Ecology and Development Series No. 10, 2003 - ZEF

Floristic analysis of the undisturbed forestThe strong negative correlation between altitude and A. macrophylla indicates that theforest area is perhaps near the upper limit of the altitudinal range of the species. TheDracaena fragrans group, on the other hand, holds an intermediate position in the gradientsof the major environmental variables influencing the distribution patterns of the communitytypes. The plots representing the group have steeper slopes than those of the C. arabicagroup, and are at higher altitude and greater distance from the Geba river than that of the A.macrophylla group.The observed species composition in the three community types is similar to thegeneral trend of the forest vegetation types or zones occurring in SW Ethiopia.Argomuellera macrophylla, the most dominant and best indicator species of Group 2, is oneof the characteristic species of the dry peripheral semi-deciduous Guineo-Congolian forests(Friis 1992). Even though Yayu forest is located at a much higher altitude than the areaswhere such forests have been described (450-600 m), a large proportion of the study plots(37%) fall under Group 2. This can be due to the drying effect of steep slopes (Tewolde1986), which may give a comparative advantage to the plant species from a relatively loweraltitude and drier area enabling them to dominate the vegetation on the very steep areasalong the Geba river.The most dominant and best indicator species of Group 3, D. fragrans, is also oneof the characteristic species in the small tree and shrub layer of the transitional rain forest(Friis 1992). Many tree and shrub species associated with the group are also characteristicspecies of this forest type. Beside the similarity of the species composition of the vegetationin this group to that in the transitional rain forest, the altitudinal range and overall climaticconditions also match the description of this forest type (Friis 1992). The most dominantand best indicator species of Group 1, C. arabica, is considered as one of the characteristicspecies of the Afromontane rain forest vegetation (Friis 1992). The majority of the tree andshrub species associated with the group are also characteristic species of this forest type.Even though the mean altitude of the group is slightly lower than the lower limit of thealtitudinal range of the Afromontane rain forest, many plots are located at sites higher thanthe 1500 m lower limit of this forest type. This group represents the major community type,with about 40% of the plots investigated in the study belonging to the group. Many61