A study of altitudinal zonation in the montane forest belt of Mt. Elgon,Kenya/Uganda

Altitudinal changes in the vegetation of the Montane Forest Belt of Mt. Elgon, East Africa, were investigated by analysis of aerial photographs and by sample plots along two transects, one on a relatively wet, and the other on a relatively dry aspect.Classifications for both tree and herb/shrub plots show that the forests along the two transects become increasingly similar floristically with altitude. The total number of tree species recorded is greater for the transect on the moister aspect. The mean altitudinal range for tree species increases on both transects with altitude, while that for herb/shrub species remains constant. It is suggested that the upper altitudinal limits of many species are determined principally by temperature-related factors, while the degree to which species extend their ranges downwards is much influenced by competition.Structural heterogeneity is particularly well marked at altitudes corresponding to the bamboo zone on the wetter slopes and, even in the absence of bamboo, tree density here is much reduced. it is suggested that thicket-forming species, including bamboo, have enhanced competitive ability in intermediate altitude montane forests.Previously advanced classificatory schemes for montane forest vegetation in eastern Africa are examined in the light of the findings.Nomenclature follows Agnew (1974) and Dale & Greenway (1961) with minor modifications after Chapman & White (1970), Mabberley (1973) and Pennington & Styles (1975).The work was funded by a grant from the Natural Environment Research Council (U.K.). We are grateful to the Government of Kenya and to the Forest Department, Kenya, for permission to work on Mt. Elgon. Dr. A. D. Q. Agnew and Miss C. Kabuye very kindly identified some of the plant specimens. The illustrations were drawn by Mr. J. Shaw and the manuseript was critically read by Dr. D. Riley.