Impacts of alien plant clearance on vegetation in the mountain catchments of the Western Cape

SYNOPSIS Monitoring plots were established in two mountain catchment areas in the Western Cape to assess the impacts of dense alien plant stands and alien plant clearance on indigenous Fynbos vegetation. In both areas Pinus pinaster was the dominant alien species with Hakea sericea also common. Indigenous vegetation persisted in the understorey of invaded stands, indicating that the high density of aliens was of recent development. Species richness of indigenous vegetation was lower in invaded, compared to uninvaded plots. After clearance by felling and burning, the aliens were effectively controlled, except for pines at Genadendal, where unfelled, older generation pines released seeds to recruit in the post-fire environment. The post-fire recovery of indigenous vegetation at both sites was rapid, with no significant differences measured in projected canopy cover, plant density and functional guild richness between invaded and uninvaded control plots. This improved condition of the indigenous vegetation was reflected in an increase in community similarity between invaded and uninvaded plots after fire. However, species richness and diversity remained lower for invaded plots compared to controls two and a half years post-fire and guild structure also differed between treatments. Clearance of serotinous alien species by felling and burning is an effective method for controlling the aliens and initiating Fynbos recovery in relatively young vegetation (8 years in this study). It is recommended that alien clearance teams are coordinated to reduce time lags between the clearance of younger generation aliens and older established or inaccessible trees. Biological control should be developed, where not already available, to lower the re-invasion risk from clearance escapes. At sites where soil damage may result from fire through heavy slash (e.g, older vegetation), steps should be taken to minimize this risk and contingency plans developed to prevent soil erosion and accelerate post-fire vegetation recovery.