropical woodland, woody thickening.
INTRODUCTION
State-and-transition models are useful for describing the range of complex ecosystem processes and multiple vegetation states observed in tree-grass ecosystems (Westoby et al. 1989; Angassa & Oba 2008). According to state-and-transition theory, vegetation can shift from one stable state to another, and this shift can be irreversible without dramatic intervention. Substantial changes in composition and structure may occur within
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