Abstract
The ability to map burn severity and to understand how it varies as a function of time of year and return frequency is an important tool for landscape management and carbon accounting in tropical savannas. Different indices based on optical satellite imagery are typically used for mapping fire scars and for estimating burn severity. However, cloud cover is a major limitation for analyses using optical data over tropical landscapes. To address this pitfall, we explored the suitability of C-band Synthetic Aperture Radar (SAR) data for detecting vegetation response to fire, using experimental fires in northern Australia. Pre- and post-fire results from Sentinel-1 C-band backscatter intensity data were compared to those of optical satellite imagery and were corroborated against structural changes on the ground that we documented through terrestrial laser scanning (TLS). Sentinel-1 C-band backscatter (VH) proved sensitive to the structural changes imparted by fire and was correlated with the Normalised Burn Ratio (NBR) derived from Sentinel-2 optical data. Our results suggest that C-band SAR holds potential to inform the mapping of burn severity in savannas, but further research is required over larger spatial scales and across a broader spectrum of fire regime conditions before automated products can be developed. Combining both Sentinel-1 SAR and Sentinel-2 multi-spectral data will likely yield the best results for mapping burn severity under a range of weather conditions.
Highlights
Fire is an integral component of savanna ecosystem functioning and is a key driver of vegetation structure in tree–grass systems around the globe [1]
The “deltaNBR” and “deltaSAVI” indices that we derived from Sentinel-2 detected the experimental fire event and suggested that burn severity was higher within the E2 plots than the E1 plots (Figure 3)
Results based on indices derived from Sentinel-2 optical satellite imagery suggest overall higher burn severities for plots which are burnt every second year compared to the more frequently burnt plots
Summary
Fire is an integral component of savanna ecosystem functioning and is a key driver of vegetation structure in tree–grass systems around the globe [1]. The tropical savannas of northern Australia are flammable, with many regions burning in two out of every three years [2] These regions are characterised by distinct dry and wet seasons, and the lack of precipitation from May to October results in very dry and highly flammable fuel-loads [3,4,5]. Fires late in the dry season are of higher intensity than those earlier on in the season and cause greater structural change [6]. These late season fires may result in higher greenhouse gas emissions, and as such, there are policy frameworks in place to encourage a reduction in the frequency of late season burns (the Australian Government’s Emissions Reduction Fund [7]). As such, mapping savanna fires and their severity is important ecologically for Remote Sens. 2020, 12, 49; doi:10.3390/rs12010049 www.mdpi.com/journal/remotesensing
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