SPATIAL AND TEMPORAL VEGETATION CHANGES ALONG A MOISTURE GRADIENT IN NORTHEASTERN SUDAN

Vegetation changes in Sahel have been pardy explained by overgrazing. Some authors have queried the importance of herbivory as a cause of succession in arid areas, but this requires a differentiation between life-forms. In this study abundances of phanerophytes and chamaephytes were estimated on 30 hills on an arid mountain plateau in eastern Sahel, influenced by mist from the Red Sea. Their relative abundances were compared with a similar study in 1953 when the area was partly protected against browsing. Significant spatial and temporal trends were found by ordination techniques. Succession from an open woodland to a very open shrubland was revealed. Hardwood trees suitable for fuel and other plants of direct commercial value decreased. Plants with small, sclerophyllous leaves, and/or toxic chemicals increased. The changes are attributed to an increased human population and intensified browsing. These biotic factors interacted with drought periods. Species that also grow in the surrounding Sahelian transition zone were relatively more stable than those with a disjunct distribution in the region, which occur in die area of more moist regional climate found in the Ethiopian highland.