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

Abstract

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.