Using GEDI Data to Evaluate the Impact of the Australian 2019-2020 Fire Season on the Structure and Biomass of Gondwana Rainforests
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Wildfires are increasing globally in their intensity, frequency, and range. A unique yet globally significant example is the devastating eastern Australia 2019-2020 fire season, which resulted in an unprecedented burnt forest area with > 21% of Australia’s temperate forests affected, and, in an unprecedented event, over 50% of Gondwana Rainforests in Australia were impacted. Currently, optical satellite imagery is used to detect active fires and assess burnt area, as well as impact. Assessing the impacts of fires solely through these techniques lacks information on vegetation structural changes, especially subcanopy changes. Lidar (light detection and ranging) can be used to evaluate the impacts of fire on vegetation structure more comprehensively. This study uses full waveform lidar data from NASA’s Global Ecosystem Dynamics Investigation (GEDI) to study the Gondwana Rainforests of Willi Willi National Park and Werrikimbe National Park, in New South Wales, Australia. GEDI footprints were filtered to remove those incident upon steep slopes (>25°) and collected more than 9 months before and after the MODIS recorded burn date. GEDI footprint level canopy structure measurements (Level 2A and 2B data products) were aggregated into 5 km2 grid cells across the study sites and showed an average decrease of 19% in Plant Area Index, and a 15% decrease in Canopy Cover Fraction. Vertical profiles of Plant Area Volume Density (PAVD) grouped into 5 meter vertical profile height bins showed an average decrease of -15.6% (STD = 8.1) for bins from 0-5 meters to 35-40 meters, with the largest decrease of -27.63% occurring in the 10-15 meter vertical profile height bin. GEDI relative height (RH) metrics were lower post fire: RH50 decreased by average of 12.7%, RH75 by 5.1%, and RH90, RH95, RH98, and RH100 all by <1%. GEDI footprint level aboveground biomass density (Level 4A product) showed a -1.5% decrease, correlating closely with percent change in RH75 and the 20-25 meter PAVD vertical profile height bin. The PAVD metric change therefore captured the impact of the fire more effectively than RH metrics, which showed limited change. Consequently, the footprint-level GEDI biomass which is largely derived from RH metrics show minor changes after the fire. These results indicate that GEDI was uniquely positioned to directly quantify the effects of vegetation disturbance on vertical canopy structure. The structural damage resulting from these fires ranged from entire stand-replacing disturbance to primarily crown or understory fires. We discuss the potential implications of these findings to understand the impact of fire severity and post-fire recovery on carbon stocks at regional scales.