The effect of adjacent vegetation on fire severity in Afrotemperate forest along the southern Cape coast of South Africa

Abstract

Recently two large wildfires (around the towns of Knysna in 2017 and George in 2018) jointly burnt over 80 000 ha in the southern Cape, South Africa. Here, patches of closed-canopy Afrotemperate forest occur within a matrix of fire-prone (native) fynbos shrublands or adjacent to commercial plantations of invasive alien Pinus trees (a transformed state). We compared the distances that fire penetrated into forest margins adjacent to fynbos to those adjacent to plantations. Furthermore, we used a satellite-derived index of fire severity, the differenced Normalised Burn Ratio (dNBR), to compare fire severity in forest margins adjacent to fynbos to those adjacent to plantations. Results were inconsistent between the two fires. In the Knysna fire, no significant difference was noted in fire penetration into forest, while forest margins adjacent to plantations burnt at significantly lower severity than those adjacent to fynbos. In the George fire, penetration into and fire severity in forest margins adjacent to plantations were significantly greater than in those adjacent to fynbos. Not only were fire severity and penetration in forest margins with plantations adjacent dissimilar between the George and Knysna fires, but variability in fire severity was greater in forest margins with plantation adjacent than in those with fynbos adjacent. Our study provides evidence that fire severity in forest margins is associated with the adjacent vegetation type and that plantations potentially expose forests to greater variability in fire severity and extent than the forest may experience under natural conditions. Other important factors that may affect the severity and extent of burn in forest margins are forest patch size and topographical position, the nature of the ecotone between forest and the adjacent vegetation, and the fuel load and structure of the adjacent vegetation which varies considerably in relation to stand age and management practices in plantations.