Some ecological side-effects of chemical and physical bush clearing in a southern African rangeland ecosystem

• Some ecological side-effects of bush thickening treatments were evaluated.
• No effect on non-targeted tree species was found 24–36 months post treatment.
• Both treatments were colonised by undesired sagebush Pechuel-loeschea leubnitziae.
• Perennial grass species were particularly abundant in physically treated sites.
• A side-effect of arborocide treatment: facilitating fast-growing, undesired species

Bush thickening is a major concern to farmers of arid and semi-arid rangelands; reactive intervention remains the norm. Here we compared some of the short-term ecological implications of chemical and physical removal of the bush encroacher Acacia mellifera in the central Highland savanna of Namibia. We selected 21 invaded sites, 7 had been chemically cleared, 7 had been physically cleared and 7 had never been cleared. From each site, we recorded grass species composition, as well as the densities of A. mellifera, the undesired perennial shrub Pechuel-loeschea leubnitziae and a non-targeted tree Acacia erioloba, 24–30 months post-treatment. We fitted one-way ANOVA models to test for the effect of treatment on grass species richness and density of targeted and non-targeted tree species. A canonical correspondence analysis was used to determine how treatment affected grass species composition. While both treatments reduced the density of the bush encroacher, P. leubnitziae effectively replaced A. mellifera in chemically treated sites, where die-back presumably happened faster and nutrient and water competition decreased more rapidly. In such sites, perennial grass species were effectively outcompeted by P. leubnitiziae.