What limits fire? An examination of drivers of burnt area in Southern Africa

TLDR

Fire extent in southern Africa was investigated using 500‑m satellite‑derived burned area maps and environmental data. The study aims to elucidate the physical, climatic, and human drivers of fire across southern Africa and to develop an initial predictive framework for burnt area. A random‑forest regression tree was applied to quantify the relative importance of environmental, climatic, and human factors on burned area. The model explained 68 % of the variance, with tree cover, recent rainfall, and rainfall seasonality as the strongest predictors, while human activities influenced burnt area only in certain climatic contexts and ignitions were not limiting, indicating that fuel load and moisture drive most spatial variation.

Abstract

Abstract The factors controlling the extent of fire in Africa south of the equator were investigated using moderate resolution (500 m) satellite‐derived burned area maps and spatial data on the environmental factors thought to affect burnt area. A random forest regression tree procedure was used to determine the relative importance of each factor in explaining the burned area fraction and to address hypotheses concerned with human and climatic influences on the drivers of burnt area. The model explained 68% of the variance in burnt area. Tree cover, rainfall in the previous 2 years, and rainfall seasonality were the most important predictors. Human activities – represented by grazing, roads per unit area, population density, and cultivation fraction – were also shown to affect burnt area, but only in parts of the continent with specific climatic conditions, and often in ways counter to the prevailing wisdom that more human activity leads to more fire. The analysis found no indication that ignitions were limiting total burnt area on the continent, and most of the spatial variation was due to variation in fuel load and moisture. Split conditions from the regression tree identified (i) low rainfall regions, where fire is rare; (ii) regions where fire is under human control; and (iii) higher rainfall regions where burnt area is determined by rainfall seasonality. This study provides insights into the physical, climatic, and human drivers of fire and their relative importance across southern Africa, and represents the beginnings of a predictive framework for burnt area.