Population structure and regeneration of the main tree species in the acacia woodlands of the Rift Valley of Ethiopia

The goal of this study was to characterise the tree population structure under different land use practices in the acacia woodlands of the Rift Valley of Ethiopia. Topics addressed are the species composition, the height and diameter distributions, the number of trees and the regeneration of major tree species. The relationship between the occurrence of regeneration and population characteristics such as number of trees, basal area and crown closure was studied. Results indicate that within the study area the numbers of seedlings (height <150 cm), saplings (> 150 cm height and diameter < 50 mm) and mature trees (> 150 cm height and diameter > 50 mm) are 98, 278 and 63 ha'1respectively. Acacia tortilis was found to have the highest number of seedlings (65 ha"1) and mature trees (33 ha'1). The highest number of saplings was from A. senegal with 139 individuals per hectare. The diameter distribution was J-shaped with most of the trees less than 30 cm in diameter. The arithmetic average height of trees (> 50 mm in diameter) was found to be around 6 m. With reference to regeneration, most of the seedlings/saplings were found to be concentrated in a few parts in grazing lands with relatively well stocked sites. A positive partial relationship (p = 0.07) between number of mature trees and occurrence of regeneration was found. Given a certain number of trees, crown closure was found to have a negative partial relationship (p = 0.06). With regard to land use, the categories "open grazing" and "cultivated land" were foundto have a significantly lower number of seedlings. The relatively low frequency of mature trees, with very few trees remaining in the upper diameter classes, the concentration of seedlings to limited parts of the study area, and the low species diversity are some of the indicators of the degradation that has occurred over the past decades. Evidence implicates uncontrolled grazing, browsing, and tree felling for commercial fuelwood and charcoal as the major causes of the degradation.