Evidence that shrublands and hummock grasslands are fire-mediated alternative stable states in the Australian Gibson Desert

Abstract

Alternative stable state theory predicts that different disturbance regimes may support contrasting ecosystem states under otherwise analogous environmental conditions. In fire-prone systems, this theory is often invoked to explain abrupt ecotones, especially when adjacent vegetation types have contrasting flammabilities and differing tolerances to pyric perturbation. Despite being well-documented in forest-savanna transitions, unambiguous examples of fire-driven alternate stable states (FDASS) in arid systems are rare. The current study examined whether flammable spinifex (Triodia spp.) grasslands and fire-sensitive waputi (Aluta maisonneuvei subsp. maisonneuvei) shrublands in Australia's Gibson Desert represent FDASS. Specifically, analyses of soil and topographic variables assessed whether environmental differences explain habitat zonation. To determine whether different flammabilities of Aluta and Triodia systems may perpetuate alternative states via vegetation-fire feedback processes, community-level fuelloads were quantified to provide an indirect measure of flammability. To determine the propensity for fire to trigger 'state-shifting', community responses to a single high-severity fire were evaluated. Habitat segregation did not relate to between-site environmental differences, and the fuel-load study indicated that the more pyrophytic community (Triodia grassland) is more flammable, and hence more likely to experience higher frequency fire-cycles. Fire was identified as a potential vector of 'state-change', because although both systems regenerated well after fire, Triodia reproduced more prolifically at a younger age than Aluta, and hence should tolerate shorter fire-return intervals. In the absence of between-community topographic and edaphic differences, or herbivores that consume either plant, it is likely that Aluta shrublands and Triodia grasslands represent fire-mediated alternative equilibrium states.