The Ecology and Biogeography of the Endulen-Laetoli Area in Northern Tanzania

We have mapped the vegetational variability of the Endulen-Laetoli area of northern Tanzania based on the premise that heterogeneity at the landscape level is a function of differences in geology, soils and topography. Vegetation habitats have been described in relation to these factors, particularly soils and topography, and the effects of climate changes are estimated in relation to these other factors. We have made voucher collections of the plants in the Laetoli region, an area of about 250 km2, and we have recorded distributions of plants by habitat, climate, soil and topography. Vegetation structure has been measured in terms of canopy cover, species richness, and canopy height. Sixteen species associations and structural types have been identified within the study area, and these have been mapped at a scale of 1:120,000. The whole area has been much disturbed by human activity, with large areas of weed growth resulting from cultivation, burning and overgrazing. Results show that areas of low relief have soils with impeded drainage or seasonal waterlogging and dense Acacia drepanolobium woodland, low stature when disturbed by human action, high when not. Shallow brown soils on volcanic lavas have five woodland associations, two dominated by Acacia species, two by Combretum-Albizia species, while a fifth to the east on volcanic soils has a different woodland association with Croton-Dombeya-Albizia species. Elevated land to the east, also on volcanic soils, has two associations of ­montane-edge species, one with Croton-Celtis-Lepidotrichilia, and the other with Acacia lahai, grading into the eastern highlands montane forest at 2,750 m. Seasonal water channels ­flowing from east to west have three Acacia riverine woodland associations. Three deep valleys to the north of the area have dense riverine woodland with Celtis, Albizia, Euclea, Combretum, and Acacia spp. Emergence of springs at Endulen feed a perennial stream with closed gallery forest with Ficus-Croton-Lepidotrichilia. Finally, recent ash falls from Oldonyo Lengai have produced localized immature alkaline soils with calcrete formation and short grass vegetation. Projections of increased or decreased rainfall and how this may potentially have affected the different environments form the basis for predictions about vegetation and habitat change.