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

Abstract

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 <1.0 represents a lesser threat). We considered the future responses of invasive species by increasing the age of enclosures. We identified five vegetation states and ten transitions described in terms of densities of mature trees, saplings and seedlings, bush cover, woody species composition, and the IWSR. Greater densities of invasive woody species and higher IWSR were found in the enclosures than in the open grazed system, but this was not true for bush cover. Doubling the age of enclosures had a negative impact on the majority of invasive woody species, the decline probably being related to self-thinning mortality. The evidence that regeneration of invasive woody species is nonlinear in relation to age chronosequence suggests that bush encroachment is probably controlled more by stochastic rainfall events than by livestock grazing alone. Vegetation states differed in terms of tree density, IWSR and species composition, suggesting that the threat of invasive woody species is greater in the enclosures than in the open grazed areas.