The Acacia-Commiphora ecosystem is known for its varying soils, topography, and diverse biotic and ecological elements. These plant species are with either small deciduous leaves or leathery persistent ones. The density of trees varies from ‘high’, in which they form a closed canopy to scattered individuals to none at all forming open grasslands. The grasses do not exceed more than one meter, thus, no true savannah is formed.
The altitudinal range in which the ecosystem is found is 900-1900 meters above sea level. The rainfall ranges from 600-1600 mm and the temperature varies from 18-27o C270c (100-800 mm, 210-27.50c for arid, 300-800 mm, 160c-270c for semi-arid, 700-1000 mm, 160c-280c for dry sub-humid respectively) (Ensermu Kelbessa et al., 1992). This is especially concerning the semi-arid moisture zone in which the ecosystem is found, where there are distinct differences in characteristics between the semi-arid plains, the semi-arid lakes and Rift Valley, and the semi-arid mountains and plateaus showing characteristic climatic variations.
It is a dry ecosystem; at times, the dry season lasts as long as 10 months in a year. In moisture zone classification, the ecosystem extends from dry sub-humid to arid zones. Accordingly, a harsh and hot temperature with a low and uneven rainfall distribution generally characterizes the ecosystem. In such areas, evapotranspiration is very high due to the high temperature with strong occasional winds in the rainy season. This makes the monthly precipitation to exceed rainfall in some areas of the ecosystem (Engeda Mersha, 2000). Most of the areas in the ecosystem are defined as areas in which the ratio of annual precipitation to PET falls with in 0.05-0.065 ranges. 180c-
Soil types vary depending on the locations and altitudes. For instance, the Southern and Central Hararge, the parts of the lowlands of Shewa and Wollo have predominantly vertisols and cambisols. On the other hand, eutric cambisols is common in Tigray. A soil types that is medium to shallow in depth, stony with low organic matter and deficient in nitrogen content. The Northern Rift Valley (starting from the footsteps of Northern Hararge highlands towards the north areas of lowland of Afar) is covered by shallow soils such as regosols, solonchaks and fluvistols. Such soils are usually affected by salinity (EPA, 1998).
In the ecosystem, there is a substantive amount of water resources. There are seven major lakes located in the ecosystem, namely, Ziway, Langano, Abijata, Shalla, Awasa, Abijata and Chamo. These lakes are used for commercial fisheries, irrigation, recreation and for industrial purposes. In the lakes region of the ecosystem, most streams are perennial and the depth to ground water is 0- 150 m. A moderately large quantity of ground water is also a characteristic of some areas of this ecosystem. For instance, Southern parts of Oromia, Eastern parts of Afar and Northeastern Tigray are good examples. Most of the streams in these areas are intermittent and some are perennial. The ground water has a fair- to- poor chemical quality with a depth of 0 –270 m.
The ecosystem mainly occurs in Southern, Eastern and in some parts of the Rift Valley. It is located in Oromia, Afar, Harari, Somali, and Southern Nations, Nationalities and Peoples Regional States.
The ecosystem is characterized by vegetation type that includes drought-tolerant tree and shrub plant species such as Acacia tortilis, A. millifera, Balanites aegyptiaca Acalypha spp., Aerva sp, Combretum spp., Terminalia spp., Capparis spp., etc. These plant species are with either small deciduous leaves or leathery persistent ones.
The wild life species by which this ecosystem is characterized are large wild animals including Oryx, Zebra, Hartebeest, Kudu, Gazelle, etc. Thus, it is sensitive to overgrazing.
5.4 Use and values
The ecosystem is traditionally a grazing area. Since the past few decades, the ecosystem has become a major source of fuel wood and charcoal supply for populations of metropolitan centres including Addis Ababa.
Plants with high traditional values and economic importance are found in this ecosystem Codeauxia edulis, which has very nutritious fruit and highly favoured by the local people, and several species of Boswellia and Commiphora, which provide the frankincense myrrh of local and international trade are some important plants in this ecosystem.
5.5 Threats and rates of change
The Acacia-Commiphora deciduous woodland is currently under strong environmental stress. Extraction of fuel wood and charcoal for major towns in the country has increased the rate of deforestation and natural resource depletion. The ever increasing of woodland clearance for rain-fed agriculture and irrigation under takings further enhanced the vulnerability of the ecosystem. From some studies conducted in the area, it has become evident that in all affected areas, certain species, including those that are of medicinal values might have probably been affected irrecoverably (Zerihun Woldu and Mesfin Tadesse, 1990). The current estimate is that over 40% of the area is affected due to human interference of various nature. The combined effects of these activities has left the soil bare and exposed it to erosion. Since the soil is alluvial and very fine in nature, which actually makes it susceptible to both wind and water erosion.
Currently, very little of the vegetation remains except for small patches in Lake Langano area and further west along the Rift Valley. Wild fire is one of the repeatedly experienced problems in the ecosystem. It has an impact on the diversity by forming fire resistant climax species.
In this ecosystem, over forty taxa are threatened, much of which are found in the Bale, east and west Hararge and Borena Zones. Several of this plant species are highly adapted life forms with extreme cases such as Euphorboa piscidermis. In addition to the habitat destruction, the highly lucrative trade in succulents threatens the vegetation (Enssermu Kelbessa et al. 1992).
5.6 Conservation status
Some the areas lying in this ecosystem are conserved. The vegetation is relatively in good condition along the Abernosa cattle ranch, for instance, where grazing and tree cutting is comparatively controlled. A small patch of the ecosystem is protected in the areas of Abijata-Shalla Lakes Park and around the Awash National Park.Most of the National Parks in the country are found in this ecosystem. Of these parks, only the Awash National Park is gazetted. All the other conservation areas (Abijata-Shala Lakes National Park, Nechisar National Park, Omo National Park, etc.) attempt to function without proper legal recognition. The exclusion of human activity from protected areas is not satisfactory and very often poor. This implies that human encroachment in its negative sense is manifested in this area. Moreover, disregard of community participation in the establishment and further activity of these areas resulted to land use conflict and development of negative attitudes.