DIVERSITY AND DISTRIBUTION OF THE AFROALPINE FLORA OF EASTERN AFRICA WITH SPECIAL REFERENCE TO THE TAXONOMY OF THE GENUS PENTASCHISTIS (POACEAE)

The African tropic−alpine system, consists of a number of geographically separated ‘sky−islands’ and constitutes an excellent system in which to investigate the interaction between the geographical and ecological components of differentiation. The mountain systems harbouring the Afroalpine zone act as giant water towers that are source of almost all rivers that are an important source of water for domestic, industrial and agricultural use. Due to inaccessibility caused by extreme isolation and harsh conditions little research has been done on the diversity, species richness and evenness among other aspects in the Afroalpine zone. For effective conservation of these important zones it is important to understand their ecology and the conservation status of the species of these zones. The aim of this study was to carry out ecological analysis of Afroalpine species in general and addresses species delimitation in the genus Pentaschistis. The vegetation of Afroalpine zone of Mount Kenya, Elgon, Kilimanjaro, Ruwenzori, Bale and Simen were sampled for ecological analysis and taxonomic revision of the genus Pentaschistis. A total of 75 plots (100 × 100 m) from five vegetation types were analysed using both univariate and multivariate analyses. From the study a total of 46 families, 124 genera and 278 species were recorded from rock outcrops, bogs, grassland, Dendrosenecio forest/woodland and Alchemilla vegetation communities. Six families, namely Asteraceae, Poaceae, Caryophyllaceae, Lamiaceae, Cyperaceae and Rosaceae constitute 56% of all the species sampled. Five communities were described from the vegetation types studied: Carex−Deschampsia bogs, Festuca−Pentaschistis grassland communities, Dendrosenecio−Alchemilla forest/woodland, Alchemilla communities and Helichrysum−Festuca−Koeleria−Pentaschistis rock outcrop communities. The species richness and diversity is higher in older mountains such as Mt. Simen, Bale and Elgon (over 20 MYBP) compared to younger mountains such as Mt. Kenya (3.5 MYBP), Kilimanjaro and Rwenzori (less than 3 MYBP) mountains. Similarly Ethiopian Mountains were more species rich and have higher diversity than mountains of East Africa. Simen was the most species rich as well as exhibiting highest species diversity while Rwenzori was the most species poor and with the least species diversity. The East Africa Mountains were similar both in their species composition and community structure, as were the Ethiopian Mountains. Factors such as the geographical distance, age, geological history, and position of the mountains along the Rift Valley, climatic and edaphic factors are thought to be responsible for the observed patterns. The five vegetation communities were recognizable from each other though rock outcrop and grassland communities were barely separable (R< 0.25). The rest of the vegetation communities were overlapping but separable especially between bogs vs. Dendrosenecio (R > 0.5). The mainly southern African grass genus Pentaschistis was represented on all Afro−alpine mountain systems but due to the complex ecological and geographical variation patterns, the number of species recognized is widely disputed. A classification based on a well–supported evolutionary hypothesis for the genus is necessary. In order to obtain this, morphological studies of both natural populations and herbarium specimens have been conducted during this study. A total of 38 characters were used to carry out phenetic analysis of 37 specimens from nine taxa. The results from morphological observation as well as the cluster and principal component analysis produced two main groupings viz. (1) the two widely distributed species P. borussica and the P. pictigluma species complex, and (2) two narrow endemics P. dolichochaeta and P. chrysurus. Similarly, phylogenetic analysis of three gene regions (TrnL-F, rpL-16 and ITS) based on 83 sequences were done in this study to infer the phylogeographic history of this genus. The results from this study indicate two independent events of colonization each corresponding roughly to the two broadly distributed species and the two narrow endemics. The morphological, phenetic and phylogenetic analysis of this study support the current taxonomic classification of the tropical species of the genus Pentaschistis.