Delaying a prescribed burn to scale up the restoration of alien-invaded Lowland Sand Fynbos in South Africa

Abstract

Fire-adapted species invading fire-prone ecosystems in Mediterranean climate regions are difficult to control because they are equally responsive to restoration treatments. We assessed the efficacy of delaying a prescribed burn to promote the recruitment of native species in a Lowland Sand Fynbos ecosystem invaded by alien Acacia saligna. Acacia stands were felled and fallowed for 2 years before burning the slash and sowing pre-treated fynbos seeds. We hypothesized that sowing pre-treated fynbos seeds after fallowing cleared areas before burning would improve the regeneration of native species. Fallowing cleared areas would reduce the density of acacia seeds and post-fire acacia recruitment while allowing felled acacia biomass to dry and decay over time leading to improved prescribed burn properties. Sowing pre-treated fynbos seeds improved the recovery of native cover but had little effect on the recovery of native species diversity after 2 years. Although the rapid recruitment of annual native species did not affect acacia density, it might have contributed to reduced post-fire acacia cover and allowed the native perennial species to establish. After 2 years of fallowing cleared areas, the density of acacia seeds and post-fire acacia recruitment was reduced by ≈50% and 80% respectively. Although too dense to be outcompeted by recruited native species alone, this reduced acacia recruitment helped to decrease and delay the need for follow-up acacia clearing as it was where burning occurred soon after felling acacia stands. The recruited native species managed to establish before the acacia emergents were removed unlike where burning occurs soon after felling and where native species were outcompeted resulting in the need for follow-up sowing. In conclusion, building resilience against acacia resurgence by increasing the pool of sown species to recover a good cover of native species in combination with the timely removal of acacia recruits can help to scale up restoration.