Verification of the differenced Normalised Burn Ratio (dNBR) as an index of fire severity in Afrotemperate Forest

Abstract

Temperate evergreen broad-leaved forest seldom experiences fire, and little is known about the severity at which they burn. During October-November 2018, 70 000 ha of fynbos, thicket, Afrotemperate forest and other lesser vegetation types burnt in the southern Cape of South Africa. This study set out to verify through field observations the accuracy of the differenced Normalised Burn Ratio (dNBR) as an index of fire severity, derived from Sentinel 2 images, in Afrotemperate forest. Stem fire severity and char height were observed for 1 648 trees in 87 plots and linear regression used to test whether these observed measures were related to dNBR. Stem fire severity and char height were collinear, and the latter dropped from further analysis. The strong linear relationship between dNBR and stem fire severity (r2 = 0.69, p < 0.001) confirmed the accuracy of dNBR as a measure of fire severity in Afrotemperate forest. The verification of dNBR enables the mapping and characterization of fire severity in Afrotemperate forest. In the fire investigated, a total of 4 628 ha of Afrotemperate forest burnt of which 67 % burnt at low severity, 21 % at medium severity and 12 % at high severity. Such information on the distribution of fire severity facilitates investigation of further questions regarding past and future fire regimes and the fire ecology of Afrotemperate forest.