GRADIENTS IN FIELD-LAYER VEGETATION ON AN ARID MISTY MOUNTAIN PLATEAU IN THE SUDAN

Earlier studies have described how moist, on-shore winds cause meso-scale vegetation patterns on arid mountains near the sea. However, all protruding objects such as trees, micro-relief, and hill slopes influence the distribution of sea-mist. The influence of the tree-canopy, aspect, and distance to the sea on the field-layer vegetation in montane savanna was investigated on 16 hills in the Red Sea Hills, at 34 - 38 km from the sea. At 32 sites, total field-layer cover, species cover, and species number were estimated in a sub-canopy plot and in a nearby open plot on seaward and leeward slopes. Cover and species number in the understorey are significantly higher than in the open. The difference is highest on seaward slopes. Detrended correspondence analysis reveals short species-axes of ca. 2 SD-units. Differences between plots are mainly in species cover. This fits a principal components ordination model. PCA and its constrained version RDA give concordant results. The explanatory variables, Tree-cover and Relative Radiation Index (aspect), have similar indirect influences on plants, and are significantly correlated with axis 1, which is interpreted as a moisture and temperature gradient. The moist seaward plots show an independent trend in species composition along axis 2, which correlates with distance to the sea. On a presence basis the variables, all representing different spatial separation, correlate on the first axis. Presumably, the species composition, at all spatial scales, is directly or indirectly related to the variation in temperature and moisture.