Abstract
The study was conducted to assess the influences of edaphic and topographic variables on plant diversity on the slopes of Gra-kahsu forest area. Collection of vegetation data was made using systematic sampling methods, laying 19 transects and 62 quadrats, each with 20 m × 20 m for trees and 5 m × 5 m for shrubs. In each quadrat, heights (1.5 m), diameters (2.5cm) and numbers of woody species, level of grazing intensity, level of human disturbance and topographic variables were recorded. Analysis of one way using R-software was used to analyze the mean of plant diversity across the edaphic and topographic variables. The highest species richness and diversity appeared in the upper altitude gradients and the variation was significant (p<0.05). The nil and slightly grazed sites had significantly higher woody plant species diversity, density and basal area compared to the heavy grazed sites (p<0.001). Grazing pressure and human disturbance had an effect on the density and number of woody species as well as on the vegetation structure. This research concluded that, changes in woody plant vegetation, density and regeneration status are caused by interactions edaphic and topographic variables and these interactions determine the ecological of plant diversity. Key words: Altitudinal pattern, disturbance, plant species diversity, slope.  
Highlights
Patterns of plant species diversity are influenced by latitudinal, altitudinal and soil gradients (Huston, 1994)
Vegetation patterns are determined by environmental factors that exhibit heterogeneity over space and time, such as climate and topography as well as human disturbances
Upper altitude had significantly higher Shannon and Weaner diversity index value (2.2) than the other two-altitudinal gradients (Table 1). Such result with higher diversity in the upper altitude might be due to relatively favourable climatic, environmental condition and soil characteristics specially soil texture, soil organic matter (Desalegn and Carl, 2010) and slope (De Lafontaine and Houle, 2007)
Summary
Patterns of plant species diversity are influenced by latitudinal, altitudinal and soil gradients (Huston, 1994). Understanding vegetation and species diversity patterns is fundamental for conservation of natural areas; these patterns have frequently been the focus of ecological studies (Loreau et al, 2001; Fetene et al, 2006; Muhumuza and Byarugaba, 2009). Vegetation patterns are determined by environmental factors that exhibit heterogeneity over space and time, such as climate and topography as well as human disturbances Topography has climatic influence with respect to prevailing sunshine, air currents and water bodies. Altitude has a marked influence on the kind, nature and productivity of rangeland and it creates ecologically diverse vegetation (Herlocker, 1999)
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