Encroachment of Acacia brevispica and Acacia drepanolobium in Semi-Arid Rangelands of Ethiopia and their Influence on Sub-Canopy Grasses

摘要

This study was conducted in the semi-arid Borana rangelands of Ethiopia and focused on the distribution pattern of Acacia brevispica and Acacia drepanolobium and their influence on sub-canopy grasses. Both species had significantly greater total densities on communal lands than on a government ranch and on the nearest site than on the furthest site from water points. A total of 23 grass species were identified in the sub-canopy and open habitats surrounding A. brevispica and A. drepanolobium. Cenchrus ciliaris and Chrysopogon aucheri were dominant species surrounding A. drepanolobium in both habitats. For grasslands surrounding A. brevispica, Themeda triandra was the dominant sub-canopy grass species, while C. aucheri, Panicum turgidum and Loudetiaflavida dominated open habitats. Sub-canopy habitats in both species had significantly higher yields of total, highly and intermediately desirable grasses than open habitats. Although A. brevispica and A. drepanolobium have encroached due to prolonged heavy grazing, they did not negatively impact on sub-canopy grass productivity and, therefore, their control should be considered with caution. Future research is required to examine if changes in total tree density or cover may alter results of this study. Research is also needed on determinants of changes in sub-canopy grass productivity.