Scaling up restoration efforts by simulating the effects of fire to circumvent prescribed burns when preparing restoration sites in South African fynbos ecosystems.

Abstract

The method of clearing alien species and the nature of the soil seedbank influence the quality of restoration outcomes, particularly in fire-prone ecosystems heavily invaded with fire-adapted alien species. One of the challenges encountered is reducing the likelihood of reinvasions when the invading species are equally responsive to restoration treatments. By simulating the fire effects that are required to regenerate native vegetation, the study tested whether the recovery of the native species could be initiated without conducting a prescribed ecological burn. In a case study of South African Cape Flats Sand Fynbos with a heavy invasion of Acacia saligna, the felled acacia were stacked into brush piles, with the litter raked off from the sowing areas and the collected seeds were pre-treated for germination. Despite the lack of a fire, the sowing of pre-treated seeds on raked plots led to good recovery of native vegetation over time. This was indicated by the recovery of higher density, cover and richness of native species in sown plots compared with unsown treatments. However, the recovery of native species had not approached the vegetation structure comparable to a reference site after 2 years; that is, only partial fynbos structure was recovered. The recruitment of acacia was less dense without fire, as hypothesised, and independent of treatment. However, over time, control plots had higher acacia cover than other treatments. Despite this sparse recruitment of acacia, the acacia seedbank decreased naturally to about 50% of the initial size over 2 years in control plots. Raking off litter during site preparation removed 50% of the acacia seedbank which decreased slightly thereafter. Consequently, the residual acacia seedbank was relatively similar across treatments after 2 and a half years. In conclusion, circumventing prescribed burns in heavily degraded fynbos ecosystems is a scalable restoration strategy, as recruitment of alien acacia was minimized, its seedbank declined significantly, and good native cover developed after clearing and sowing.