Aim Globally, most landscape burning occurs in the tropical savanna biome, where fire is a characteristic of the annual dry season. In northern Australia there is uncertainty about how the frequency and timing of dry season fires have changed in the transition from Aboriginal to European fire management. LocationIn the tropical eucalypt savannas that surround the city of Darwin in the northwest of the Northern Territory of Australia. MethodsOur study had three parts: (1) we developed a predictive statistical model of mean mass ( µ g) of particulates 10 µ m or less per cubic metre of air (PM 10 ) using visibility and other meteorological data in Darwin during the dry seasons of 2000 and 2004;(2) we tested the model and its application to the broader air shed by (a) matching the prediction of this model to PM 10 measurements made in Darwin in 2005, (b) matching the predictions to independent measurements at two locations 20 km to the north and south of Darwin and (c) matching peaks in PM 10 to known major fire events in the region (2000-01 dry seasons); and (3) we used the model to explore changes in air quality over the last 50 years, a period that spans the transition from Aboriginal to European land management. ResultsWe demonstrated that visibility data can be used reliably as a proxy for biomass burning across the largely uncleared tropical savannas inland of Darwin. Validations using independent measurements demonstrated that our predictive model was robust, and geographically and temporally representative of the regional airshed. We used the model to hindcast and found that seasonal air quality has changed since 1955, with a trend to increasing PM 10 concentrations in the early dry season. Main conclusionsThe results suggest that the transition from Aboriginal to European land management has been associated with an increase in fire activity in the early months of the dry season.

Allosyncarpia ternata S.T.Blake is a large tree endemic to the rugged western edge of the Arnhem Land Plateau, northern Australia, with most of the species conserved in Kakadu National Park (KNP). A. ternata stems suffer substantial mortality following wildfire but the species resprouts prolifically from root stocks. Nonetheless, there is concern about the persistence of A. ternata rainforest patches following breakdown of traditional Aboriginal landscape burning that generated a mosaic of burnt and unburnt areas. Generalised linear modelling was used to identify the landscape features associated with the fragmentary distribution of A. ternata rainforest. We randomly sampled 12 areas that together made up 12.6% of the total coverage of A. ternata in KNP (12 191 ha) that spanned the geographic range of this vegetation type within the Park. The modelling of these data showed that A. ternata forests were most likely to occur at sites with fire protection, as inferred from the small number of fire scars apparent on sequences of satellite imagery, steep slope angles and proximity to drainage lines. Analysis of historical aerial photography revealed that, despite considerable negative and positive variation, there has been a 21% expansion of A. ternata forests over the last 50 years. Expansion occurred by incremental growth from existing forest boundaries and not by nucleation, reflecting the poor seed dispersal of the tree. The forest expansion was negatively correlated with fire activity. A regionally wetter climate since the mid-20th century may be an important cause of the expansion despite currently unfavourable fire regimes.

Aim Globally, most landscape burning occurs in the tropical savanna biome, where fire is a characteristic of the annual dry season. In northern Australia there is uncertainty about how the frequency and timing of dry season fires have changed in the transition from Aboriginal to European fire management. LocationIn the tropical eucalypt savannas that surround the city of Darwin in the northwest of the Northern Territory of Australia. MethodsOur study had three parts: (1) we developed a predictive statistical model of mean mass ( µ g) of particulates 10 µ m or less per cubic metre of air (PM 10 ) using visibility and other meteorological data in Darwin during the dry seasons of 2000 and 2004;(2) we tested the model and its application to the broader air shed by (a) matching the prediction of this model to PM 10 measurements made in Darwin in 2005, (b) matching the predictions to independent measurements at two locations 20 km to the north and south of Darwin and (c) matching peaks in PM 10 to known major fire events in the region (2000-01 dry seasons); and (3) we used the model to explore changes in air quality over the last 50 years, a period that spans the transition from Aboriginal to European land management. ResultsWe demonstrated that visibility data can be used reliably as a proxy for biomass burning across the largely uncleared tropical savannas inland of Darwin. Validations using independent measurements demonstrated that our predictive model was robust, and geographically and temporally representative of the regional airshed. We used the model to hindcast and found that seasonal air quality has changed since 1955, with a trend to increasing PM 10 concentrations in the early dry season. Main conclusionsThe results suggest that the transition from Aboriginal to European land management has been associated with an increase in fire activity in the early months of the dry season.

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