The fire ecology of Callitris intratropica. Tracing the legacy of Aboriginal fire management to inform contemporary responses to a conservation crisis on the Arnhem Plateau, northern Australia

摘要

The decline of Northern Cypress Pine (Callitris intratropica) throughout the tropical savannas of northern Australia has become an iconic example of the ecosystem-wide effects of destructive fire regimes. The persistence of C. intratropica, a conspicuous, long-lived, obligate-seeding conifer with limited fire-tolerance, in one of the world’s most fire-prone ecosystems is ecologically puzzling. An appealing hypothesis is that habitat mosaics created by Aboriginal burning maintained enough long unburnt patches throughout the landscape for fire-sensitive plant species like C. intratropica to successfully recruit. However, widespread depopulation of Aboriginal lands within the past century across much of northern Australia has resulted in the shift from small-scale patch burning to landscape-scale wildfires. On the Arnhem Plateau, this change in fire pattern is widely blamed for current declines in C. intratropica as well as other species, notably native mammals and granivorous birds, which were able to persist for more than 50,000 years of continuous Aboriginal occupation. Within the last several years, in an attempt to simulate Aboriginal fire regimes, contemporary managers across much of the Arnhem Plateau have implemented programs consisting of frequent burning, largely from helicopters, in the early dry season when higher fuel moisture and cooler temperatures result in smaller, patchier fires. However, continued and dramatic species declines, especially in the World Heritage listed Kakadu National Park, urgently signals the need for some means by which to better direct and monitor current strategies. If, as the above hypothesis implies, the presence of C. intratropica in Australian savannas is a relict of past fire regimes, then maintaining the species in the landscape may provide a powerful benchmark for evaluating contemporary management and conservation efforts. Therefore, this research examines the utility of C. intratropica as a proactive indicator both of ecologically beneficial fire regimes and critical habitat for other fire-sensitive species by addressing four key components: ud1) Exploring the role of human management in driving habitat heterogeneity in flammable ecosystems worldwide using fire simulations and the spatial scale of C. intratropica grove occurrence as a case study ud2) Patch-level assessment of the interaction between C. intratropica groves and savanna fires and its implication for vegetation heterogeneity and diversity ud3) Population modelling of C. intratropica to understand the role of patch dynamics in driving population stability under prevailing and hypothetical fire regimesud4) Landscape-scale survey of C. intratropica grove condition to examine patterns of habitat heterogeneity and plant diversity across differing management regimes in Kakadu National Park (KNP) and central Arnhem Land (CAL)udThe results demonstrated that under low-intensity fire regimes, fire exclusion by C. intratropica groves effectively creates small-scale fire refugia, which have implications for both the persistence of C. intratropica populations as well as increasing the heterogeneity, diversity, and structural complexity of savanna vegetation. Disturbance and population models further implicated human intervention – specifically via the reduction of high intensity fires – in maintaining conditions that favour the establishment and persistence of C. intratropica, despite high fire frequencies in this savanna. Better overall C. intratropica grove condition in CAL provided evidence that continued Aboriginal fire management – and possibly the presence of feral water buffalo – supports greater savanna heterogeneity and diversity than in neighboring KNP.