The Western Woodland of Ethiopia:A study of the woody vegetation and flora between the Ethiopian Highlands and the lowlands of the Nile Valley in the Sudan and South Sudan

The western woodlands of Ethiopia have been less studied than the highlands, with a first description published in 1940. Based on fieldwork and herbarium studies we intend to increase the knowledge of these woodlands. Some authors describe them as one continuous zone from south to north, spanning ca. 10° latitude and dominated by species of Combretum and Terminalia but sometimes with various names applied to the vegetation at its limits. The Atlas of the Potential Vegetation of Ethiopia (2010) followed the continuous model. In the Sudan and South Sudan, the same vegetation is described as distinct types defined on soil and rainfall. Reviewing species composition, environment, research-history and human influence, we present our own observations of the woody species and map their distribution in Ethiopia, review their ecological adaptations and model the total distribution in Africa of twelve characteristic species. We use our data for clustering and principal component analyses to study continuity and discontinuity of the vegetation and the drivers of variation. The appearance of the western woodlands is variable, and cluster analyses indicate that a high number of small and narrowly defined units might be identified. A significant number of trees are widespread from north to south, but some are restricted to the north, a few to the centre and south, and many just enter the woodlands with a main distribution elsewhere. At a high level, our analysis defines two or three weakly defined clusters, (1) the northern woodlands to just south of the Abay River and continuing far south as a brim along the western escarpment of Ethiopia, (2) woodlands south of the Abay at altitudes above cluster no. (1), and (3) dry woodlands of the upper Tacazze and Abay Rivers. Many environmental factors explain small but important parts of the variation. The clusters relate to variables such as latitude, altitude, climate and soil types, while slope, fire frequency and other parameters are