Fire Regime Shifts in the Blue Mountains, NSW, During the Twentieth Century: Insights from Charcoal Records in Temperate Highland Peat Swamps

A major concern about the 2019-2020 Australian ‘Black Summer’ bushfires, along with other recent wildfire events worldwide, is whether they signal a shift toward a more extreme fire regime characterized by greater frequency, intensity, or burned area. Although fire has shaped Australia’s terrestrial ecosystems over evolutionary timescales, climate variability, and increasingly severe fire weather, perhaps exasperated by human-induced climate change or decisions regarding natural resource management, may be contributing to more extreme wildfires. Charcoal preserved in undisturbed, well-dated sediments holds significant potential for reconstructing long-term fire history. This study employed high-resolution ¹⁴C dating, charcoal accumulation (CHAR), and of a calibration experiment between Raman spectroscopy and Eucalypt species burnt in a calorimeter under controlled energy conditions to simulate a gradient from low-intensity to high-intensity wildfires. Our focus was on examining changes in fire intensity, severity and area burned in the upper Blue Mountains of NSW, in eastern Australia, over the Twentieth Century. We evaluated several previously proposed Raman-derived indicators of thermal maturity, including Raman band separation (RBS or G-D), the ratio of peak maximum intensities in the D- and G-bands (ID/IG), the ratio of the area under these bands (AD/AG), and the ratio of the full width at half maximum for the D- and G-bands (WD/WG). AD/AG produced the best relationship with increasing applied energy, but all these Raman-derived parameters were found to be less capable at higher fire intensities. To address this issue, a chemometric (backward interval partial least squares (PLS) regression) modelling approach was used which provided a more robust model linking Raman spectra and fire intensity. The application of this model across multiple upper Blue Mountains sites does not support the hypothesis that fire is becoming more severe. In contrast, CHAR results suggest that area burned across the region is increasing. We present a consideration of the drivers of these changes across the Twentieth Century, and further work seeks to place these trends in the context of the characteristics of fire regimes over the many thousands of years (represented by the sediments in the mires of the Blue Mountains).