Local distribution of C3 and C4 grasses in sites of overlap on Mount Kenya.

A zone of C3-C4 grass overlap has been identified on the northern slopes of Mount Kenya at elevations of 2,800-3,200 m. At higher elevations, C3 grasses predominate; below, C4 grasses predominate. At each of three elevational sites of overlap, a series of transects was surveyed for C4 and C3 grass composition. All transects were located in structurally similar vegetation (high elevation grassland) within 150 m of each other. Transects were run in a variety of topographic situations. Within each transect, several environmental parameters were measured. Both unweighted (species presence/absence) and weighted (by frequency) measures of %C4 composition were significantly correlated with dry season soil moisture. Drier transects contain significantly more C4 species, in greater frequency, than wetter transects. Other correlations of C4 composition (e.g. factors which tend to parallel soil moisture) were also found. This local trend parallels the larger scale tendency of C3 grasses to replace C4 grasses with increasing elevation. Finer scale habitat partitioning may account for C3-C4 overlap within transects.