Effects of management and time on mechanisms of bush encroachment in southern Ethiopia

Bush encroachment poses a threat to livestock management in southern Ethiopia. Despite this, the mechanisms of bush encroachment in response to protection and the time of protection compared with continuous grazing are rarely investigated. In this study, we used the state-and-transitional model (STT) to investigate the mechanisms of bush encroachment using six traditional enclosures along age chronosequences of 12-14, 17-24 and 26-30 years compared with open grazed system immediately adjacent to each enclosure (as control). The woody vegetation was sampled by randomly locating 10 plots (5 x 5 m) in each of the six enclosures and the adjacent open grazed areas. We compared the enclosure management with the open grazed system in terms of densities of mature trees, saplings and seedlings, bush cover, woody species composition, and the invasive woody species ratio (IWSR). The IWSR is the ratio of density of invasive woody species to non-invasive woody species (i.e. a ratio of >1.0 represents a high invasive threat, while