Abstract
Characterizing the spatial variability of the severity of wildfires is important to assess ecological and economic consequences and to coordinate mitigation strategies. Vegetation indices such as the differenced Normalized Burn Ratio (dNBR) have become a standard tool to assess burn or fire severity across larger areas and are being used operationally. Despite the frequent application of dNBR-like vegetation indices, it is not yet fully understood which variables exactly drive the variability in dNBR observed by multispectral satellites. One reason for this is the lack of high quality prefire information about vegetation structure and composition. Consequently, the influence of prefire vegetation composition and other potentially influential variables such as cast shadows has hardly been examined. Here, we use very high resolution Unmanned Aerial System (UAS) orthoimages collected briefly before and after the large wildfires in Central Chile in the fire season 2016/2017 to derive variables related to the pre- and postfire landscape composition and structure. The variables are used as predictors in Generalized Additive Models (GAM) to explain the spatial variability in dNBR and RdNBR pixel values as observed by Sentinel-2. Our models explain more than 80% and 75% of the variability in dNBR and RdNBR values, respectively, using a sparse set of five predictors. The results suggest that in our study area the largest fraction of variability in Sentinel-2 based dNBR and RdNBR values can be explained by variables related to the fraction of consumed canopy cover while the vegetation composition before the fire does not have a large influence on dNBR and RdNBR.Our results further suggest that cast-shadows of snags and standing dead trees with remaining crown structure have a notable influence on the dNBR signal which may have been underestimated so far. We conclude that spatially continuous, very high spatial resolution data from UAS can be a valuable data source for an improved understanding of the exact meaning of common vegetation index products, operationally used for monitoring the environment.
Highlights
Wildfires are an integral part of many ecosystems, shaping their structure and functions (Attiwill, 1994) and having significant global impacts on terrestrial, aquatic and atmospheric systems (Lentile et al, 2006)
The results suggest that in our study area the largest fraction of variability in Sentinel-2 based differenced Normalized Burn Ratio (dNBR) and relative dNBR (RdNBR) values can be explained by variables related to the fraction of consumed canopy cover while the vegetation composition before the fire does not have a large in fluence on dNBR and RdNBR
The best subset selection results indicate that best models with 5 to 10 predictors show hardly any difference in model performances when compared to the model using all predictors (Fig. 4)
Summary
Wildfires are an integral part of many ecosystems, shaping their structure and functions (Attiwill, 1994) and having significant global impacts on terrestrial, aquatic and atmospheric systems (Lentile et al, 2006). G., Kasischke et al, 1992; Nioti et al, 2011) to regional (e.g.,BourgeauChavez et al, 1997; Pu et al, 2004) and even global scales (e.g., AlonsoCanas & Chuvieco, 2015; Giglio et al, 2006) using mostly airborne and spaceborne multispectral and Radar data in combination with simple thresholding procedures or more complex classification approaches Such approaches often achieve very high accuracies when compared to visually delineated fire scars and field-reference data and are nowadays used operationally (Hua & Shao, 2017). Relating the remotely sensed information about fire or burn severity to the actual processes and changes that occurred on the ground remains challenging (Lentile et al, 2009). It is further important that the meaning of both terms may vary drastically depending on the examined ecosystem and neither of them is generally (per definition) connected to defined measurable ecological variables, even though this may be the case in individual studies
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