Human-Imposed, Fine-Grained Patch Burning Explains the Population Stability of a Fire-Sensitive Conifer in a Frequently Burnt Northern Australia Savanna
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Woody plant demographics provide important insight into ecosystem state-shifts in response to changing fire regimes. In Australian tropical savannas, the switch from patchy landscape burning by Aborigines to unmanaged wildfires within the past century has been implicated in biodiversity declines including the fire-sensitive conifer, Callitris intratropica. C. intratropica commonly forms small, closed-canopy groves that exclude fire and allow recruitment of conspecifics and other fire-sensitive woody plants. C. intratropica groves provide a useful indicator of heterogeneity and fire regime change, but the mechanisms driving the species’ persistence and decline remain poorly understood. We examined the hypothesis that C. intratropica population stability depends upon a regime of frequent, low-intensity fires maintained by Aboriginal management. We combined integral projection models of C. intratropica population behaviour with an environmental state change matrix to examine how vital rates, grove dynamics and the frequency of high- and low-intensity fires contribute to population stability. Closed-canopy C. intratropica groves contributed disproportionately to population growth by promoting recruitment, whereas singleton trees accounted for a larger proportion of adult mortality. Our patch-based population model predicted population declines under current fire frequencies and that the recruitment of new groves plays a critical role in the species’ persistence. Our results also indicated that reducing fire intensity, a key outcome of Aboriginal burning, leads to C. intratropica population persistence even at high fire frequencies. These findings provide insight into the relationship between ecosystem composition and human–fire interactions and the role of fire management in sustaining the mosaics that comprise ‘natural’ systems.
KeywordsIntegral projection model (IPM) Patch mosaic burning Tropical savanna Fire management Anthropogenic disturbance Population dynamics
We are grateful to the Rostron family, Wesley Campion and the Djelk Aboriginal Ranger group for support and permission to work in Arnhem Land. These analyses would not have been possible without the initial guidance of Orou Gaoue and Carol Horvitz, made possible by a grant to CT from the National Climate Change Adaptation Research Facility of Australia, as well as the patient feedback of Lisa Mandle. Thanks to Talia Portner, Fay Johnston and Clancy Bowman for help in the field. Grants from the Commonwealth Environmental Research Facilities (CERF) Program of Australia, ANZ Holsworth Endowment, Australian Geographic, the Ecological Society of Australia and the Australian Research Council (LP110200006 and DE130100434) funded this research.
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