Stacking and Burning Invasive Acacia saligna Trees in South African Fynbos: Can Native Plant Species Passively Recover?

The management of invasive taxa is crucial to mitigating their negative impacts and facilitating native biodiversity recovery. A common feature of clearing invasive trees in fire-prone systems is the 'stack burning' method. The intense and long duration of the heat experienced during burning, with areas at the centre of the stack reaching over 300 degrees C and 175 degrees C at the edge, often leaves persistent stack burn scars, limits native biodiversity recovery, facilitates secondary invasion, and alters soil biotic, chemical and physical conditions. Despite such negative impacts, stack burning remains a method for clearing invasive plants because it is an economical way to dispose plant biomass. Our study investigated the recovery of native species in 80 stack burn scars in lowland fynbos (Cape Flats Sand Fynbos) and mountain fynbos (Peninsula Sandstone Fynbos) of South Africa over 3 years after clearing. Our results indicate that the (1) native species recovery in stack burn scars was limited and (2) stack burn scars showed higher native species richness and cover in lowland than mountain fynbos. To encourage the recovery of native soil seed banks and biota in stack burn scars, active seeding and/or by covering them with topsoil is recommended.