Ecology and Development Series No. 10, 2003 - ZEF

Floristic analysis of the undisturbed forestslopes are influenced by bedrock and tend to be less moist and less acidic (Tewolde 1986).Hence, slope strongly affects the composition and structure of forest vegetation. Maximumtree size tends to decrease with increasing slope, while abundance of stems tends toincrease (Takyu et al. 2002). However, different species respond differently to gradients.Distance from river bank and soil moisture content are also known to be inversely related(Carr 1998), and hence play an important role in zonation of vegetation along riversides.Carr (1998) observed different vegetation zones along the Omo river in SW Ethiopia.The patterns of occurrence of the sites or plots representing the three groups orcommunity types are influenced by the three terrain variables mentioned above and theresulting micro-climate and soil characteristics (texture, moisture content and chemicalproperties) as a result of their direct or indirect impacts. The dominance of coffee in Group1 (Coffea arabica group) is, for instance, not random, but related to the gradients in theenvironmental variables; this group has the lowest mean slope and the highest meanaltitude. It also has the longest mean distance from the Geba river. Similar to the results inthis study, Tewolde (1966) observed that the abundance of coffee decreases withincreasing. On the other hand, contrary to Tewolde’s observations at Yassera and Tobaforests (Tewolde 1986), it appears that the coffee occurrence at Yayu increases withincreasing altitude. This is quite possible, since Yayu forest lies in the lower half of thealtitudinal range in which coffee occurs in the natural forests of SW Ethiopia, whileYassera and Toba are located in the upper half, at altitudes ranging from 1600 m to 1850 m(Tewolde 1978; 1986). The altitudinal distribution range of the wild coffee populations inSW Ethiopia is between 1000 m and 2000 m (Meyer 1965; Friis 1979). Similarly, theabundances of coffee trees decrease with increasing altitude in Bonga forest at areas higherthan 1700 m (Abayneh 1998). On the other hand, the upper altitudinal limit for coffee inHarrena forest, SE Ethiopia, is 1700 m and the lower limit is defined by changes invegetation composition and moisture (Mesfin and Lisanework 1996). The maximumdevelopment of the coffee trees and abundance in Harrena, however, occurs between 1390m and 1450 m, and 1550 m and 1600 m (Mesfin and Lisanework 1996).Contrary to the Coffea arabica group, the Argomuellera macrophylla group ischaracterized by very steep slope, lower altitude and shorter distance from the Geba river.60