A burning issue: community stability and alternative stable states in relation to fire

Abstract

Fire regimes have long been thought to drive plant community change in biomes by altering feedbacks that maintain "stable" community assemblages (see Jackson, 1968; Mutch, 1970). The occurrence of contrasting vegetation types in otherwise comparable environments, where flammable communities are juxtaposed with those that rarely burn, is often explained by alternative stable state (ASS) theory, where shifts in equilibrium are triggered by catastrophic fire (Petraitis & Latham, 1999; Scheffer & Carpenter, 2003). When high-intensity fires burn into less flammable communities, compositional change is thought to occur because, firstly, an ecological threshold is reached beyond which the disturbance is large enough to remove species that perpetuate the exclusion of fire, secondly, space is then opened up for colonization by more flammable species and finally self-reinforcing pyrogenic dominance is achieved (Figure 5.1). Alternative stable states are often invoked in fire-prone regions and climates where there are sharp boundaries between communities (Figure 5.2). One of the most cited examples of alternative stable states is the fire-triggered transformation of rainforest to more flammable assemblages (Jackson, 1968; Webb, 1968; Bowman, 2000; Beckage et al., 2009; Hoffmann et al., 2009; Warman & Moles, 2009). Other examples of ASS include the conversion of boreal forest to deciduous forest (Johnstone et al., 2010), tropical savanna to grassland (Hoffmann & Jackson, 2000) and the replacement of arid shrublands by more flammable grassland (Nicholas et al., 2011). Fluctuations in community composition are the norm for most plant communities and in this sense they are "meta-stable" at Holocene timescales even under the effects of severe disturbance. In contrast, transformation in structure and complete floristic turnover at decadal timescales are those associated with ASS.