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

Abstract

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