Vegetation structure of a heavily grazed range in northern Kenya: tree and shrub canopy

The structure of the shrub and tree layers of a semi-arid rangeland in northern Kenya was investigated. A physiognomic classification revealed four categories: woodland, dwarf shrub bushland, shrubland and thickets. Woodland can only develop with subsidiary water supply along channels and permanent rivers. The differentiation of bushland and shrubland is correlated with soil properties. While the former covers loamy and sandy sites, the latter is confined to clay soils which are characterised by low infiltration rates. Thicket formation especially occurs on eroded valley sediments. The bushland is characterised byAcacia tortilis, Commiphora africana, Grewiaspecies and dwarf shrubs such asLippia carviodoraandVernonia cinerascens. There are indications that the present high cover ofAcacia tortilisis a result of an increase in herbivore pressure during recent years. Similar conclusions are drawn in the case of thickets which are mainly composed ofAcacia horridaandA. reficiens. In contrast, there is no evidence for an impact of livestock on the structure of the shrubland which is characterised byAcacia mellifera, A. paoliiand several species of the Capparaceae family. Exotic trees or shrubs have not invaded. Compared to the ground layer which is analysed in a subsequent paper, the effects of livestock on the bush and tree layer are low.