Cattle, mining or fire? The historical causes of recent contractions of open forest in the wet tropics of Queensland through invasion by rainforest

Abstract

Changes to Aboriginal fire regimes since European occupation are thought to have affected the range and demographic structure of many vegetation communities. This study shows a contraction by 49% of the area of fire-prone open forest through rainforest invasion between 1945 and 1991-94 in the northern wet tropics of Queensland, Australia. Relative Growth Rates (RGR) for open forest areas varied from -0.112 to -0.005. Collaborative historical research with the Aboriginal traditional owners, the Kuku-Yalanji people, investigated possible linkages with alterations to their fire practices. A multiplicity of human impacts is associated with the measured vegetation change, including clearing for agriculture and mining, logging for timber and firewood, and the introduction of cattle and horses. Some rainforest expansion since 1945 represents a recovery following clearing from earlier mining operations. Contraction of open forest through rainforest invasion was most rapid (RGR = -0.124) where there was a continuation of Aboriginal fire management with cattle grazing. The contraction of open forest was nine times slower in an ungrazed area (RGR = -0.005) than in a nearby area grazed by horses (RGR = -0.045). Aboriginal fire regimes may act synergistically with cattle or horse grazing to accelerate the invasion of rainforest into open forest. Management prescriptions currently focus on active fire management to prevent further open forest contraction. However, fire management may have unexpected outcomes when rainforest-open forest dynamics are complicated by recent historical factors such as cattle grazing, logging, and tin mining, and possible synergies between these factors and fire regimes. Managers need to understand the histories of particular sites when formulating plans, and monitor the consequences of their actions to enable an adaptive approach.