Fire regimes and management options in mixed grassland-fynbos vegetation, South Africa

Abstract

Fire regimes in South African fynbos shrublands have been quantified in the western (winter rainfall) and central (aseasonal rainfall) parts. They have not been quantified at their eastern extremity (summer rainfall), where fynbos transitions to grassland and is embedded in other fire-resistant vegetation types. We assessed fire regimes at the eastern extremity of the fynbos biome and discussed the ecological and management implications of our findings. Using remote sensing, we assessed fires over 21 years in a 350,000-ha study area at the eastern edge of the fynbos biome. Only 9% of the study area burned, almost exclusively in mixed grassland-fynbos vegetation. The largest proportion (44% of the total area burnt) burnt in late winter and early spring, but fires occurred throughout the year. Fires only occurred in 39% of the mixed grassland-fynbos vegetation. Of the mixed grassland-fynbos vegetation that did burn, 44%, 11%, 26%, and 19% experienced one, two, three, and four fires, respectively. Areas with multiple fires had return intervals ranging from 3 to 15 years, with 76% of the area with multiple fires burning every 4 to 6 years. Most large fires were preceded by two relatively dry months, when fuels were drier and more flammable. The mixed grassland-fynbos vegetation at the eastern extremity of the fynbos biome is surrounded by fire-resistant vegetation and exhibits a varied fire regime, with different parts experiencing frequent, infrequent, or no fire over 21 years. Fires were largely aseasonal, occurring throughout the year, although data over more years may reveal that a greater proportion of fires will occur in winter, when grasses are cured and rainfall is low, as well as after relatively long (> 2 months) periods of below-average rainfall. As most fires would burn out safely against fire-resistant vegetation, we conclude that lightning-ignited fires should be allowed to spread unhindered to approximate a natural and heterogeneous fire regime, provided that no neighboring properties or infrastructure are threatened. The response of the biota to fires is not well understood for this transition area, and further research is needed.