Influence of topographic aspect on floristic diversity, structure and treeline of afromontane cloud forests in the Bale Mountains,Ethiopia

We studied the influence of east and west aspects on floristic composition, diversity, structure and treeline of afromontane cloud forests at Rira in the Bale Mountains, southeast Ethiopia. In addition, we studied how aspect relates to and/or interacts with other topographic and edaphic factors in influencing vegetation diversity. Strati- fied systematic plot sampling was used to survey the floristic composition, diversity and structure of forests on east- and west-facing slopes. The sample plot size was 20 x 20 m and a total of 36 plots were inventoried. A total of 72 composite soil samples were collected and analysed. Woody species richness of the forest on the east-facing slope was 1.7 times higher than on the west-facing slope. Shannon, Simpson and Log-series alpha diversity indices and evenness of forests on the east-facing slope were sig- nificantly higher than on the west-facing slope. NMDS ordination indicated that the east- and west-facing slopes formed two clusters of species and aspect explained 55.2 % and 10.4 % of the variation in species richness and abun- dance, respectively. There was no significant difference between aspects in stand structure except in dominantheight, which was higher on the east-facing slopes. There was significant interaction between aspect and elevation in influencing woody species diversity. The four plant com- munity groups, which were identified using cluster and indicator species analysis were represented differently on the east and west aspects. The treeline on the east-facing slope （3352 m） was located about 110 m higher than on the west-facing slope （3240 m）. Soil moisture deficiency was unlikely to be a limiting factor on either site. Near the equator, east-west aspect was shown to have considerable impact on floristic composition, diversity, structure, and treeline position of montane forests. Diurnal cloud move- ment patterns and its impact on microclimate of slope aspect should be taken into account in future studies of cloud forest diversity, structure, and treeline position.

Woody species composition,structure and environmental determinants in a moist evergreen Afromontane forest,southern Ethiopia

Ethiopian moist evergreen Afromontane forests are vital for the conservation of plant diversity,including genetic pools of Coffea arabica L.However,these forests are continuously shrinking and need empirical scientific studies for their effective conservation and sustainable management.This study was,therefore,conducted to assess woody species composition,diversity,endemicity,structure,plant communities,and environmental determinants in Sirso moist evergreen Afromontane forest of southern Ethiopia.Environmental variables such as altitude,slope,aspect,disturbance levels,and geographical location were recorded for each main plots.Vegetation structure and importance value index(IVI),diversity,vegetation classification,and correlation with environmental factors were analyzed.A total of 74 woody species representing 70 genera and 34 families were identified.Seven species were endemic.Rhus glutinosa A.Rich subsp.glutinosa and Prunus africana(Hook.f.)Kalkm.were species categorized as vulnerable according to the IUCN red list category.The highest number of species was recorded for the Rubiaceae family(8),followed by Euphorbiaceae(6),and Asteraceae(6).Five plant communities were identified:Maesa lanceolata-Dombeya torrida,Galiniera saxifraga-Cyathea manniana,Vepris dainellii-Triumfetta tomentosa,Dracaena fragrans-Rytigynia neglecta,and Arundinaria alpina.The distribution of these communities depends on altitude,slope,and disturbance factors.Syzygium guineense(Wild.)DC.,Schefflera abyssinica(Hochst.ex A.Rich.)Harms,and Galiniera saxifraga(Hochst.)Bridson were species with the highest IVI,while P.africana,Cordia africana Lam.,Hagenia abyssinica(Bruce)J.F.Gmelin,Bridelia micrantha(Hochst.)Baill.,and C.arabica had the lowest.Population structure of woody species showed an inverted J-shape,a J-shape,a bell-shape,and an irregular shaped pattern.These latter three population structure revealed signs of disturbances,and were negatively correlated with altitude and slope.The study showed that the Sirso moist evergreen Afromontane forest is species rich and should be given conservation priority to maintain endemic and native species.