Woody-grass ratios in a patterned (mulga-spinifex) arid system are mediated by unexpected competitive interactions driven by fire, resources, and life-history

Predicting changes in woody-grass ratios in arid and semi-arid ecosystems is fraught with uncertainty because the causality of both coexistence maintenance and breakdown in these systems is poorly known.  Patterning in Mulga (Acacia anuera) shrubland and Spinifex (Triodia spp.) grassland mosaics in arid Australia may comply with Bowman’s (2000) ‘alternate biome’ model for the coexistence of juxtaposed resource-controlled forest and ‘consumer’-controlled (sensu Bond 2005) savanna but this has not been tested.  We therefore determined, using reciprocal transplant experiments, the relative importance of resources, competition and fire in the maintenance of habitat boundaries.  Our results indicated that the expansion of grassland species into shrubland habitat is negated by physical limitations but not necessarily vice versa. Woody thickening of the grassland is mediated by strong competitive effects of spinifex grass combined with removal of obligate seeding shrubs by fire. These results are not consistent with the expectations of the alternate biome model because overall, interactions were strongly negative in the grassland and neutral to slightly positive in the shrubland.  Instead, we propose that it is fundamentally the life history of the community dominants interacting with resource gradients and fire regime that are the proximal drivers of woody-grass ratios both within and between habitats in these arid mosaics.