Vegetation Composition , Biomass Production , Carrying Capacity and Grassland Types in Odolla Area of Shinile Zone , Eastern Ethiopia

A study was conducted to characterize vegetation composition and grassland types as well as to estimate biomass production and carrying capacity of the rangeland in Ordolla areas of Shinile zone, eastern Ethiopia in October 2004. The Ordolla grasslands covered 22,621ha on alluvial plain composed of sand and silt texture and clay in minor cases in small hydromorophic depressions. Seventeen vegetation types are identified in Orodolla areas based on soil types, vegetation structure and density, and drainage pattern
described using LANDSAT image and geographical map of the area. The dominant grass species were Cynodon dactylon, Cynodon nlemfuensis, Eragrostis cilianensis, Cenchrus ciliaris, Andropogon greenwayii, Brachiaria leersiodes, Setaria pumila and Sorghum arundinaceum. Acacia tortilis, Acacia nilotica, Acacia mellifera and Acacia seyal were the most dominant shrubs with scattered Caddaba rotundifolia, Caddaba furmisa, Seddera bagshawei, Tamarix nilotica, Dobera glabra and abundant Parthenium hysterophorus, Cissus rotundifolia and C. quadrangularis . The grass biomass estimated in enclosed site near Ordolla village was 4.5 t DM/ha/yr and the total grass biomass produced in Ordolla rangeland was estimated around 27 534 t DM/year. The theoretical, dry and wet seasons carrying capacities were 0.16, 0.06 and 0.09 TLU/ha, respectively. There were good grazing lands in hydromorphic depression and well drained facets in Ordolla areas but the palatable grasses, trees and shrubs are decreasing due to overgrazing, runoff and drought and replaced by Parthenium, Calotropis, Solanum and Xanthium species, which are invader category of the plant community, indicating overgrazing and rangeland degradation. Keywords : Biomass Production; Carrying Capacity; Grassland Types; Grazing Lands; Vegetation Composition East African Journal of Sciences Vol. 1 (2) 2007: pp. 148-159


Introduction
The rangeland of Ethiopia covers about 60% of the total area (about 62 million ha) of which 88% receive less than 600 mm rainfall per year, and is the major source of livestock feed (BLPDP, 2004;PFE, 2004).The pastoral areas of Ethiopia are located around the peripheries almost surrounding the central mass below 1500 masl on degraded shallow soils and fallow lands, which cannot be successfully cropped because of physical constraints (Alemu and Lemma, 1991;Alemayehu, 1998)).They are characterized by lowland plains and have relatively harsh climate with low, unreliable and erratic rainfall and high temperature (Coppock, 1994).The grasslands include sparse vegetation of grasses, bushes, shrubs, small trees and bare land with low level of surface water.The pastoral areas are sparsely human populated and are occupied by temporary and seasonal settlements with semi nomadic pastoral transhumant mode of life (Coppock, 1994).
The Somali Region in Ethiopia is one of the pastoral areas of the country covering about 285,000 km 2 and is divided into 9 administrative zones with 52 districts and the people depend on pastoral activity and in some extent agro pastoral agriculture (SORPARI, 2003).Evaluation and monitoring of rangelands is a useful strategy to document the capacity of the rangeland that could accommodate a given number of livestock species and to develop proper interventions for future pasture utilisation.The vegetation compositions of rangelands vary depending on topography, climate and soil fertility (Coppock, 1994).The current study was designed to characterize vegetation composition and grassland types as well as to estimate biomass production and carrying capacity of the rangeland in Ordolla areas of Shinile zone, eastern Ethiopia in which information is lacking for designing and implementing range and natural resources management and development interventions.
into lacustrine deposits (Corra, 1993).There is wide alluvium expanding plain stripped by fast moving hydrographical net south of Errer in the direction of Awash Valley and Djibouti.
Ordolla Kebele (peasant Adminstration) and most of the settlements are located along the main Ordolla River that joins Asbuli river to the west, 20 km to the north of Ordolla.On east side, the biggest river is found crossing Aydora Kebele, coming from Tebi and Sedeti mountains and all the rivers in Ordolla areas are seasonal and do not have permanent flood.The hydro net, in addition of these main rivers, presented plenty of tributary rivers, hydromorphic veins and progressive gullies created by soil erosion (Map 1).The range land was mainly open grassland covered by short growth cycle annual vegetation which was originally predominant compared to woody stratus.Woody stratus was found in the southern part of the area on sandy soils near the hills surrounding Errer district but decreasing bit by bit when going to the north due to heavy grazing.The rangeland was mainly opened grassland covered by annual vegetation (Map 2).

Study Procedures
Vegetation and grassland characterization was done by interpretation and primary zoning of LANDSAT image based on the appearance of images and using GPS by clustering and delineation of similar grasslands and vegetation types together and taking samples of representative points to be surveyed.Twelve aerial photographs available for the area issued from the Ethiopian Mapping Agency (1: 50000) (edited 1999), Asbuli and Aydora topo maps (1:50000) (1999) and Dire Dawa geological map (1: 250000) (1985) were used during the field work (Map 1).During the field survey, visits were made to each representative grassland priory identified on LANDSAT image and moving to each site using GPS for validation in early October 2004.The latitude, longitude and altitude of the grasslands identified on the map were read on a hand held GPS during the fieldwork.The data include location, soil type, grazing land exploitation and grassland types, percentage of vegetation cover, grazing lands and barren soil, proportion of grasses, shrubs and trees and range and soil degradation.

Vegetation Composition, Biomass Production and Carrying Capacity
In the long rainy season of 2004, five half-hectare (50m x 100m) pasture sites were enclosed in all direction around Ordolla village on degraded rangeland dominated by weeds and barren soil to sensitize livestock herders and authorities about the impact of overgrazing and to estimate vegetation composition and potential biomass production.For biomass yield estimation of the vegetation inside the enclosure, four samples of 1m x 1m quadrates in each site, a total of 20 samples were taken randomly, in which all the herbaceous species were cut at full maturity.All the plant species inside the quadrat were harvested and weighed and samples dried in an Oven at 65 0 C for 72 h to constant weight for dry matter yield determination (Tarawali et al., 1995).In addition, in each grassland types and in the enclosures, all plant species were listed and identified on site primarily based on their morphological, structural and floristic characteristics while unknown species were collected for further identification at Haramaya University Herbarium in comparison with preserved specimens and using taxonomic keys (FAO, 1989;Hedberg and Edward, 1989;Sylvia, 1995).
In Ordolla areas, the rainfall pattern allows two vegetation growth cycles per year.The first rainy period called "Gu" which is the shortest one from mid-March to mid-May and "Karan" is the long rainy season from July to September.The enclosure biomass yield from the five random sites and 20 samples across Ordolla areas represented the Karan vegetation growth only, which is the main biomass production period.The Gu vegetation growth has not been assessed but elders in the community considers that the vegetation growth in Gu period is roughly half of the Karan season due to short and small rainfall even if the limitation in exactly estimating the biomass in Gu period is clear.Thus, the total vegetation productivity of the area was obtained by adding the GU season production onto the Karan season biomass production.
Estimation of livestock numbers grazing in Ordolla areas was done through community interview on average number of animals per household and number of household heads for each Kebele settlements and the TLU conversion factors for cattle, sheep and goats, donkey and camel which are 0.7, 0.1, 0.5 and 1, respectively (ILCA, 1990).The theoretical carrying capacity (CC) of a given grazing area is calculated as CC = (B*K)/PD x 6.25 (Stoddart, 1976), in which B is the biomass edible in kg DM/ha, K is the animal use ratio which varies between 0.1 and 0.9 but usually 0.3 is considered as the grazing lands has limit for pasture regeneration properly and PD is the period duration in days and 6.25 is kg of DM consumed by one TLU/day.The pasture available for livestock is only for 3 to 4 months in Ordolla areas and the two rainy seasons are separated by a short dry spell resulting in creating two short vegetation growth cycles which limits the amount and duration of biomass production.During this dry spell the pastoralists move and displace their cattle by remaining sheep and goats and donkey.Therefore, the CC adapted to the transhumant condition for cattle in Ordolla areas was calculated for two short rainy seasons.
Five composite soil samples of each 1 kg were collected from hydromorphic veins, upper sandy areas and eroded gullies separately which constitute a total of 15 samples.A composite sample was prepared at each sampling site for texture, pH (Bouyoucos, 1962), organic matter (Walkley and Black, 1934), total N (Jackson, 1970), available P, exchangeable K, Ca, Mg and Na (Olsen et al., 1954).Electrical conductivity was determined using sodium saturation ratio (SAR) as Van Reev Wijk, 1992) and the results of the soil test are presented in Table 1  and 2.

Grassland Distributions
Seventeen grassland types are identified (Map 2) in Ordola areas based on soil type, vegetation structure and species composition according to vegetation classification of Pratt and Guwyanne (1977).The detail description of each grass land is indicated in Table 3 but some unique characteristics of each grassland type are discussed in brief as follows:

Barren Land with Scarce Caddaba rotundifolia and Restricted Pockets of Mixed Perennial and Annual Herbaceous (ORD 1)
The grassland was covered by barren land (95%) with scattered shrubs (1%) and perennial grasses (4%).The dominant shrub was Caddaba rotundifolia and rare Seddera bagshawei and Cissus quadrangularis and the grass species were Brachiaria leersioides and Dactyloctenium scindicum.

Acacia nubica Open Shrub Land (ORD 2)
It was open shrub vegetation composed of Acacia nubica syn.Oerfota and rare Caddaba with 40 and 10% canopy cover, respectively, and some weeds along the shallow gullies.The vegetation was covered 90% by barren soil and 5 to 10% scattered C. rotundifolia.

Hydromorphic Fast Land Covered by Tribulus zeryheri (ORD 14)
The vegetation was highly influenced by hydromorphic condition and covered only by barren soil without grasses but it was covered by T. zeyheri during the rainy season.

Xanthium strumarium Monospecific Formation on Degraded Riverbanks (ORD 15)
The vegetation area was characterized by 30% barren soil land and exclusively covered 65% by X. strumarium and 5% by C. procera but no herbaceous species at all.

Calotropis procera Dominated Shrub Land on River Overflow Area (ORD 15)
The vegetation was dominated by tall and densely populated C. procera, followed by X. strumarium, Amaranthus hybridus, S. somalense and P. hysterophorus, respectively.

Alluvial Terrace Barren Land with Scarce Caddaba Shrub and Annual Vegetation
This vegetation was dominated 15% by Caddaba species and 85% by barren land and some pockets of T. zeyheri.

Acacia and Caddaba Dominated Open Vegetation (PS 008)
This vegetation was characterized by distinct patches of A.oerfota, C. rotundifolia and T. nilotica and the presence of rare Barleria eranthemoides, Lasiocorys abyssinica, C. quadrangularis and stunted Panicum coloratum, Chrysopogon plumulosus and Eragrostis cilianensis.

Ranking of Vegetation Types
The grassland types were ranked into six pastoral values (Map 3) based on area coverage of palatable species mainly perennial grasses, species diversity and productivity of the pasture follows: -

High Pastoral Value Grassland
It covered 3 275 ha (14.4%) representing the best pastures of the area including pure vegetation (ORD 6), the mixed vegetation (ORD 10) and the very productive grass and bush mixed land (ORD 12).These are the most productive rangelands because of more than 80% perennial vegetation coverage.Moreover, this vegetation was mainly composed of dense perennial grass carpet (ORD 6 and 10) except in the small over trampled part (central part of hydromorphic veins, depressions-ORD 10).

Medium Pastoral Value Grassland
It covered 5 361 ha (23.6%) of Ordolla area and it included the hydromorphic vein vegetation (ORD 7), the close alluvial terrace (ORD 9) and the open shrub and vegetation (PS008).They were areas where some perennial and annual mixed grasses and weeds species were found in relatively dense coverage (ORD 7 and more weeds in ORD 9).It also included the PS008 grassland where only annual grasses and T. zeyheri were found but constituted an interesting potential area by its wide coverage and the presence of some Acacia shrubs.However, these rangelands have quite lower pastoral interest than the first group, as the vegetation coverage was limited to restricted areas (hydromorphic veins) or pockets (PS008).

Preferred Shrubs Dominated Grassland
It covered 1 561 ha (almost 6.9 %) of Ordolla area and included all woody grasslands dominated by A. nubica syn oerfota, along the tributary riverbeds.The herbaceous coverage was absent or limited (ORD 2, 4, 5) and mainly composed of T. zeyheri and some rare annual grasses in secondary erosive gullies.These grasslands have limited potential due to the absence of grass and even herbaceous species but have pastoral interest as they constitute an important browse for camels.

Barren Soil Dominated Grassland
It occupied 11 418 ha (almost 50.4%) of Ordolla area representing ORD 1, 8, 13, 14 and the wide terrace.The contribution of these grasslands to grazing was very low to null as they were covered by more than 90% barren soil and 10% by Caddaba shrubs.Some of the barren soil was completely sterile, but remaining wide areas covered by T. Zeheri, the only vegetation growing for few weeks.Due to its short-lived behaviour of T. Zeheri, it is partially grazed by camels before blooming.Therefore, these grasslands are not considered as significant grazing lands.

Altered and Degraded Grassland
This grassland covered 606 ha (almost 2.7%) of Ordolla area representing grasslands invaded by C. procera (PS003) and X. strumarium (ORD 15) where they were forming pure colony.These grasslands were located along the river, where trampling by animals during watering and do not have interest for animal grazing since there are no perennial grasses and preferred browse species.

T. nilotica Formation
It covered 400 ha (almost 1.8%) of Ordolla area.This vegetation (PS007) was isolated as it was pure population of T. nilotica.Moreover, it does have small value for grazing since there is no herbaceous species for animals.

Grazing Land Categorization
According to the ranking done, the grazing areas in Ordolla surroundings are categorized by considering the surveyed areas as a whole, based on the palatability of herbaceous vegetations (proportion of perennial grasses, shrubs and invaders available), total vegetal coverage and barren soil as follows: 1. Good and permanent grazing which represents only 15% of the surveyed area (grasslands of the first ranked group) 2. Complementary grazing lands, which represent additional 30% (medium pastoral value + browsing grassland), wide areas dominated by low productive barren soil as well as taking probably low part on livestock grazing 3.About 55% of the rangeland can be considered as non-grazing areas (barren soil dominated grassland + altered grassland + Tamarix colony) due to their limited values and it does have low value for livestock grazing or browsing.

Vegetation Composition, Biomass Production and Carrying Capacity
After enclosing the pasture, the difference between the inside and outside on vegetation composition was very clear and demonstrative.Although the place was looking degraded, barren, sandy and invaded by weeds, the grasses grown showed an unforeseen diversity and density.By protecting seasonal grazing, essential grass species has grown up, flowered and set seeds with in short period demonstrating high capacity of regeneration and presence of good soil seed bank.Moreover, it was impressive for herders to observe the grasses at full maturity as grasses were continuously grazed even up to the roots in the unprotected area.This showed that the area has good potential and capacity to support different livestock species if properly managed.The main grass species observed in the enclosure were Cynodon dactylon, Cynodon nlemfuensis, Andropogon greenwayii, Brachiaria leersiodes, Setaria pumula and Cenchrus ciliaris in order of abundance and rarely Cyprus involucratus Table 3. Enclosures are good sources of biodiversity of grass and shrub vegetations as indicated by different authors (Blench and Sommer, 1999;Emiru, 2002).The grasses are naturally palatable and important for livestock feeding and the height of these grasses reached around 25-50 cm.Similar research findings are reported by many authors in other parts of Ethiopia (Coppock, 1994;Baars, 1996;Ayana and Baars, 2000;Lema, 2001;Amsalu and Baars, 2002;Alemayehu, 2004;Yihalem, 2004;Yvan and Tessema, 2005).
Biomass produced measured inside the enclosure during the main rainy season was 4.5 t DM/ha.This biomass yield was obtained in the Karan rainy season (July to September) only herbaceous species due to the absence of trees and shrubs in the enclosure.The total grazing land annual biomass production in Ordolla areas may be estimated as 4.5 + 2.25 = 6.75 t DM/ha/year by taking the two short rainy seasons.This figure is comparable to most managed natural pasture (Tessema, 2005) and even higher than most of the communally grazed pasture in Ethiopia as reported by many authors (Coppock, 1994;Ayana and Baars, 2000;Lema, 2001;Amsalu and Baars, 2002).The productivity estimation for high pastoral value grazing lands, with no grass cover was 3 275 ha * 6.75 t DM/ha = 22 106 t DM/year and for medium pastoral value grazing lands, with 15% herbaceous cover was 5 361 ha * 0.15 * 6.75 t DM/ha = 5 428 t DM/year, and the total annual biomass produced by different grazing lands in Ordolla areas was estimated to be around 27 534 t DM/year.
For the 390 households in twelve settlements in the Ordolla areas, the number of different animal species was estimated as follows: shoats 39 340, cattle 11 396, camels 3 293 and donkey 315, total of 54 344 heads of mixed livestock or 15 361.7 TLU (Table 4).According to the estimated biomass production of the pasture and available livestock species in TLU, the carrying capacity of the rangeland in Ordola areas would be estimated as 0.16 TLU/ha/year (27 534 000 * 0.3/365 * 6.25) or 6.25 ha grazing land for 1 TLU or 1 camel or 10 shoats without affecting vegetation composition and pasture productivity.The estimated carrying capacity of Ordolla areas with respect to transhumant pastoral condition was 0.06 and 0.09 TLU/ha in dry (5 months) and in wet seasons (7 months), respectively.

Conclusions
In general, large proportion of Ordolla rangelands was barren land without herbaceous vegetation except some scattered shrubs and trees.Seventeen grassland types were identified in Ordola areas.Vegetation composition, grassland types and other phyto ecological characteristics of other districts of Shinile zone near Ordola area has been reported by Corra (1993).The dominant grass species in Ordolla areas were C. dactylon, C. nlemfuensis, E. cilianensis, C. ciliaris, A. greenwayii, B. leersiodes, S. pumila and Sorghum arundinaceum. A. tortilis, A. nilotica, A. mellifera and A. seyal were the most dominant shrubs with scattered C. rotundifolia, C. furmosa, S. bagshawei, T. nilotica, D. glabra and abundant P. hysterophorus, C. rotundifolia and C. quadrangularis.The grass biomass estimated in enclosure near Ordolla village was 4.5 t DM/ha/yr and total annual biomass produced in Ordolla rangeland was around 27 534 t DM/year.The theoretical and transhumant context carrying capacities of Ordolla rangeland in dry and wet seasons were 0.16, 0.06 and 0.09 TLU/ha, respectively without affecting the pasture.There were good grazing lands in hydromorphic depression in Ordolla areas but the palatable grasses, trees and shrubs are decreasing due to overgrazing, runoff and drought and replaced by Parthenium, Calotropis, Solanum and Xanthium species, which are invader plant species and replaced palatable plants (decreasers and increasers mainly perennial grasses) indicating overgrazing and rangeland degradation as observed in most parts of Ethiopia.

Acknowledgment
The authors would like to thank Handicap International for financing the study and Haramaya University for the expertise and the pastoralists for their assistance during the fieldwork.

Altered by Invading Species in Lower Hydromorphic Valley (ORD 3)
It was bushed grassland covered 40% by shrubs and 50 by various perennial grasses and 10% barren land.The grass species were Cynodon, Setaria pumila and B. leersioides and C. rotundifolia and some S. bagshawei and A. nubica were also found.The grassland was cinvaded by Xanthium strumarium, Solanum somalense and Parthenium hysterophorus.

Table 1 .
Soil physical and chemical characteristics in Ordolla areas of Shinile zone, eastern Ethiopia OM= Organic matter; EC=Eelectrical conductivity; Tot N = Total nitrogen

Table 2 .
Some soil macro nutrients in Ordolla areas of Shinile zone, eastern Ethiopia

Widely Opened Bushed Rangeland Striped with Perennial Grasses along Small Hydromorphic Veins (ORD 7)
This grassland was composed of two sub-grasslands: the long veins formed due to hydromorphic conditions and presented various perennial and annual grass mixed with weeds and the interfluves between hydromorphic veins were the upper land with dry and barren soil covered by scattered C. rotundifolia and T. zeyheri.

Table 5 .
Average and total of number of livestock species in Ordolla areas of Shinile zone, eastern Ethiopia