Fire and cattle disturbance affects vegetation structure and rain forest expansion into savanna in the Australian monsoon tropics

Abstract

AbstractAimsTo detect changes in area and vegetation dynamics of monsoon rain forests in relation to disturbance and an observed wetting trend.LocationThe Mitchell Plateau and the Bougainville Peninsula (north Kimberley, Australia).MethodsGeo‐rectified aerial photographs acquired in 1949 and 1969 and a pre‐existing map from 2005 were used to detect changes in rain forest boundaries. To ground‐truth rain forest expansion, we established 20 transects running across rain forest‐savanna boundaries and recorded plant species, stand basal area, grass and rock cover, cattle impact, and canopy cover. Generalised linear models and Akaike's information criterion were used to detect differences in these variables between locations.ResultsOn the Bougainville Peninsula average fire frequency was low (0.11 per year) and cattle entirely absent, while on the Mitchell Plateau average fire frequency was high (0.58 per year), and cattle were common and associated with lower seedling density in savannas. Rain forests expanded more on the Bougainville Peninsula (69%), where patches were bigger and more convoluted, than on the Mitchell Plateau (9%). Rain forest expansion was positively associated with rainfall and topographic complexity, and on level terrain it occurred only on the Bougainville Peninsula. Rain forests were floristically and structurally similar in the two locations, while savannas on the Bougainville Peninsula had denser vegetation and more abundant rain forest elements. The frequency distribution of canopy cover was bimodal on the Mitchell Plateau, signalling the presence of two distinct vegetation states, and unimodal on the Bougainville Peninsula, consistent with the blending of the two states.Main conclusionsWetting trends are likely strong drivers of rain forest expansion, but at a landscape scale their effect is probably modulated by fire activity and the presence of mega‐herbivores, which may also be pivotal in maintaining sharp floristic and structural distinctions between rain forests and savannas.