Composition, structure, and regeneration patterns in a gallery forest along the Tana River near Bura, Kenya

This study used classification and ordination techniques to characterize the composition and distribution of woody vegetation along the Tana River floodplain near Bura in eastern Kenya. Results obtained from cluster analysis of tree and shrub vegetation corroborated the results from non-metric multidimensional scaling (NMS), separating the forests into seven fairly well-defined assemblages of species. The primary vegetation gradient summarized by the ordination was significantly correlated (0.257-0.394, p < 0.01) with soil texture and soil carbon at depths of 50-120 cm. The secondary vegetation gradient was significantly correlated (0.480-0.483, p < 0.01) with indicators of river flood regime. Measured environmental variables, however, only partially explained observed vegetation patterns. Many overstory species were well represented in the regeneration layer of low-lying point-bar and oxbow forests, but were poorly represented in the higher elevation levee forests. There were significant correlations (0.278-0.320, p < 0.01) between the first ordination axis for the regeneration layer and flood regime of the river, and between the second ordination axis and soil texture (0.321-0.346, p < 0.01) in the top 20 cm where seedling roots are likely to be settled. Spirostachys venenifera and Acacia elatior had the widest environmental tolerance, occurring in 78% and 60% of all sample plots, respectively. Populus ilicifolia had the narrowest environmental tolerance, occurring in less than 1% of plots, all located on point-bars.