Impacts of clearing invasive alien plants from 1995 to 2005 on vegetation structure, invasion intensity and ground cover in a temperate to subtropical riparian ecosystem

The impacts of invasive alien plants (IAP) and their subsequent clearing by the Working for Water Programme (WfW) on (a) overstorey (woody plant) vegetation structure, (b) invasion intensity (overstorey aerial cover of woody alien plants) and (c) ground cover, in a temperate to subtropical riparian ecosystem were studied in 1996/7 and again in 2005, in order to provide a longer-term perspective on the effectiveness of WfW clearing. Forty 1000 m2 plots were surveyed and resurveyed, comparing between (a) higher altitude Grassland and lower altitude Savanna, (b) high (> 50% invasion intensity) versus low (< 50% invasion intensity) alien invasion sites, and (c) WfW cleared versus uncleared sites (the three ‘treatments’). Pre-clearing estimates from cut stumps in 1996/7 indicated high alien invasion intensities of 72 ± 8% in Grassland and 69 ± 11% in Savanna. From 1996/7 to 2005 there was a large decrease in aerial cover of alien trees of > 5 m and to a lesser extent 2–5 m in height, and a large increase in alien plants of < 2 m. Hence WfW was initially successful, with the original tall Eucalyptus grandis tree layer largely removed. However, total invasion intensity remained unchanged over the first decade (30.4 ± 4.6% in 1996/7, 31.9 ± 3.2% in 2005). From 1996/7 to 2005, grass and herbaceous cover decreased, while bare soil and litter increased, indicating reduced surface stability. This was in response to (a) the major flood event of February 2000, (b) the effects of IAP invasions and (c) WfW clearing. Total ground vegetation cover was negatively related to alien aerial cover in both biome reaches in 1996/7 and 2005. By 2005, there were no longer any differences in the aerial cover of woody alien plants in response to the original 1996/7 invasion intensity or clearing ‘treatments’, and hence progressive homogenization of IAP cover. Aerial cover of woody indigenous plants also responded negatively to increasing alien aerial cover in 1996/7 and 2005 in both Savanna and Grassland. In conclusion, the nature of the IAP problem has changed from dealing largely with relatively few large E. grandis trees in the mid-90s, to the present large suite of invasive species with numerous smaller individuals. This has implications for the time needed for clearing. This is one of few studies to have assessed the initial and longer-term (1995–2005) effectiveness of WfW clearing operations. It shows that improved IAP clearing protocols are needed. More follow-up treatments are recommended to ‘capture’ alien resprouts and new seedlings before they establish and reproduce. Secondly, integrating clearing with restoration of the tall indigenous riparian canopy tree species in heavily invaded sites would help to shade out many alien recruits.