Secondary invasion after clearing invasive Acacia saligna in the South African fynbos

Abstract

It is often assumed that clearing invasive alien species will lead to the dissipation of their negative impacts and recovery of native plant diversity. However, this is often not the case because clearing of primary invasive alien species can lead to secondary invasion by non-target species. We investigated the effects of vegetation type and application of fire during management of biomass after clearing invasive acacias on secondary invasion in the South African fynbos. Furthermore, we determined how these effects change with years after clearing. We sampled vegetation in lowland and mountain fynbos cleared of invasive Acacia saligna using the “fell, stack and burn” method. During burning of the stacked slash, the area at the centre of the stack experiences a high severity fire while the area at the edge experiences a low severity fire. After fire, burn scars remain in place of the stacked slash. We sampled in and outside of 80 burn scars over three years after clearing. Overall, we set out to (1) identify species that are secondary invaders; (2) determine whether secondary invader richness and cover differ between where there were high and low severity fires and no fires, and how these differences change with years after clearing; and (3) determine whether secondary invader richness and cover differ in and between lowland and mountain fynbos, and how these differences change with years after clearing. We identified 32 secondary invader species. Mean secondary invader richness was lower where there were high severity fires (2.75) compared to where there were low severity fires (3.28) and no fires (3.24). Mean secondary invader proportion cover was lower where there were no fires (0.14) compared to where there were high severity fires (0.19) and low severity fires (0.2). Three years after clearing, secondary invader richness and cover had not changed or was now higher than in the first year, while secondary invader richness was similar between lowland and mountain fynbos. Secondary invader cover was similar between lowland and mountain fynbos up to two years after clearing but was 58% lower in lowland fynbos in the third year. Fire application after clearing invasive acacias can have positive (i.e. reduction of Acacia soil seed banks by triggering mass germination) and negative (i.e. favors the dominance of secondary invaders) effects. As a result, slash should be spread throughout the restoration site instead of being stacked and then burnt to reduce Acacia soil seed banks. To avoid the establishment of a second generation of invasive acacias, the seedlings that germinate can be controlled through manual weeding, mowing and herbicide application. Due to the persistence and abundance of secondary invaders up to three years after clearing at levels similar to or higher than in the first year, we conclude that practicing restoration ecologists must manage these species to ensure successful restoration of native plant diversity.