The effect of changes in human drivers on the fire regimes of South African grassland and savanna environments over the past 100 years

Fire is a key regulator of tree cover in grassy ecosystems, but century-long changes in fire regimes have not been explicitly quantified in South Africa. This study aimed to determine changes in the fire regimes of South Africa’s grassy biomes over the past century in response to widespread human-induced changes. Using spatially explicit data, we employed random forest models to examine the importance of climatic and human drivers on the extent, frequency and intensity of fire. We then modelled how fire regimes are likely to have changed over the past century using historical trends in key human drivers (population growth, livestock density, road density and land cover change) as inputs. Results showed that human drivers, particularly land transformation and human population density, were important determinants of fire regime. Random forest predictions showed a decline in burnt area (mean of 10.5% in 1910 to 5.1% in 2020), and a shift to fire regimes with smaller burnt areas, longer fire return intervals and lower intensity fires over the past century, particularly in the mesic east. These dynamics have important implications for our understanding of the widespread increases in woody cover observed in the region over this period.