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The floristic composition and structure of the vegetation of Afromontane forest remnants on the Central Plateau of Shewa, Ethiopia are described and related to environmental factors. The study was conducted in four forests: one humid forest, the Jibat forest, and three dry forests, the Chilimo, Menagesha Suba and Wof-Washa forests. 146 relevés were analysed, 77 in Jibat, 30 in Chilimo, 20 in Menagesha Suba, and 19 in Wof-Washa.
The sample plots were usually 30 m × 30 m for trees and shrubs, and 2 m × 2 m for herbaceous plants. For each species the cover/abundance value was estimated. Height and diameter at breast height of all woody individuals taller than 2 m and thicker than 2 cm were measured. Profile diagrams were made in selected transects in two of the forests, Menagesha and Wof-Washa.
The following environmental variables were measured in each sample plot: altitude, slope, and exposure. Representative soil samples from each sample plot were analysed for chemical properties: pH, Electrical conductivity, Na, K, Ca, Mg, N, P, cation exchange capacity, and organic matter, and for physical properties: sand, silt and clay.
The relevés were classified with the clustering and relocation program TABORD, and the two-way indicator species analysis program TWINSPAN. The resulting clusters were interpreted as community types and given provisional names after usually two dominant or characteristic species. Community-environment relationships were analysed with the ordination program Canonical Correspondence Analysis (CANOCO program) and the Discriminant Analysis program DISCRIM.
Eighteen community types were described: Arundinaria alpina type, Ilex mitis-Rapanea simensis type, Syzygium guineense-Psychotria orophila type, Olea hochstetteri-Olinia aequipetala type, Croton macrostachyus-Ficus sur type, Olea welwitschii-Carissa edulis type, Syzygium guineense-Vepris dainelli type, Erica arborea type, Juniperus procera-Myrsine africana-Ehrharta erecta type, Juniperus procera-Maytenus arbutifolia-Peucedanum winkleri type, Erica arborea-Myrica salicifolia type, Spiniluma oxyacantha-Scolopia theifolia type, Scolopia theifolia-Podocarpus gracilior type, Podocarpus gracilior-Olea europaea type, Podocarpus gracilior-Allophylus abyssinicus type, Juniperus procera-Sideroxylon gillettii type, Podocarpus gracilior-Maytenus arbutifolia type and Euphorbia obovalifolia-Podocarpus gracilior type. There is a strong contrast in species composition between the humid and dry forests: the humid forest is characterized by a mixture of deciduous species while the dry forests are dominated mainly by the two conifers Juniperus procera and Podocarpus gracilior.
Analysis of community-environment relationships revealed strong correlations with altitude, soil chemical factors – especially organic matter and total cations – and the physical factors sand and clay content. It was deduced from the structural comparisons and vegetation history of the forests that Wof-Washa is an old forest while Jibat, Chilimo and Menagesha Suba are forests in different stages of secondary development.
Seven general pattems of species population structure were recognized for the forests, and interpreted in terms of the dynamics of the species: (1) good reproduction and continuous recruitment, (2) poor reproduction, but relatively many individuals in medium-size classes, (3) good reproduction but bad recruitment, (4) good reproduction but irregular recruitment, (5) good reproduction and continuous recruitment, but under-representation of medium-sized individuals, (6) poor reproduction but relatively many individuals in higher size classes, and (7) no reproduction, and only very large and old individuals present.
Floristic comparisons with other montane forests showed that the Jibat forest is more related to the southwestern humid forests of Ethiopia, while the forests of Chilimo, Menagesha and Wof-Washa were related to each other, and other dry forests on the Southeastern Plateau of Ethiopia.