Declining High Temperature Bushfire in Australian Tropical Savanna Following Arrival of European Pastoralists

The Australian tropical savanna is among Earth’s most fire-prone regions. For millennia, Aboriginal Australians used prescribed burning to improve habitats for food plants and herbivores and to mitigate high intensity fires ignited by lightning in the late dry season. However, these practices were rapidly and profoundly interrupted beginning in the late 19^th and early 20^th centuries with the arrival of European pastoralists. Some studies have suggested that as a result of this reduction in early dry season, low intensity burning, late dry season, high temperature fire activity increased, with deleterious effects on ecosystems. However, as Aboriginal burning was curtailed, the introduction of cattle (as well as sheep and donkeys) reduced the grassy fuel layer. Developing a clear picture of baseline fire activity prior to the pastoralist era is important because bushfire intensity modulates greenhouse gas emissions from tropical savannas and modulates savanna and rainforest ecosystem dynamics. Reconstructing bushfire frequency and intensity is complicated by limited historical records of burning prior to the late 20^th century, and few naturally-occurring, high-resolution, fire-sensitive archives.

In order to place 20^th century bushfire into a long-term context, we reconstructed fire activity at sub-decadal resolution for the majority of the last millennium using polycyclic aromatic hydrocarbons (PAH) in three precisely-dated and fast-growing stalagmites from cave KNI-51, located in the tropical savanna of northeastern Western Australia. The molecular weights of PAH are tied to combustion temperature (i.e., higher molecular weights (HMW) form at higher temperature fires), and thus our record preserves evidence of both the timing and temperature of bushfire. In order to integrate the multiple stalagmites used to construct this composite record, we normalized each PAH class (low and high molecular weight) to the total PAH abundance in each sample.

The KNI-51 stalagmite record reveals that high temperature fire was a regular component of the Australian tropical savanna throughout the last millennium, suggesting late dry season fires were commonplace. However, soon after the arrival of European pastoralists in the 1880s, the frequency of high temperature fires decreased markedly and remained low until the record end of the KNI-51 record in 2009 CE. This shift in bushfire regime, which is apparent based on decadal averages of normalized HMW PAH and through breakpoint analysis, occurred despite severe reductions of early dry season burning by Aboriginal Australians. It also occurred during a monsoon rainfall regime, determined using oxygen isotope ratios from the same stalagmites, that was close to the last millennium average. Thus, after discounting prescribed burning and hydroclimate, we ascribe this decrease in high temperature bushfire to reductions by cattle of grassy fuel loads. The anomalous nature of the 20^th century Australian tropical savanna pyroscape in the area of KNI-51 highlights the complexities associated with re-establishing the pre-pastoralist era bushfire regime in this region.