Fynbos vegetation recovery twelve years after removal of invasive Eucalyptus trees

Abstract

Alien plant invasions are increasing in extent worldwide and diverse measures are being applied to reduce impacts. Besides removing the invasive plants, follow-up measures are often needed to restore diversity and functionality of invaded ecosystems. Effective restoration requires long-term commitment and monitoring to determine the success achieved through different measures.In 2019 we resurveyed a site in South Africa's Cape Floristic Region (CFR) which had been used as a Eucalyptus plantation, which consisted of a heterogenous mixture of Eucalyptus conferruminata, E. cladocalyx and E. gomphocephala interspersed with Acacia saligna and Paraserianthes lophantha. It was restored twelve years prior to our survey using “active” (clearing followed by sowing of native species and soil restoration treatments) and “passive” (clearing and burning, only) approaches in separate plots. The last survey that monitored recovery took place in 2010. The aim of our study was to determine how the restored fynbos vegetation had changed since 2010. In 2019 we conducted a vegetation survey on the restored sites and on a reference site with native fynbos vegetation. We tested whether invasive species had been successfully controlled and whether active and passive restoration approaches differed in effectiveness.Although diversity, evenness and species richness for native taxa had increased significantly since 2010, the decrease in alien species cover was insignificant. Compared to the restored site, the reference site had a significantly greater cover of native plants, and higher species richness, diversity and evenness. There were also significantly more alien species with a significantly greater cover on the restored site, implying that further attention is required to maintain the site free of invaders. The passively restored plots were more diverse than the actively restored plots, largely due to the dominance of one of the sown native species (Searsia laevigata) on the actively restored plots. The vegetation in passively restored plots was dominated by weedy native grasses.We conclude that the restoration was mostly successful in terms of re-establishing native species diversity, but that there is room for improving restoration techniques. The tenacious invader A. saligna still occurs on restored sites and ongoing efforts will be needed to eliminate this species.