The distribution of C3 and C4 grasses and carbon isotope discrimination along an altitudinal and moisture gradient in Kenya

More than 500 species of the Poaceae are found in Kenya, East Africa. Eighteen of twenty-seven tribes are exclusively (except the Paniceae and Danthonieae) of the C₃ photosynthetic type. A floristic analysis of low altitude grasslands suggests that nearly all species at these low altitudes are of the C₄ photosynthetic type. At high altitudes, however, nearly all grasses are of the C₃ photosynthetic type. Open grassland vegetation was sampled along a transect from arid low altitude sites to the top of Mt. Kenya in an attempt to document the general distributions of the photosynthetic types. The major tribes illustrated three general patterns of distribution. The C₄ tribes Chlorideae, Eragrosteae, Sporoboleae, and Aristideae were abundant at low altitudes (or low indices of available soil moisture). The Paniceae and Andropogoneae were also exclusively C₄ but were more common at intermediate altitudes. The C₃ tribes Aveneae, Festuceae, and Agrostideae were found only at high altitudes. In these open grasslands there were no C₃ species below 2,000 m and no C₄ species above 3,000 m. The variation in δ₁₃C of the live grass vegetation with altitude confirms these distributional patterns and suggests a sharp transition zone between these two photosynthetic types. The photosynthetic type accounts for broad distributions within the Poaceae but these distributions are further modified by characteristics which may be inherent in the tribal groups. Ecological and paleoecological significance of these patterns of distribution are discussed.