Detecting Burn Severity across Mediterranean Forest Types by Coupling Medium-Spatial Resolution Satellite Imagery and Field Data

Luigi Saulino,Antonio Saracino,Antonello Migliozzi,Angelo Rita,Emilia Allevato,Antonio Pietro Garonna,Carmine Maffei

doi:10.3390/rs12040741

Luigi Saulino, Antonio Saracino + Show 5 more

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https://doi.org/10.3390/rs12040741

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Abstract

In Mediterranean countries, in the year 2017, extensive surfaces of forests were damaged by wildfires. In the Vesuvius National Park, multiple summer wildfires burned 88% of the Mediterranean forest. This unprecedented event in an environmentally vulnerable area suggests conducting spatial assessment of the mixed-severity fire effects for identifying priority areas and support decision-making in post-fire restoration. The main objective of this study was to compare the ability of the delta Normalized Burn Ratio (dNBR) spectral index obtained from Landsat-8 and Sentinel-2A satellites in retrieving burn severity levels. Burn severity levels experienced by the Mediterranean forest communities were defined by using two quali-quantitative field-based composite burn indices (FBIs), namely the Composite Burn Index (CBI), its geometrically modified version CBI (GeoCBI), and the dNBR derived from the two medium-resolution multispectral remote sensors. The accuracy of the burn severity map produced by using the dNBR thresholds developed by Key and Benson (2006) was first evaluated. We found very low agreement (0.15 < K < 0.21) between the burn severity class obtained from field-based indices (CBI and GeoCBI) and satellite-derived metrics (dNBR) from both Landsat-8 and Sentinel-2A. Therefore, the most appropriate dNBR thresholds were rebuilt by analyzing the relationships between two field-based (CBI and GeoCBI) and dNBR from Landsat-8 and Sentinel-2A. By regressing alternatively FBIs and dNBRs, a slightly stronger relationship between GeoCBI and dNBR metrics obtained from the Sentinel-2A remote sensor (R2 = 0.69) was found. The regressed dNBR thresholds showed moderately high classification accuracy (K = 0.77, OA = 83%) for Sentinel-2A, suggesting the appropriateness of dNBR-Sentinel 2A in assessing mixed-severity Mediterranean wildfires. Our results suggest that there is no single set of dNBR thresholds that are appropriate for all burnt biomes, especially for the low levels of burn severity, as biotic factors could affect satellite observations.

Highlights

Throughout the Mediterranean basin, the year 2017 was catastrophic due to huge wildfire damage in several Mediterranean countries [1]
The delta Normalized Burn Ratio (dNBR) map obtained from Sentinel 2A reflectance data provided moderate-high correlation with geometrically modified version CBI (GeoCBI) field-based estimates of burn severity and likewise provided moderate-high statistical accuracies in classifying burn severity levels, in the heterogeneous Mediterranean forest landscape in question
This accuracy implies that the combination of GeoCBI and Sentinel-2A dNBR may be suitable for the burnt Mediterranean vegetation communities studied

Summary

Introduction

Throughout the Mediterranean basin, the year 2017 was catastrophic due to huge wildfire damage in several Mediterranean countries [1]. This information can be summarized by a map of burn severity, where the term ‘severity’ is used to indicate holistically the magnitude of change, caused by aggregate effects of the fire on the vegetation community, with respect to the pre-fire condition [10]. Overall effects of fires are classified into first and second order, which correspond respectively to short- and long-term post-fire effects [14]. Both effects can be quantified in terms of burn severity [13,15]. For the purposes of this work, “burn severity” defines the degree of fire-induced changes, which in turn fits well with ecological post-fire restoration requirements of burnt forest areas [13,14]

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