Effects of human disturbance on composition and structure of Brachystegia woodland in Arabuko-Sokoke Forest, Kenya

We examined tree species diversity, density, dispersion patterns and size class distributions in Brachystegia woodland of Arabuko-Sokoke Forest, Kenya. The metrics varied with human disturbance (disturbed versus relatively undisturbed areas). Julbernardia magnistipulata Harms occurred only in the undisturbed site. Brachystegia spiciformis Benth. had the highest importance value (IV) at both study sites, whereas the IVs for the other tree species were greater within the undisturbed than the disturbed sites. Simpson's Diversity Index and tree densities were greater in the undisturbed site than in the disturbed site. All seven tree species exhibited random dispersions in the disturbed site, but three species were clumped in the undisturbed areas. Smaller individuals of B. spiciformis were over-represented in the disturbed habitat relative to the undisturbed habitat. In contrast, J. magnistipulata, Lannea schweinfurthii Engl. and Mimusops obtusifolia Wall. exhibited a greater proportion of smaller trees in the undisturbed site. The tree size class distributions may provide an index of regeneration for these four tree species in the disturbed and undisturbed sites respectively. Active management and restoration may be desirable for J. magnistipulata but appears unnecessary for the remaining common tree species. Illegal logging has persisted in the forest despite its conservation status over many decades. If logging activities cease, it would be instructive to document changes in vegetation composition and structure over time.