Acacia mearnsii is an invasive alien plant in riparian fynbos habitats in the Western Cape of South Africa. This study focused on the difference in success of native plant species in invaded and cleared reaches of the Holsloot River in the Western Cape. The South African government sponsors a conservation program named Working for Water in order to clear A. mearnsii to facilitate native plant recovery. Two reaches were compared, one that had been recently cleared by Working for Water, and one that was invaded following flood disturbance in 2013. Previous studies suggest that water use by A. mearnsii may facilitate rapid growth resulting in reduced light availability to the slower growing native plants. We predicted that A. mearnsii would be taller than individuals of native species, that native species density would be inversely correlated with density of A. mearnsii, and that light levels would be lower in invaded vs cleared reaches. We established seven 100 m transects and recorded density, height and diameter of A. mearnsii and native tree species in cleared and invaded reaches in 50 4m2 plots along each transect.
Densities of native species were generally higher in the invaded area relative to the cleared area despite the presence of A. mearnsii. There was a negative correlation between A. mearnsii density and density of native species in the invaded area. The height of A. mearnsii was not correlated with lower densities of native species. Light levels were much higher in the cleared reach. The results of this study suggest that A. mearnsii reduces light availability for native species and that increased numbers of A. mearnsii are negatively correlated with native species density; however, these results also suggest that the geomorphology of the river may have more of an effect on the native plant species density than invasion. The cleared reach was adjacent to a narrow channel with a narrow floodplain, and the invaded area was in a broad floodplain. Future studies should consider the influence of fluvial geomorphology as a factor in determining the extent to which invasion influences the density of native species.