Vegetation structure of a heavily grazed range in Northern Kenya: ground vegetation

Classification of the herbacous vegetation of a heavily grazed rangeland in northern Kenya revealed four communities and a complex of extremely disturbed vegetation units. According to the ordination of relevés, two of these communities, theChrysopogon–Hibiscusand theSporobolus–Indigoferaassociation, represent a continuum that graded from stone covered and moderately grazed sites to smooth and heavily grazed pastures. From these, aSetaria acromelaena–Digera muricataassociation and anAchyranthes–Commelinaassociation were distinctly separated. Whereas the former reflects moderately grazed pastures on clay soils, the latter occurs in shaded livestock resting places and in gallery woods. Finally, the relevés of the extremely disturbed vegetation originate from cattle tracks, watering places and livestock enclosures. On light-textured soils, herbivore pressure caused a change in dominance among perennials and an increase of annual grasses. The bunch grassChrysopogon plumulosuswas replaced by the hemicryptophytic grassSporobolus nervosusand the dwarf shrubIndigofera spinosa. Annual grasses, such asTetrapogon cenriformis, increased in cover and frequency, or invaded i.e.Tragus berteronianus. On clay soils the perennial grasses were completely replaced bySetaria acromelaenaand annual forbs. The latter achieved dominance on extremely disturbed and heavily defecated sites. Here, many of the therophytes are widespread weeds or species found on roadsides and waste grounds. The herb layer was far more sensitive to herbivore pressure than the shrub and the tree layer investigated in a previous paper.