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

N.S Haussmann,J.M Kalwij,S Bezuidenhout

doi:10.1016/j.sajb.2015.07.012

Abstract

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, 24–30months 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.

Highlights

IntroductionBush thickening forms a major agricultural concern in arid savanna ecosystems (de Klerk, 2004; Ward, 2005; Wiegand et al, 2006), where the associated suppression of palatable grasses, formation of impenetrable thickets and consequent reductions in carrying capacity have dire economic consequences (Bovey, 2001; de Klerk, 2004)
Grass species richness was affected by treatment (F2,18 = 7.933, p = 0.003), but only sites where A. mellifera was physically removed showed a significantly higher richness per plot (Fig. 2D)
This increase in grass species richness for physically treated sites was due to an increase of perennial species, while there was no change in annual species richness (Fig. 2E and F)

Summary

Introduction

Bush thickening forms a major agricultural concern in arid savanna ecosystems (de Klerk, 2004; Ward, 2005; Wiegand et al, 2006), where the associated suppression of palatable grasses, formation of impenetrable thickets and consequent reductions in carrying capacity have dire economic consequences (Bovey, 2001; de Klerk, 2004). Vahl) Benth (synonym: Senegalia mellifera (Benth.) Seigler & Ebinger). This deciduous species has an extensive root system, concentrated in the top of the soil profile, and is considered a pioneer on shallow soil (Adams, 1967). Whereas manual hacking is labour intensive and slow, there are ecological concerns associated with the use of arborocides. These concerns are mostly related to their environmental persistence and effects on non-target species (Bovey, 2001; du Toit and Sekwadi, 2012; Emmerich, 1985)

Methods

Results

Discussion

Conclusion