AbstractMulga (Acacia aneura) and spinifex (Triodia spp.) are common throughout arid Australia, often dominating the landscape as discrete, abruptly alternating dominants of vegetation. The floristic, soil and fire frequency characteristics of the Mulga and spinifex dominated vegetation in the flat sandplain country at the southern limit of the Tanami Desert were explored to identify factors that are contributing to the distribution and possible dynamics of these vegetation types.
A floristic survey revealed that most woody species occurred on both sides of the Mulga-spinifex boundaries. The mulga to spinifex transition sat sharply across a diffuse soil surface textural gradient from loams through to sands. The woody species in both mulga and spinifex vegetation types shared functional life history and regeneration traits. Spinifex vegetation burnt on average three times more than Mulga vegetation. A positive correlation was found between Mulga structural diversity and time since fire. Summer fires are associated with stands dominated by younger Mulga trees. The species richness of woody plants was higher in spinifex than mulga vegetation at the local scale, but similar at the landscape scale.
The landscape was overwhelmingly characterized by the dominant mulga and spinifex. I argue that the boundary between mulga and spinifex vegetation types is created and maintained by fire, with possible dynamics associated with a fire-driven soil gradient. The fire effect was deemed most important because of the high degree of shared soil and other floristic features across the Mulga-spinifex boundary. The soil features argued to be reinforced by fire across the Mulga-spinifex boundaries may be contributing to conditions that heighten Mulga’s capacity to regenerate after fire.
The reduction of structural complexity in frequently or intensely burnt Mulga stands is of concern for the current mulga-spinifex vegetation matrix in the southern Tanami Desert. It is conceivable that it could indicate a reduced structural state that has future consequences for the current mechanisms that mostly allow Mulga to maintain its boundary position under present conditions.
|Date of Award
|Donald Franklin (Supervisor), David Bowman (Supervisor) & Guy Stuart Boggs (Supervisor)