Contrasting changes in vegetation structure and diversity with time since fire in two Australian Mediterranean-climate plant communities

Mallee-heath and mallee communities occur in a mosaic across large areas of south-western Australia, in topographically subdued and fire-prone landscapes. Consequently, it could be expected that these communities would have historically experienced similar fire regimes, and would respond similarly to variation in aspects of the fire regime. We studied the response of mallee-heath and mallee to time since the last fire, measuring species density, species-area relationships, diversity indices and vegetation structure. Floristic responses to time since fire accorded with the initial floristic composition model of plant succession, with declining species density and Shannon diversity with age in mallee-heath. Mallee-heath exhibited structural senescence when > approximately 45-55years since fire, with increasing standing dead vegetation, bare ground and stagnating or declining size in sprouting Eucalyptus spp. Mallee showed no such evidence of senescence, and indeed continued to increase in stature beyond the mean fire interval but without the compositional change required to provide support for the relay floristic model of plant succession. These results indicate that mallee-heath is a fire maintained community and as such is reliant upon periodic burning to maintain diversity and vigour. Mallee, in contrast, is modified but not maintained by fire (at least over the period of time since fire examined) and hence is less susceptible to fire interval effects. Indeed, structural attributes likely to be significant for fauna habitat and carbon sequestration continue to develop in mallee unburnt for 55years or more. Different responses to time since fire will create challenges for management, particularly in fragmented landscapes where fire potentially interacts with other threatening processes.