Origins Under the Blowtorch: Frequent Fire Shifts the Balance Between Sunda-Origin and Sahul-Origin Plant Species in a Tropical Savanna.

Modern savanna fire management is based on climatic season and fire frequency. However, the different biogeographic origins of savannas across the world influence their ecosystem functioning, making them floristically highly dissimilar. We examine the effect of fire frequency on the composition of vegetation of different biogeographic origins, to understand how frequently an ecosystem can be subject to fire but still retain its evolutionary diversity. We surveyed savanna subject to a long-term fire-frequency experiment in Darwin, Australia, and analysed vegetation changes according to species' biogeographic origin as either Sunda (Southeast Asia) or Sahul (ancient Australia and New Guinea). We found dramatic structural and biogeographic change in less than 20 years of frequent burning. Vegetation transformed from dry tropical woodlands into savannas, with the lens of biogeographic origin revealing deeper trends. Plots subject to fire every 1-3 years caused a shift from mostly Sahul-origin, multi-strata vegetation into simple tree-grass systems dominated by Sunda-origin Andropogoneae annual grasses, with ramifications for all Sahul-origin taxa. Despite the common physiognomy of the world's savannas, no single fire-frequency regime suits all-local ecosystem composition and dynamics need to underpin all prescribed burning regimes. In northern Australia, fire management that ultimately promotes the shift to Sunda-origin grasses threatens the continued existence of ancient Sahul-origin plants and animals which had not evolved with grass-fire cycles. We call for inclusion of species origin in analyses of ecosystems wherever modern and ancient elements cohabit. In a policy and management sense, the promotion of annual grasses by frequent fire also affects the management of savannas.