Woody plant diversity along disturbance gradients in the northern afro-montane forests of the bale mountains, Ethiopia

The effects of human disturbances on woody plant species diversity was assessed by comparing species richness, Shannon diversity and evenness, population abundance and composition along three disturbance gradients in the northern Bale Mountains, Ethiopia. Data on woody plant species were collected in six forest patches along three levels of disturbances in 20 by 20 m-2 quadrates in each forest patches. Plant species richness peaked at the Moderately Disturbed (MD) site either when trees and shrubs were treated separately or pooled together, but similar between the Low disturbed (LD) and Heavily Disturbed (HD) sites. Overall population density of woody plants was also significantly higher in the MD site, followed by the LD site. The three site groups had distinct species assemblages. Species that contributed most to the differences between the MD and the others were shrubs, which were degradation tolerant; while the difference between LD and HD sites were due to degradation and/or local disappearance of some tree species in the HD site. This result suggest that the consequence of human disturbance on woody plant diversity appeared to be both positive and negative depending on the type and intensities of the disturbances. In comparison to the LD sites, disturbances such as high selective logging and grazing had resulted in increased richness and density of woody plants in the MD sites, while these and crop cultivation and settlement encroachments in the HD site resulted in decreased population abundances. However, the increased diversity in the moderately disturbed site was due to additions of shrub species, which have affinities to disturbed habitats. Since the central goal of conservation is to maintain maximum diversity of native species, but there is a potential of non-native species displacing the native ones in the long-run, and hence the high diversity reported here in the MD site should, therefore, be interpreted cautiously.