Diversity and distribution of the afroalpine flora of Eastern Africa with special reference to the taxonomy of the genus pentaschistis (poaceae)
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Date
2013-08-14
Authors
Ahmed, Abdikadir Abdi
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Abstract
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.
Description
QK 495.G74A2