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.

火灾是草地生态系统中树木覆盖的关键调节因子，但南非尚未明确量化长达一个世纪的火灾制度变化。本研究旨在确定过去一个世纪南非草地生物群落的火情变化，以应对广泛的人为引起的变化。使用空间明确的数据，我们采用随机森林模型来检查气候和人类驱动因素对火灾范围、频率和强度的重要性。然后，我们使用关键人类驱动因素（人口增长、牲畜密度、道路密度和土地覆盖变化）的历史趋势作为输入，模拟了过去一个世纪火灾状况可能发生的变化。结果表明，人类驱动因素，特别是土地转变和人口密度，是火灾状态的重要决定因素。随机森林预测显示，在过去的一个世纪里，烧毁面积下降（1910 年平均下降 10.5% 至 2020 年 5.1%），并转向燃烧面积更小、火灾返回间隔更长、火灾强度更低的火灾制度，特别是在东方。这些动态对于我们理解这一时期在该地区观察到的木质覆盖的广泛增加具有重要意义。