An Ecological Study of the Forest Vegetation of Southwestern Ethiopia

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Addis Ababa University


'DIe floristic composition and stmcnIre of tbe forest vegetation of southwestem Etluopia between altinIdes 1050 to 2500 m are described. Sample plots of 30 m x 30 m were taken for woody plants and 2 m x 2 m for herbaceous and bambeo plants. A total of 101 sample plots were analyzed. The cover-abundance values for trees, shrubs and herbs witltin the sample plots were estimated. All trees and shrubs with diameter at breast height (DBH), i.e, 1.3 m of <0 2 cm were measured for height and diameter. The leaf size of all tree species Witll heights <0 6 m was measured. The presence of epiphytes on each woody individuals and tlle presence of woody climbers (lianas) in tlle sample plot was noted. Altitude, slope, and exposure were measured and soil samples from surface and sub-surface were taken and analyzed for pH, electrical conductivity, soil colour, texture, organic carben, total nitrogen, available phosphorus, exchangeable bases, and cation exchange capacity. A total of 139 species of vascular plants, 15 species of vascular epiphytes, and 12 species of woody climbers (lianas) were recorded. Average linkage clustering procedure was followed to classify the vegetation data and 9 clusters were recognized and tlle clusters were identified as local plant community types and given names after one or two dominating and/or characteristic species. The conl1l1wuty types are: Arulldinaria a/pilla, -Manilikara blltllgi-Coffea arabica, Syzygilllll guineense-Maytelllls gracilipes, IIex lIIitis-Galiniera saxifraga, Celtis' qfi1calla-Dracaella ajrolllolllana, Allop/zylus abyssinicus-JIIsticia schill/periana, Allingeria adolfi:friederici-ChiollClIlthes mildbraedii, Syzygillm gllilleellse-Dracaella ajl'OlIIOlllalla, and Olea lVellVitschii-Oliollallfhes mildbraedii. Analysis of variance (ANOY A) was performed to see if there is any significant variation among the community types with respect to anyone enviromnental parameter. The result obtained shows tltat tlle community types significantly vary for aU the envirol1ll1ental parameters except for slope, exposure, exchangeable sodium and potassium, cation exchange capacity and available phosphorus for subsoil (2()'60 em). Altitude is the enviromnental parameter that differentiate most of the community types. Some of the enVirOll1l1ental perimeter are significantly correlated. The structural 3.ltalysis of the community types showed tltat the density of trees in tlle > 10 cm DBH class is significantly higher than density in the> 20 em DBH class. Forty-seven per cent of the trees in Manilikara blltllgiCoffea arabica community type, and more tltan 60% of the trees in the remaining types belong to the lowest diameter class (5-20 em). More than 50% of the trees in Mallilikara blltllgi-Coffea arabica and SyzygiulIl gllineellse-Maytenus gracilipes community type and over 70% of the trees in the remaining community types raU into the lowest height classes (6-9 & 9-12 m). Mosses are the most abundant epiphytes in all community types 3.lldLando/phia buchananni is the abundant liana in all conununity types except in AllopJzylus abyssiniclIs-JlIsticia schimperiana type. The predominant leaf size class in all conmlwtity types is mesophyll (450()'18,225 mm'), while microphylls (225-2025 mm'), notophyUs (2025-4500 rom'), macrophylls (18,225-164,025 mm'), and megaphylls (> 164,025 mm') leaf sizes are shared by less than 20% of tlle species. Only mesophylls are present in Arundinaria a/pina community type.