1 This annex has been adapted from text in Assefa Eshete et al (1986: pp 3-18).
Information gathered in the south-western portion of the Borana Plateau during 1982-86 by ILCA included details related to climate, soils and vegetation. This information has been used to produce an ecological map (see Figure B1, presented as a fold-out at the back of this volume). This annex covers two objectives: (1) to present the map legend; and (2) to provide some interpretation of the map to assist development planning. Readers interested in more interpretative details should consult Assefa Eshete et al (1986).
The map does not duplicate the AGROTEC/CRG/SEDES Associates map of Sidamo that was based on phyto-sociological concepts (AGROTEC/]CRG/SEDES Associates, 1974d). The map presented here was based on satellite imagery as well as ground-truthing (see Section 3.2.1: Ecology and land-use map).
There were problems of interpretation of satellite imagery, however. Ecological subunits did not always have distinct boundaries and the quality of satellite images was not consistent. Scale distortions across images occurred and registration of images (i.e. transferring results to a topographic map) was difficult. Soil background also changed the apparent category of vegetation because of colour distortions. For example, trees varied from dark red to bright orange; shrubs were grey or blue on volcanics and yellowish on basement-derived soils; and grass appeared pale blue on volcanics and pale orange on basement-complex formations.
The first level of differentiation among mapping units is the combined influence of climate and soils on the environment. Subunits were primarily designated from variation in land form or vegetation physiognomy associated with soil features. The resulting legend is therefore not a simple one-axis presentation.
1. Subhumid zone
2. Upper semi-arid zone
3. Lower semi-arid zone on basement-complex substrates
4. Semi-arid zone on mixed soils
5. Arid environments
6. Bottomlands
The following material should be used in conjunction with Figure B1, Annex B.
The units in this zone are either densely wooded or previously wooded lands which have been exploited for fire wood and building materials or cleared for cultivation. The climate has favoured establishment of small towns (such as Arero, Yabelo and Mega) in the vicinity of these forests and this has traditionally put local pressure on resources. The present situation (i.e. 1986) seems to be stable and no changes in location or extent of remaining forest could be observed over the last 10 to 15 years from comparisons between the map of AGROTEC/CRG/SEDES Associates (1974d) and more recent satellite imagery.
1.A Juniperus forest
Intact forest can still be observed in two locations which are far from towns. The facets are:
1.A.1 = Group of valleys with steep walls within the Mega-Dokole massif. The site can be observed from the air. It is well protected and does not reveal significant human influence.1.A.2 = Large area (150 km2) north of Arero with human influences limited to the periphery.
1.B Juniperus woodland
Juniper trees are still abundant but there is evidence of moderate human exploitation. Shrubs have started to grow in interspaces between juniper trees. Plant associations also include elevational transition types. Facets include:
1.B.1 = This is the more open part of the Arero forest.1.B.2 = Most of the higher ground of the Yabelo mountains have stands of juniper trees which are protected. The stands are separated by patches of shrubs.
1.C Mixed woodlands
Although woody cover is high, juniper trees are increasingly rare. Better indicator plants are Olea, Euclea and Dodonea spp. The most common shrub is often Tarconanthus camphoratus, usually considered an invader, which can be seen near the summit of the Mega range. Facets include:
1.C.1 = These communities cover the slopes of Gamedu and Gabela; juniper is present in small pockets.1.C.2 = Rocky and steep slopes around Mega are covered with this very variable woodland; Tarconanthus sp is abundant.
1.C.3 = These facets correspond to slopes on the Yabelo range as well as those in the northwestern portion of the Teltele plateau. Juniperus and Olea spp are accompanied by Tarconanthus sp, Acacia brevispica and Croton macrostachyus.
1.C.4 = This unit has been differentiated from 1.C.2 because of the numerous campsites and settlements associated with small, cultivated fields. It occupies a large valley and covers rugged hills at the northern end of Mega Mountain.
1.D Terminalia sp woodlands
These sites are intermediate between the subhumid and upper semi-arid zones and are found on the lower slopes of mountains with occasional Juniperus sp, but more commonly Terminalia sp, Combretum sp and A. brevispica. The following facets are differentiated by land form:
1.D.1 = Very rocky slopes below Arero town, dissected by small river beds. Also occurs along the eastern fringe of the Arero massif.1.D.2 = Previously cultivated slopes south of Yabelo town, now invaded by A. brevispica and Commiphora spp on shallow soils.
1.D.3 = Very rocky terrain south-east of Yabelo, isolated from the other woodlands. Occurs on Lithosols.
1.D.4 = Rugged terrain with granitic outcrops, possibly derived from unit 1.C.4 after cultivated fields have been abandoned.
1.E Cropped areas
These sites benefit from higher rainfall and cooler temperatures which allow regular cultivation. They are situated on top of mountain ranges, in valleys or at the base of mountains:
1.E.1 = Cropland around Yabelo town, presently expanding eastward along drainages into various acacia communities (especially A. tortilis, A. nilotica and A. seyal).1.E.2 = Mosaic of fields and grasslands around Mega, with colluvial red soils from surrounding hills.
1.F High ground influenced by volcanism
The mountains described to this point are derived from basement-complex formations but volcanics have intruded into the Mega massif from two craters (at Medecho and Soda). The volcanics are often weathered and have contributed to building mixed soils. Volcanics also occur as scattered rocks. Facets include:
1.F.1 = Vegetation similar to that of 1.C.1 and 1.C.4 except for large volcanic rocks and granitic cliffs where basement-complex mountains were broken by volcanism to produce cuts oriented in a north-westerly or south-easterly direction.1.F.2 = Juniperus and Tarconanthus spp on geology similar to that for 1.F.1 between Mega and Medecho.
1.F.3 = Olea and Terminalia trees on brown soils developed from volcanics along the valley where Mega town is located.
1.F.4 = Grasslands with temporary water logging and dominance of Vertisols.
Broad-leaved trees occur in the most mesic sites of these units and mean annual rainfall is around 600 mm. Cultivation is practiced on deeper soils but success of cultivation depends on annual rainfall. The zone includes most of the northeastern part of the mapped area (i.e. the teltele slopes and the top of the Moyale escarpment along the Kenyan border) where sudden changes in elevation (i.e. from 700 to 1200 m) results in an orthographic source of moisture.
2.A Combretum and Acacia spp woodlands
Combretum and Terminalia spp are usually accompanied by A. etbaica or A. brevispica. The latter may be replaced by A. tortilis, A. nilotica and A. bussed. Soils are mostly derived from granite parent materials. Facets include:
2.A.1 = Dense, heterogenous woodlands south of Hidi Lola. Large granite rocks are apparent within wooded valleys which correspond to deeper soils.2.A.2 = Contiguous to 2.A. 1, this unit differs in terms of steep cliffs and slopes that give rise to numerous small parallel drainages.
2.A.3 = This unit exists at elevations 1500 m in the same area as 2.A.2 on very rocky hills with red-yellow soils from nongranitic basement-complex substrates and extends to the east around Web town.
2.A.4 = A comparable unit to 2.A.3, but this occurs on a mixture of granites and schists from Das to Did Hara.
2.A.5 = Hills with deep reddish soils in Did Hara, very sandy with many pebbles and scattered termite mounds occur. Variations occur according to topography. Soils are slightly acidic.
2.B Acacia savannahs
These occur in Did Hara. Acacia tortilis and A. nilotica dominate the tree component as commonly observed for group 2.A. However, the units can often be considered as degraded and A. brevispica and Commiphora spp are substituted for broad-leaved trees. The following facets have been established on the basis of decreasing shrub density:
2.B.1 = Dense woody vegetation on eroded soils. Soils are sandy, fine grained and pale red in colour. The unit occupies both sides of a valley west of Arero and is heavily grazed. Grass cover has been greatly reduced and formation of gullies has occurred.2.B.2 = Lower hills and small drainages east of Yabelo, with bare ground, sheet erosion and exposed bedrock. Attempts at cultivation have been made in the proximity of towns, but soils are acidic and poor for cultivation.
2.B.3 = Less eroded and bushy area in the same part of the zone as 2.B.2. Soils are yellow-red and similar to those in 2.B.2 except they are deeper. Vegetation is variable with dense woody vegetation occurring in small drainages.
2.B.4 = Facets are with those of units of 2.B.3 and occur on hill tops comprised of basement-complex materials (i.e. granites, gneisses and schists). Grass cover is moderate and young Commiphora spp trees are abundant.
2.B.5 = Occurs as one isolated patch in Did Hara region. Similar to 2.B.4 except this occurs on darker soils.
2.C Euphorbia spp woodlands
These units are scattered across the eastern portion of the upper semi-arid zone and may indicate very old examples of bush encroachment. Woody cover is dominated by Acacia spp but acacias are mixed with less competitive plants such as Hibiscus or Euphorbia spp. Facets include:
2.C.1 = Dense bushlands on rocky terrain having numerous outcrops.2.C.2 = Same as units 2.C.1 but on eroded sites. Facets have been heavily grazed over a long period of time and are still heavily utilised.
2.C.3 = Same as unit 2.C.1 but on yellow sandy soils. Little grass cover.
2.C.4 = A southern variant of 2.C.3 in which soils are whitish or grey in colour.
2.C.5 = Lower slopes of the Yabelo mountains on the western-facing aspect. Colluvions have been cleared for settlement and crop cultivation.
2.D Acacia spp on volcanic substrates
These units occur along the Mega-Dokole range. They differ from 2.B or 2.C units because their soils are derived from, or influenced by, volcanics. They also show a more sparse and extensive grass cover. Dominant woody species are similar to those reported for 2.C units. Facets include:
2.D.1 = Isolated rocky volcanic hills in the Dokole area.2.D.2 = Large volcanic outcrops within granitic formations surrounding Mega town.
2.D.3 = A facet specific to Mega town, where mixed colluvions and rocks are covered with a shrubby plant community.
2.D.4 = Equivalent to the 2.D.3 formation, but occurring in the Dokole region.
2.E Acacia spp on limestone
The Hidi Lola area occurs on a large gently undulating portion of the plateau where limestone was deposited and has been either partially eroded or covered by sandy deposits. Soils are darker than those derived from granites, vegetation is characteristic of the upper semi-arid zone and the region is suitable for cultivation. Scattered limestone formations are also found to the north in the Did Hara region. Facets include:
2.E.1 = Higher elevations dissected by erosion. Cultivation is often restricted to gentler slopes on deeper soils.2.E.2 = This unit includes lower elevations having a reduced plant cover. There are numerous isolated cultivated fields and small settlements.
2.E.3 = Hilly landscape in Did Hara, close to a Vertisol valley. Soils are mixed with volcanics. Vegetation consists of grasslands with scattered Acacia spp trees.
Soils derived from granites have a more favourable water regime than soils derived from volcanic rocks and the eastern region of this zone (which coincides with the basement-complex as a geological formation) is more productive than the western region. It is typified by dry savannahs dominated by Acacia and Commiphora spp. It is rather homogenous in terms of flora. Subdivisions are largely based on vegetative physiognomy.
3.A Wooded and elevated sites
These rocky slopes and hills contain various mixtures of shrubs and grass which produce a high plant cover overall. The most common Acacia spp are A. bussed, A. etbaica, A. senegal, and A. mellifera. Commiphora spp are also abundant. Two facets occur on the central mountain ridge south of Yabelo and two more occur in scattered locations on granitic rocks. Facets include:
3.A.1 = Granitic steep hills and hill tops.3.A.2 = Lower slopes of granitic hills.
3.A.3 = Eastern, high-elevation area having rocky and shallow red soils.
3.A.4 = Isolated large rocks subtended by extensive bare ground at higher elevations.
3.B Wooded plateaus
At the periphery of units which comprise the 3.A group, this group covers most of the central part of the eastern zone, being generally similar in plant composition to that of the 3.A group. Facets are differentiated on the basis of location and plant cover:
3.B.1 = Colluvial plains associated with 3.A.2 or 2.C.5; the surface is slightly sloped and grass cover is low.3.B.2 = Eastern woodlands with red soils and low grass cover, immediately below 3.A.3 or 3.B.5 sites in terms of elevation.
3.B.3 = Slightly elevated ground. Resembling 3.A.3 sites but flatter and more dissected. Grass cover is high in spite of heavy utilisation. Trees are less abundant than in other 3.B facets.
3.B.4 = A variant of 3.B.1, comprised of a mosaic of sloping plains and small drainages.
3.B.5 = This constitutes most of the central eastern plateau with a plant physiognomy that varies from dense bushland to open grasslands. Woody cover is usually less than in 3.B.2, but these sites appear to be regularly invaded by shrubs.
3.C Open savannah
The Dubluk area is remarkable in terms of the low density of trees and shrubs; this may have occurred because of pressure from small ruminants being taken to water at nearby wells. Four facets have been described:
3.C.1 = An open flat savannah south of Dubluk with small cultivated fields along the tarmac road.3.C.2 = Similar to 3.C.1 but a high grazing pressure throughout the year has probably encouraged bush encroachment.
3.C.3 = Degradation is more pronounced here than in facets 3.C.1 or 3.C.2, although partial recovery of vegetation has occurred as a result of the creation of the Dembel Wachu Ranch.
3.C.4 = One small isolated hill used for settlement. It is currently heavily grazed.
3.D Grasslands
Grasslands are situated in the vicinity of the 3.C units. Soils are slightly acidic. Differentiation of units is based on landscape. Facets include:
3.D.1 = These flat grasslands lie close to 3.A or 3.B units, west of the main tarmac road.3.D.2 = Terrain is gently undulating along the Mega range.
3.D.3 = Rolling landscape. Trees increase in abundance along small drainage lines.
The geological formation described as Quaternary sediments (see Section 2.4.1.1: Geology) includes hydromorphic surfaces (Fluvisols and Vertisols in valleys) and more variable units on alluvial or colluvial material. This is a tentative classification of the latter group. Most facets appear to be more representative of the lower semi-arid zone, but soils are deeper and richer in nutrients that allow cultivation. Similarly, woody cover (at least at lower elevations) often appears higher than that expected based on annual rainfall.
The term "mixed" soils refers to material from granitic (or metamorphic) and volcanic origin. Soil variability is often very high and localised; this group of units articulates the transition between the eastern granitic plateau and the western volcanic depression which can be considered part of the Rift Valley, linking lakes Abaya and Chamo to the Chalbi desert and Lake Turkana. Subdivisions are established from the varied intensity of volcanic influence that increases from 4.A to 4.C and from vegetative physiognomy.
4.A Wooded savannahs
Acacia spp are the major component of the woody strata with A. bussed, A. mellifera, A. nubica, A. tortilis, A. reficiens and A. horrida common. Five facets occur:
4.A.1 = The south-eastern woodlands, on top of the Chalbi escarpment, correspond to a highly eroded landscape. Dense woodlands occur in small valleys and the facet is intermediate between 2.A.1 and 2.E.1 units in terms of vegetative cover. Cultivation could be attempted here.4.A.2 = This is a similar unit to 4.A.1 that occurs on the Mega hills. Cropping is presently expanding.
4.A.3 = This degraded plateau borders the western hills and presents a mosaic of woody patches, dense bush and open savannahs on rocky terrain. Grass cover is highly variable. Erosion is common.
4.A.4 = Northern woodlands which border 2.A.4 units. They include grasslands where soil moisture is higher.
4.A.5 = These woodlands are aligned on the plateau from the Yabelo mountain range to Kancharo and border western volcanics. The volcanic influence is pronounced. Soils are of poor quality.
4.B Grasslands
Grasslands have established on either water-logged sites or shallow soils. The group on waterlogged sites includes four facets:
4.B.1 = Bordering the Vertisols in the Medecho area, these sites are slightly sloping and contain patches of dense bush.4.B.2 = A bushy grassland, again adjacent to Vertisols near the Sarite Ranch.
4.B.3 = A unit near the 4.A.6 series. Volcanic rocks are common.
4.B.4 = This is a facet having a northern exposure. Soils are variable.
The group on shallow soils occurs on rocky hills adjacent to a large lava flow. There are two facets:
4.B.5 = Shallow soils on volcanic hills and slopes. Includes colluvial material, possibly due to proximity of hills representing the 3.A.4 type.4.B.6 = This is an extension of unit 4.B.5, although this facet has higher grass cover and fewer rocks.
4.C Transition units
These units can be ranked based on vegetative physiognomy (from woodlands to grasslands). Physiognomy also reflects topography, as woodlands are closer to mountain slopes, and grasslands correspond to lower elevations nearer to lava flows. Seven facets have been mapped:
4.C.1 = Dense woodlands on lower slopes of the massif. Grass cover is very reduced except on old encampment sites. Acacia spp is the dominant genus for woody plants and invading fortes are abundant.4.C.2 = These units occur along the edge of the granites. Woody plants are more scattered and bare patches of soil can be observed. The ground is often rocky.
4.C.3 = Open woodland close to volcanics, south of the Mega range. These also include volcanic outcrops, boulders and small cones.
4.C.4 = A similar physiographic unit to 4.C.3, but invaded by bush and still heavily grazed, especially near watering points (i.e. at Dilo Goraye or at mountain springs).
4.C.5 = Bushed grasslands associated with 4.C.2 that are not readily accessible to pastoralists. Further development of the tree layer would result in formations similar to those of 4.C.2.
4.C.6 = Large grassland plains, with vegetation physiognomy similar to 3.D.1 and subject to bush encroachment.
4.C.7 = Old lava flow with granitic outcrops. Vegetation is a mosaic of dense bushland and open grasslands.
The aridity of these sites is of a dual nature; partly climatic as related to reduced rainfall at lower elevations to the west but also edaphic as a consequence of shallow volcanic soils. Acacia mellifera, A reficiens and A horrida are dominant woody plants and differentiation of units considers density of trees and shrubs. In addition, lava flows have been grouped together because they exhibit low levels of grass production.
5.A Wooded plateaus
The central lava plateau is rocky and uniformly wooded. Subdivisions occur on the basis of landscape and soil features:
5.A.1 = This unit lies on deep soils and only scattered stones and boulders can be seen on the surface. Encampments have been established for wet-season grazing.5.A.2 = A highly dissected and eroded terrain on both sides of the central valley and along the western drainage line. The unit is sloped with many lava boulders and sparse grass cover. Human occupation has occurred in the past.
5.A.3 = Woodland invaded by bush. Bare ground also common.
5.A.4 = The ultimate stage of degradation in this series; only observed in one location south of Sarite Ranch.
5.B Bushed savannahs
Poorer soils have been invaded by bush, and although the vegetation is not much different from that of 5.A units, the environment appears less resilient to heavy utilisation. Facets include:
5.B.1 = Volcanic hills and slopes have shallow loose soils with a dense cover of shrubs.5.B.2 = This unit is a plateau covered by volcanic ash; soils are diverse because of occasional rock outcrops and moist swales.
5.B.3 = Similar to 5.B.2 but in a lower topographical situation where contact is made with Vertisols, especially to the south of Sarite Ranch. Grass cover is higher than in other 5.B units.
5.C Grasslands
These grasslands contain some woody vegetation. Shrubs are patchy or localised in drainages. Soils are rocky and grass cover is moderate. Facets include:
5.C.1 = A large plain at the extreme south-western part of the zone occurs on very fine soils; some bush encroachment is evident.5.C.2 = This unit is a mosaic of open and bushed grassland. Plant cover is variable.
5.C.3 = This corresponds to higher ground on volcanics and includes sloped and eroded sites. Except for drainages, soils are shallow and grass cover is low to moderate.
5.D Lava flows
Lava flows can be differentiated on the basis of their age: Older flows have more developed soils and a higher plant cover, while younger flows have a lower plant cover or are still bare. Lava flows are concentrated at lower elevations to the south of the study area. Facets include:
5.D.1 = This highly weathered lava is covered by a thin layer of ash, with exposed volcanic rocks. Vegetation consists of patches of scattered trees.5.D.2 = Like unit 5.D.1, this is associated with the Mega hills complex. The terrain is undulating with sandy ripples. Bush cover is variable and grass cover is higher compared to that for 5.D.1.
5.D.3 = This grassland is a facet near 5.D.2 at lower elevations and having deeper soils.
5.D.4 = A large and rather recent flow that remains nearly bare. Volcanic vents and miniature craters occur.
5.D.5 = The latest lava flows are completely devoid of vegetation.
Bottomlands cannot be classified on the basis of rainfall because the local water regime benefits from run-on moisture. All units in this group are temporarily waterlogged in rainy seasons. There are, however, differences among facets due to variation in soils.
6.A Vertisols
Vertisols (see Section 2.4.1.3: Soils) derived from volcanics have a fine texture and a poor water permeability. They produce impassable patches for vehicles when they are wet. They can be used to establish stock ponds. Vertisols are dark coloured and vegetation usually consists of Pennisetum spp grassland with A. drepanolobium. Other units, however, are better drained and consequently have a greater density of woody plants. Facets include:
6.A.1 = A mosaic of Vertisols and small hills covered by shrubs in the southern portion of Sarite.6.A.2 = Plains consisting of bushed grassland with an occasional thin layer of sandy soil. The grass layer varies according to the seasonal flooding regime.
6.A.3 = Lower elevations of unit 6.A.2. These are temporarily flooded. Soils have been substantially leached.
6.A.4 = The most common land facet in Did Hara. This is a bushed grassland with A. drepanolobium and A. seyal.
6.A.5 = A south-eastern variant where A. seyal is dominant. Waterlogging is restricted to lower layers of the soil.
6.A.6 = Near to unit 6.A.4, but showing less compacted soils and lower production potential for vegetation.
6.B Riverine units
This group mainly includes plant communities on coarse Fluvisols that are well drained. Units have a moderate to high cover of trees and shrubs. Some woody plants have been cleared for cultivation. Differentiation of facets is based on density of woody plants:
6.B.1 = Forest on dark alluvium, corresponding to major drainages of the Did Hara region.6.B.2 = Dense woodland in volcanic valleys. Valleys are oriented south wards and are sometimes discontinuous. Grasslands occur along banks of stream channels.
6.B.3 = Densely bushed grasslands that occur to the south-east; comparable to 6.B.1.
6.B.4 = One isolated patch where the valley is interrupted. Woody vegetation is variable.
6.B.5 = Grasslands with scattered trees and heavy grazing pressure. Units are intermediate with respect to 6.B and 6.C designations (i.e. soils in this unit are derived from limestone).
6.C Limestone valleys
Limestones are easily eroded and have produced alluvions in major valleys. Soils differ according to various parent materials, and most soils have developed on mixed allusions Soil colour ranges from whitish or light grey to red, and soil is usually fine textured. Differentiation among facets is based on vegetation physiognomy:
6.C.1 = Degraded forest in the vicinity of wells.6.C.2 = Bushland on a mixture of limestone and quartz. Grassland occurs in strips in flood plains.
6.C.3 = Grassland on very white soils, probably subtended by volcanics (a few isolated volcanic rocks occur). Shrubs and A. drepanolobium are rare.
6.C.4 = Woodland on red soils. Sites less hydromorphic.
