Moses Ooro Olang
Tana and Athi Rivers Development Authority
P.O. Box 47309, Nairobi, Kenya
There is need for another committee to be set up to review the boundaries of the existing ecological zones. The East African Range Classification based their classification on ecological potential of the land and the description of the existing vegetation with emphasis on nature and relative contribution of woody plants and grass.
Detailed work must be carried out on broad eco-climatic zones so that they are subdivided on physiographic and edaphic criteria to yield precise ecological land units. The author has tried to subdivide the existing ecological zones into 29 ecological units based on total and monthly rainfall distribution. Also used is the livestock utilisation of these areas. The latter criterion is useful when it comes to planning. Rainfall controls soil formation and its potential, and that is why it became a useful tool in the subdivision also.
The East African Range Classification Committee which was set up to classify rangeland ran into a problem because those who had tried to classify rangeland before them used grassland composition (Pratt and Gwynne, 1977). This approach was not satisfactory because the variation in grass composition in East Africa is so great that to classify rangeland solely by reference to one or two of its constituent grass species would give an incomplete idea of its character.
The committee then used three characteristics to classify rangeland; soil moisture availability, climax vegetation and land use. They came up with six ecological zones. Rangeland was divided into three zones i.e. IV, V and VI. There has always been a need to review these zones because variation in productivity is too great. For example, two areas may receive a total of 500 mm of rainfall annually.
One area may receive all that rain in one month while another area may receive it in two months. These two areas will differ in their land potential and grass composition. There has therefore been a need to review these ecological zones for proper planning and effective management.
Kenya Rangeland Ecological Monitoring Unit (KREMU) divided rangelands of Kenya into 44 eco-units based on area occupied by animals belonging to a group of people all year round. This criterion of land use was not enough to delineate rangeland into ecological units which could later be used for planning purposes. Other factors like soil types, rainfall, soil moisture, and vegetation types had to be considered.
The methodology involved looking at those factors already mentioned to see how they control or affect land potential.
Soil undergoes a series of development stages as climate and organisms act upon the original rock or parent material (Stoddard and Smith, 1955). Thus soil alone cannot influence the productivity of an area because its development is controlled by other factors. Soil which has developed through interaction of high temperature, low rainfall and little organism will be poor in plant nutrient.
Vegetation is the product of its environment. The environment includes land form, soil, climate, animal and man. Often, of course, the present vegetation represents a stage of regression from a more highly developed or rigorous community which has been brought under stress, perhaps through use by man.
Rainfall readings were divided into seven segments of 100 mm interval from 200 mm to 800 mm. Isohyets were drawn to join all areas with the same readings. Another rainfall parameter which was used was the monthly distribution.
Soil Moisture Pattern
Soil moisture is closely related to grass production. Cassady (1973) reported that storms which produced 175 mm to 280 mm of rain within 30-60 days soaked the soil profile to depths of 120-180 cm. Thus soil which is capable of retaining most of the rain water and make it available to plants has higher plant production than either sandy soil or soil with hard layer which does not allow much percolation. Soil moisture was not used because not much research has been done on the major soils.
Generally land use in most areas of rangeland is pastoralism. The area was used by either one ethnic group or a section of that group without coming out of it unless it was very necessary.
Isohyets of 500 mm, 400 mm, 300 mm and 200 mm were used as boundary lines. Another boundary line was added showing livestock movement by one ethnic group or a section of a group. Thus a few modifications were made to the isohyets to accommodate these movement areas. For instance, Ewaso Ngiro was curved out of Mandera and Khorof Harar because of increased soil moisture from the underground river.
The whole rangeland was divided into 29 ecological units, as shown in Figures 1, 2 and 3. The present 29 ecological units are manageable units as they are being used at the moment. There is some relationship between the boundaries of land potential and rainfall monthly distribution.
Figure 1. Rainfall map of Kenya.
1. Lotikipi Plain
This eco-unit covers the upper half of Turkana District. The rainfall ranges from 300-500 mm annually, and it decreases from the Sudan border to Lake Turkana. The driest months are January through March while the wettest are April through June. The other months receive a little rain. Bush density also decreases with rainfall while the Lotikipi Plain itself is open grassland. The western bushland has grasses dominated by Sporobolus confinis and Eragrostis racemosa. The central plain is dominated by palatable grasses like Echinochloa haploclada and Chloris virgata but when misused, the area is invaded by Dactyloctenium aegyptium, Aristida mutabilis, Pupalia lappacea, Barleria accanthoides and Cleome hirta (Olang, 1984b).
Figure 2. Ecological zones of Kenya.
Livestock concentrate in the highland around Lake Turkana during the dry season of January through March but move down in the plain during the rainy season. From there they move towards the Sudan border from July onwards, but cattle rustling interferes with the grazing in that area.
2. Lokichar Eco-unit
This is the lower part of Turkana District. Rainfall ranges from 500 mm on the western border to below 100 mm around the town. The dry central scrubland is divided almost in half by the Turkwel river. The area receives rainfall in April and Nay and very little along the Uganda border in June. The short rains of November and December are hardly experienced because of the prevailing wind which blows from Lake Turkana to the western border from August through December.
Figure 3. Ecological units in Kenya.
Riverine vegetation are dominated by Acacia tortilis, Hyphaene coriacea and Salvadora persica. Areas away from the river are dominated by A. reficiens, whose density and tree height increases with increasing rainfall.
The area is dominated by annual plants like Digitaria velutina, Aristida mutabilis, Dicoma tomentosa, Pupalia lappacea, Zaleya pentandra and Eragrostis cilianensis.
D. velutina dominates the western border, while A. mutabilis dominates rocky areas and E. cilianensis is the common grass found in the remaining areas. Further to the west and the southern borders perennial grasses like Panicum maximum, Digitaria milanjiana and Echinochloa haploclada are found. When the area is misused, it is invaded by A. mutabilis and Cleome monophylla.
3. Pokot Eco-unit
This eco-unit forms the northern end of the Charangani hills. Rainfall ranges between 500 mm and 700 mm annually. The area has thick bush dominated by Acacia tortilis, A. reficiens, A. brevispica and A. mellifera. Grasses are mainly desirable species like P. maximum, Chloris gayana and Cenchrus ciliaris.
4. Maralal Eco-unit
This area runs from Sukuta Marmar through Maralal town up the northern end of the district. There is a ragged valley bordering Turkana District where vegetation is still in its potential. Rainfall ranges from 300 mm to 600 mm. The area east of Maralal hill receives the lowest amount of rainfall. Rainfall falls during the months of March to April and October to November.
Desirable grasses like Themeda triandra, C. dactylon and D. milanjiana are systematically being replaced by Aristida adoensis and Harpachne schimperi which are increasers and Microchloa kunthii which is an invader.
5. Cherangani Eco-unit
This eco-unit mainly comprises the eastern footslopes of the Cherangani hills and the Kerio Valley. The western side of the river is dominated by Acacia tortilis while the eastern dry side is dominated by A. mellifera, and the grasses are mainly C. dactylon, A. mutabilis and Urochloa brachyphylla. There is high concentration of livestock in the valley because of water availability.
6. Baringo Eco-unit
This eco-unit lies between the Tugen hills and Laikipia escarpment with Lake Baringo centrally located. The vegetation cover is good to the south around Chemogoch where grasses like T. triandra, E. superba and C. dactylon are still available. The vegetation cover and palatability improves again north of Lake Baringo. The area around the lake has been overgrazed and is dominated by Acacia adoensis, Sporobolus fimbriatus, Microchloa kunthii, Tragus terrestris and Dactylocteneum aegyptium. Rainfall ranges between 300 mm and 500 mm, decreasing from south to north.
7. Laikipia Eco-unit
This eco-unit is bordered by the Ewaso Nyiro river on the north, Aberdares mountains to the south and Mt. Kenya to the east. Its rainfall ranges from 400 mm to 600 mm annually, decreasing northward. The grass cover is dominated by Themeda triandra and C. dactylon. Cynodon increases as Themeda is overgrazed, but the area is generally still in good condition as most of it is divided into ranches which are well managed. Acacia drepanolobium is the dominant woody species. Areas which remain dry for longer periods support short flat-topped A. drepanolobium while the more fertile areas that remain moist for longer periods support very tall A. drepanolobium. Areas which are overgrazed, seen mostly towards Rumuruti, are dominated by unpalatable species such as H. schimperi and A. adoensis.
8. Mukogodo Eco-unit
This is the eastern end of Laikipia district where the Ndorobo ethnic group lives. In this hilly area human population has greatly increased.
Although the area gets good rainfall, overgrazing and the hilly nature of the area has greatly accelerated soil erosion. Woody vegetation is being depleted through charcoal burning. Harpachne schimperi and Microchloa kunthii are replacing palatable species such as T. triandra.
9. Isiolo Eco-unit
This eco-unit runs from the Somali strip on the western end through Shaba and Buffalo Game Reserve to Modogashe town. Rainfall and land potentiality decreases from west to east. The long rains fall between April and May while the short rains fall in November. Herbaceous plants are dominated by T. triandra, P. maximum, Cenchrus ciliaris, Sporobolus marginatus, Panicum coloratum and Chloris roxburghiana. C. roxburghiana and C. ciliaris dominate around Garba Tula while C. dactylon dominate in the northeast of Merti town.
Areas which have been overgrazed around Garba Tula are dominated by Aristida papposa and Digitaria velutina. The woody plants are dominated by Commiphora sp. while areas around Kula Mawe through to the Shaba Game Reserve is dominated by Acacia sp.
10. Chalbi Desert Eco-unit
This eco-unit can easily be divided into three sub-units. The Kaisut area has quaternary sediment and lies south of Marsabit hill. The second is the old lake bed of Lake Chalbi which is mostly covered by saline alkaline alluvial materials. The last is the recent alluvial and aeolian material overlying the eastern and north-western edges of the Chalbi desert (Herlocker, 1979).
The desert forms a swamp of an interior drainage system with perennial springs at the edge of the desert.
Long rains fall in April and May while short rains fall in November. Evapotranspiration in this area is so high that the rains support mostly annual species. The desert supports salt-loving species like Sueda monica while other areas support annual grasses such as Aristida papposa. While one view was that the area was dominated by Chrysopogon/Aristida, Heady (1960) thought Aristida sp. was the most dominant grass. Pratt and Gwynne (1977) called the area barren land. Composition and production of grasses vary greatly from place to place and from time to time reflecting high variability in rainfall (Herlocker, 1979).
11. Marsabit Eco-unit
This eco-unit is found in two areas: around Marsabit hill and Mt. Kulal. It is restricted to the upper elevation of these mountains.
(a) Marsabit: The southern and eastern slopes of the mountain are wetter than the others (Herlocker, 1979). The deciduous trees consist of species like Commiphora spp. and Croton dichogamus. Harrisonia abyssinica is a common shrub. Fire seems to be an important factor in degrading the vegetation. Grasses are mostly Dichanthium/Themeda, but overgrazed areas have unpalatable herbs or grasses such as Eragrostis tenuifolia and Chenopodium spp.
(b) Mt. Kulal: Like Marsabit rainfall is higher and better distributed than other parts of the district. Herlocker (1979) noticed that perennial grasses grow where openings occur in the bush canopy. Grasses are especially like those found in Marsabit. Most of the pastoralists who bring their livestock up-hill in the morning during the dry season descend in the evening because of cold nights.
12. Samburu Eco-unit
This eco-unit runs from the Maralal (Karisia) hills to the Wamba and Oldoinyo hills to the east of the Elbarta plains. Animals graze in the Suare and Ilponyeki plains during the wet season but come to water at Seyia river. During dry seasons most of the animals are taken into the hills while others go to Kom area. Range conditions in this area seem to be decreasing as readings on herbaceous cover showed a change from 42% in 1961 to 31% in 1970, then 22% in 1980. Plant composition of desirable species also changed from 68% in 1961 to 49% in 1970, then to 33% in 1980 (Skovlin, 1980).
Ilponyeki plain is dominated by Cynodon dactylon and Aristida papposa.
13. Ewaso Ngiro Eco-unit
This eco-unit receives the same amount of rainfall as Mandera and Khorof eco-units except it gets more water through the drainage system. Lagh Bogal which starts from the border of Marsabit and Wajir around Tamsa swamp runs through the Boji plain and ends at the Kenya/Somalia border. The other is Lagh Awaro which forms the northern end of the unit. Lagh Dera is actually the lower reaches of Ewaso Ngiro where water from the river runs underground. It has the highest number of perennial grasses. P. coloratum, Sporobolus spp., A. adoensis, Rhynchetrum spp., Enteropogon macrostachys and Eragrostis caespitosa have been recorded (Olang and Karime, 1981).
14. Mandera and Khorof Eco-units
From the Ethiopian border the area is mostly rugged with a lot of small water streams from west to east. There are a number of watering points along the international borders with Ethiopia and Somalia. Herbaceous cover in the south is dominated by Justicia flava, Aristida kenyensis and Aristida adscensionis. Rainfall ranges between 300 mm to 400 mm annually although the central part falls below 300 mm.
15. Tupo Eco-unit
This eco-unit lies below Marsabit and Mandera/Khorof eco-units. It remains wet longer than the areas around. The good grass cover found in the Lagh bogal drainage system starts from Tamsa swamp which is in the southern end of the unit. C. dactylon is one of the most important perennial species.
16. Huri and Sololo Eco-unit
The hills are the southern end of the Ethiopian hills with the rainfall near the border being more than 800 mm. The hills do not have extensive areas of closed forest, but many valleys, gullies and steep hillsides have dense cover. Among the common trees are Croton macrostachyus, Euphorobia candelabrum, Olea africana and Bauhinia tomentosa.
Open hills and gentle slopes in the wetter western slopes carry plants like Combretum molle, Lannea spp. and Ozoroa spp. Much of the Hurri hills is grass covered which is edaphically controlled. Soils are heavy clay, with impeded drainage and frequent flooding (Synott, 1979). Themeda/Chrysopogon grassland occurs at the eastern base and to the northeast of Hurri the hills (Herlocker, 1979).
17. Ileret Eco-unit
This eco-unit includes Sibiloi National Park where the herbaceous cover is dominated by Dactylocteneum aegyptium and Aristida mutabilis which are both annual with one perennial Sporobolus ioclauds (Odhiambo, 1981). Other grass species include Enneapogon cenchroides, Aristida adoensis, Tetrapogon tenellus, Tragus berteroniaus and Stipagrostis uniplumis. Further north towards Sabarai the number of species and percent cover decreases. A. mutabilis becomes dominant. Other species like Tetrapogon tenellus and Tragus berteronianus are found in low depression areas where soil moisture is higher.
18. Kitui Eco-unit
This eco-unit covers the whole of Kitui District, the northern part of Machakos and parts of Embu and Meru districts. Livestock movement is not so conspicuous here because of cultivation and some improved ranches.
Kitui District used to support perennial grasses like P. maximum, Sehima nervosum, H. contortus, E. superba, E. macrostachyus, D. milanjiana, C. roxburghiana and T. triandra (Skovlin, 1980). Overgrazing and shifting cultivation has resulted in increased bare ground and decreased cover by perennial grasses. Annual grasses and herbs like Brachiaria leersioides, Justicia exigua, E. cilianensis, Tetrapogon tenellus and A. adscensionis dominate most of the overgrazed areas (Muchoki, 1982).
19. Nakuru Eco-unit
This eco-unit covers the Nakuru, Naivasha and Elementaita area where most of the savanna has been turned into wheat farms. Dominant grass in its potential is T. triandra, but with increased grazing, C. dactylon and Sida cuneifolia become dominant. With overgrazing, H. schimperi and Micqsrochloa kunthii increase or invade. Vegetation cover is changing from grassland to bush through the encroachment of Tarchonanthus camphoratus in the Gilgil area.
20. Narok Eco-unit
The Loit hills form the southeastern border while Mau escarpment forms the northern border. These two hills attract rain from south and west making the unit wetter than normal. The northeastern part of the plain receives fairly little rain. The Acacia/Pennisetum shrubland in the black cotton soil has a mixture of Pennisetum mezianum, C. dactylon, Achyropsis greenwayii, Justicia elliotii and Dychoristes radicans. Themeqsda triandra dominates in red or sandy loam soil (Olang, 1984b). Overgrazing in black cotton soil leads to increase in cover by J. elliotii and D. radicans. Overgrazing in Themeda-dominated areas leads to increase in woody plants such as Lippia spp., Indigofera spp., Justicia exigua and grasses such as M. kunthii and H. schimperi.
21. Magadi Eco-unit
From Suswa dam down to the beginning of Lake Magadi is the Kedong valley which has good stands of Cynodon plectostachyus and T. triandra. The area west of Lake Magadi is dominated by Sporobolus helvolus which covers the whole area during the rainy season, but almost all of it is grazed during the dry season.
The area east of Lake Magadi is covered with rock pebbles with scattered Aristida spp. during the rainy season, but all of it disappears as the dry season comes. Areas further east and southwards are dominated by T. triandra, C. dactylon, P. maximum and P. mezianum.
22. Tsavo Eco-unit
The soils are weathered and strongly leached. The area receives rainfall between 400 mm and 600 mm annually. Areas receiving the highest amount of rain are Voi hills and Mtito Andei. Then there is a gradual decrease eastwards.
Vegetation cover and composition also respond to this rainfall pattern. The tree cover is Acacia/Commiphora bushland, and grasses around Mtito Andei are dominated by C. roxburghiana and C. ciliaris. Around Voi are C. ciliaris and Eragrostis superba while further south around Mackinnon Road are E. macrostachyus, P. maximum, E. superba.
Moving eastwards towards the Galana river composition changes and Chrysopogon plumulosus and a few Tetrapogon bidentatus are present. Further still, A. mutabilis becomes dominant during the dry season.
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