Estimating prescribed fire impacts and post-fire tree survival in eucalyptus forests of Western Australia with L-band SAR data

Abstract

Prescribed burning is a technique commonly applied to reduce fire risk. Fire is introduced under specific environmental conditions, with explicit duration, intensity and rate of spread. Such conditions deviate from those encountered during the fire season. As a result, prescribed burns mostly affect surface fuels and understorey vegetation, an outcome markedly different when compared to wildfires.Information on prescribed burning outcomes is important for evaluating the extent to which land management objectives have been achieved. This study focused on the ability of Radar Burn Ratio (RBR), a Synthetic Aperture Radar (SAR)-based multi-temporal index, to appraise the outcome of prescribed burns in eucalypt forests of Western Australia. L-band datasets acquired by the Phased Array type L-band Synthetic Aperture Radar (PALSAR-2) sensor were used to relate changes in radar backscatter coefficient with the effects of prescribed burning. Two modelling approaches based on post- to pre-fire ratios were used to evaluate prescribed burn impacts. Post-fire tree survival in areas affected by high fire impacts was inferred by assessing the recovery rates of the backscatter coefficient for the first year after the burn.The impact of prescribed burns was estimated with relatively high overall accuracy (79%) using cross-polarized backscatter (HV) under dry conditions. The HH polarization and the Normalized Difference Backscatter Intensity (NDBI) showed certain ability to detect burned areas under wet conditions. The results suggest that L-band backscatter coefficient may contribute to evaluating the success of prescribed burns in Western Australian eucalypt forests through its sensitivity to changes in vegetation structure. In addition, tree survival (as opposed to mortality) and recovery towards the pre-fire state might be inferred by analysing short-term post-fire trends of the backscatter coefficient in areas affected by high fire impacts.