Evolution of the Afroalpine Flora

Abstract

The alpine enclaves of the high East African mountains provide a number of geographically and ecologically isolated temperate islands, inhabited by an afroalpine flora which is poor in species and peculiarly adapted to the extreme diurnal climate. Phytogeographically this afroalpine flora is of complex derivation. Its high degree of endemism indicates that its enclaves have long been isolated from each other and from other temperate areas. The high mountains harboring it have evidently stood isolated from each other since their origin, and Pleistocene climatic changes cannot have enabled direct contacts between the different alpine enclaves. Intermountain migration has probably occurred by independent longdistance dispersal. The frequency of apparent adaptations to different vectors is given in table 1. Establishment of newly dispersed afroalpine plants has necessitated climatic preadaptation and may have been facilitated by the occurrence of open habitats with weak competition. The vicarious plant communities occurring on the mountains appear to have been synthesized on each mountain separately after independent long-distance dispersal of the constituent species. The distribution of afroalpine plant communities seems to depend mainly on edaphic differences, and below the foreground of the glaciers there are few signs of regular successions. ACROSS THE WIDE uplands of tropical East Africa lie scattered a number of isolated high mountains, several of which reach altitudes of 4000 m or more above sea level. These mountains lie in two groups, one eastern and one western. Their vegetation displays a conspicuous zonation, starting with a montane forest belt, which is followed by an ericaceous belt, and finally an afroalpine belt, the lower limit of which falls at 3500-4000 m (Hedberg 1951). The flora of the last, the afroalpine flora, thus inhabits a number of geographically and ecologically isolated temperate islands. The montane forest and savanna vegetation separating the high mountain enclaves from each other do indeed provide better isolation than the sea between islands in an ocean. No diaspore of a mountain plant is carried across the savanna by an agent equivalent to oceanic currents, and no afroalpine plant can survive in the intervening vegetation. From a phytogeographical point of view the afroalpine enclaves are, therefore, comparable to a group of oceanic islands-which explains my participation in this symposium on insular evolution.