The influence of woody plants and livestock grazing on grass species composition, yield and soil nutrients in the Middle Awash Valley of Ethiopia

Abstract

The objectives of the study, conducted during the 2000/01 growing season in the Middle Awash Valley of Ethiopia, were to quantify the effects of two dominant woody species (Acacia tortilis and Balanites aegyptica) on grass species composition, grass dry matter (DM) yield and soil under light, medium and heavy grazing. At each study site two sub-habitats were distinguished, viz. under tree canopies and open grassland. The nutrient status of soil under the canopies of both woody species was found to be higher than the open grassland, especially with regard to soil pH, total N and organic carbon. This was more pronounced in the top 50mm of soil and on the lightly grazed site. Due to the observed association of Panicum maximum with tree canopies, the grass DM yield was higher under tree canopies, but again only at the lightly grazed site. Heavy grazing proved to be a strong overriding effect of the positive influences of the woody plants. The grass species found at the heavily grazed site were mostly annuals and less desirable. This emphasizes the importance of conservative stocking rates and proper pasture management.