The geology of the Combretum-Terminalia ecosystem is characterized by extensive Late Tertiary that covers the pre-cambrian rocks that underlie all the other rocks in Ethiopia (Mohr, 1971). Old crystalline rocks overlay the pre-cambrian rocks in the relief down to Sudan (Mesfin Wolde Mariam, 1969), and valleys are largely composed of Phyllite with uncommon Chlorite Schist (Mohr, 1971). The topography/terrain is rugged; the upper limits about 1900 meters above sea level and the lower about 500 meters above sea level. The soil erosion rate is very high especially at the onset of rains. The soils are mainly Chromic and Pellic Vertisols, with Eutric Glysols and Eutric Histosols in areas that experience extreme seasonal flooding (FAO, 1984; EMA, 1988). This ecosystem generally occurs on rockier sandy soils (Beals, 1968; Tesfaye Awas et al., 2001).
The climatic zones of Ethiopia are divided into three distinct zones (NAMSA, 2001), namely: dry climate, tropical rainy climate and temperate rainy climate. Among these, the tropical rainy climate occurs in the. A single maximum rainfall that runs from characterizes the rainfall in this region. The second highest mean maximum temperature for the country was recorded in the western lowlands of Ethiopia (35-40 oC) an area where the Combretum-Terminalia ecosystem occurs, next only to that of the Afar Depression (40 oC), an area where the Desert ecosystem type occurs (NMSA, 1996).
The vegetation in this ecosystem has developed under the influence of fire. Thus, trees have very tick bark to cop with fire while most herbs have perennial bulbs (Menassie Gashaw, 2000),
This ecosystem occurs in the north-western, western, south-western parts (Tigray, Gonder, Benishangul Gumuz, Gambella, Bench Maji, Gamo Gofa and Sidamo). It penetrates into the plateau along the large river valleys, such as the Dedessa and the Omo (Anonymous, 1992; Ensermu Kelbessa et al., 1992). Belas (1968), described the occurrence of this ecosystem in the rocky slopes of Dakota Valley between Harer and Jijiga and south of the Lake Langano, on the hills east of the Acacia woodlands.
This ecosystem is characterized by Cmbretum spp., Terminalia spp., Oxytenanthera abyssinica, Boswellia papyrifera, Anogeissus lieocarpa, Sterospermem kuntianum, Pterocarpus lucens, Lonchocarpus laxiflorus, Lannea spp. Albizia malacophylla and Enatada africana. These are small trees with fairly large deciduous leaves, which often occur with the lowland bamboo- Oxytenanthera abyssinica. The understory is a combination of herbs and grasses. The herbs include Justecia spp., Barleria spp., Eulophia, chlorophytum, Hossolunda opposita and Ledeburia spp. The grasses include Cymbopogon, Hyparrhenia, Echinochla, Sorghum, Pennisetum, etc. Usually the herbs dominate the ground layer at the beginning of the rainy season while grasses dominate toward the end of the rainy season.
Based on the floristic study made on the Combretum-Terminalia ecosystem that occurs in Gambella region, about four plant communities were recognized (Tesfaye Awas et al., 2001). They are Commelina zambesica-Hygrophila auriculata, Sorghum purpureo?sericeum–Pennisetum thunbergii, Loudetia arundinacea–Hyparrhenia pilgeriana, Combretum adenogonium–Anogeissus leiocarpa communities. The classification was further supported by the variation in environmental factors between the plant communities. Geological factors, soil, topographic features and hot climate in the region, which are believed to have contributed to the presence of these, plant communities in Gambella. The plant communities identified in the region have phytogeographical affinity to central and western Africa rather than to Central and Eastern Ethiopia and belongs to the Sudanian vegetation type (White, 1983).
6.4 Use and values
Cattle husbandry in Combretum-Terminalia woodland ecosystem is low because of tsetse flies. For food inhabitants of this ecosystem depend on agriculture, hunting, gathering and beekeeping (Kurimoto, 1996; Tesfaye Awas et al., 1997a). People traditionally grow sorghum, sesame and cotton. Shifting cultivation is the common farming practice and the area has not been used agriculturally for long time. Since 1984, however, there has been an intensification of agricultural activity together with increasing population due to schemes that settled people from famine stricken areas of the highlands (Anonymous, 1992).
Combretum-Terminalia ecosystem provides many services to the inhabitants. Mengistu Wube (1995), Tesfaye Awas et al. (1997a), Tesfaye Awas and Zemede Asfaw(1999) had studied and compiled information on the indigenous plant uses in these areas. The studies were conducted in Anywaa, Komo, Majangir and Nuer inhabited areas of Gambella Region and Berta and Gumuz inhabited areas in the Benishangul Gumuz Regions. These studies had indicated the dependence of the people on this ecosystem. The ecosystem serve as a source of non-cultivated food plants, medicinal plants and plants with other utilities.
6.5 Threats and rates of change
The area has been deforested in recent years due indiscriminate fire, which removes huge amount of biomass, intensification of agricultural activity and extraction for the supply of construction, fuel wood and charcoal for the major towns and cities (Anonymous, 1992). Soil erosion at the beginning of the rainy season was aggravated by fire since it removes soil cover.
The impact of Sudanese refuge on this ecosystem is becoming the concern of Gambella and Benishangul Gumuz regions. Both plants and wild animals around the refuge camps are highly affected. Re-aforestation efforts to reverse the impact using exotic trees are not successful because they are not adapted to the ecosystem.
6.6 Conservation status
Relatively, Combretum-Terminalia woodland ecosystemis still perhaps the least affected of the ecosystems that are described in Ethiopia. Althogh it is not demarcated, Gambella National Park is the only protacted area in this ecosytem. Beals (1968), proposed some areas for conservation, no part of the areas were officially protected so far.