The effectiveness of active restoration following alien clearance in fynbos riparian zones and resilience of treatments to fire

In 1998, a restoration field trial was initiated in a catchment near Wellington (Western Cape, South Africa) to determine whether fynbos riparian scrub vegetation cleared of woody invasive alien trees require post-clearance restoration actions to accelerate indigenous vegetation recovery. The aim was to assess the relative effectiveness of three sowing treatments for restoring indigenous vegetation cover after the widely used “Fell & Burn” method of clearing invasive alien trees. Sowing treatments included non-invasive alien grasses to determine whether they have a negative effect on recovering native vegetation. A summer fire, eight years after trial initiation, provided an opportunity to determine how resilient restoration treatments are to alien re-invasion and fire. Restoring the site after alien clearing by sowing indigenous seeds increased both diversity, by improving species presence and abundance. However, a census done 8 years later (in 2006) revealed that seedlings of woody invasive alien plants dominated all plots, and had also survived the burn by resprouting, indicating the importance of follow-up control to justify initial clearing and restoration costs. Indigenous grass density was significantly reduced in plots where alien grasses were sown, while in the control and fynbos sowing treatment, indigenous grass density increased. By 2006, alien grass density was negligible in all treatments, indicating that the two grass species sown are not persistent or invasive. Active restoration of riparian areas after alien plant clearing has potential to facilitate vegetation recovery, but must be coupled with a long-term plan for adequate follow-up removal of post-clearance and post-fire alien recruits.