Rangeland potential, quality and restoration strategies in North-Eastern Ethiopia : a case study conducted in the Southern Afar region
Vegetation dynamics and restoration strategies of degraded rangeland were investigated near a watering point in the Allaidege communal grazing area in Administrative Zone 3 of the Afar Region in the northeastern lowlands of Ethiopia. The degradation gradient formed by grazing pressure in the study area was stratified into four different areas based on the vegetation cover; severely degraded (SD), moderately to severely degraded (MSD), moderately degraded (MD) and lightly degraded (LD) areas. The study was initiated at the start of the rainy season in June 2003 and lasted untill December 2004. The objectives were to study the effects of the grazing pressure on plant species composition; on plant biomass production and basal cover; on rangeland forage quality; on the rangeland soil status and to determine and quantify viable restoration strategies for forage species in severely degraded rangelands. The botanical composition of the different degradation areas was determined by making a 250 point wheel point method survey in each of four 30 m x 30 m quadrats in each degradation area using the nearest plant approach. The botanical composition of each degradation area was determined by measuring the frequency of occurrence of the different life forms (perennial grasses, annual grasses and forbs) of the species recorded in the field. Accordingly, a significant interaction was observed in both seasons between the different degradation areas and life forms considered. A high abundance of annual grasses was evident in SD and MSD areas in both seasons. In the MD and LD areas, a three-fold increase in frequency was recorded for perennial grasses compared to the MSD area in 2003. In 2004, the frequency of annual grasses, forbs and perennial grasses in the MD area was almost similar to that of the LD area. The abundance of perennial grasses in the MD and LD areas was two- and five-fold higher compared to perennial grasses in the MSD and SD areas respectively. Biomass production was recorded by cutting the vegetation in 1 m x 1 m quadrats in each grazing area at ground level. The dry matter content of subsamples was determined in order to calculate the dry matter production of the quadrat. The differences in dry matter yield recorded in the different degraded areas was not significant for the 2003 season, although an increasing trend in yield was observed from the SD to MD areas. Significant yield differences were however recorded when one outlier in the data was excluded from the analysis. The significant differences occurred between the MD and SD areas where the MD area produced 2.4 t ha-1 more dry matter than the SD area. Similarly, in 2004 no significant yield difference was observed between the degradation areas. However, the contribution of different species to dry matter yield varied in the different degradation areas. Setaria verticillata, Sporobolus ioclados and Paspalidium desertorum were found to be the major species contributing to the dry matter producion in the SD area, S. verticillata and P. desertorum in the MSD area, Chrysopogon plumulosus and P. desertorum in the MD area and C. plumulosus and Panicum coloratum in the LD area. The percentage basal cover was calculated from the number of basal strikes recorded at 1 000 points in each plot of each degradation area using the wheel point method. The total basal cover percentage did not significantly change along the degradation gradient in any of the seasons. However, data for both seasons showed an increasing trend of total basal cover percentage closer to the watering point compared to areas further away from the watering point, except for the SD area, which had the lowest basal cover percentage. The contribution to percentage basal cover by some species decreased while it increased for some other species in grazing areas near the watering point. Forage quality was investigated by analysing sub-samples of the forage samples taken to determine biomass production. The forage samples were analysed for neutral detergent fibre (NDF), acid detergent fibre (ADF), crude protein (CP), lignin, in vitro dry matter digestibility (IVDMD), phosphorus (P), and calcium (Ca) content. The forage showed a decrease in NDF and ADF content in areas close to the watering point in both seasons. This decrease in fibre content was accompanied by an increase in CP content close to the watering point. The increase in CP was significant for the SD area in both seasons. Although a similar trend was observed in both seasons, the CP content was found to be significantly higher in 2004 than in 2003. The results of the lignin analysis were inconclusive if the data of both seasons are considered. It does appear however as if the lignin content of the forage was generally higher in 2003. The 2 years pooled average of P content of the forages showed insignificant variation along the degradation gradient. However, an increase in P concentration of the forages was evident in areas far from the watering point. Contrary to this, Ca concentration was significantly higher in the SD area compared to areas further away from the watering point. Hand clipped forage samples and esophageal collected forage samples were analysed to compare the quality of the samples. Due to the fact that only two animals were available for esophageal collection, differences were in most cases not significant at the 5% level, but trends indicate that animals select higher quality forage than what is assumed based on hand clipping. Organic carbon (OC) content, total nitrogen (N) content, available phosphorus (P) content, available potassium (K) content, exchangeable calcium (Ca) and magnesium (Mg) contents, cation exchange capacity (CEC), total exchangeable bases (TEB), exchangeable sodium percentage (ESP), soil acidity (pH) and base saturation of soils in the different degradation areas were determined by means of acknowledged laboratory methods. No significant differences in OC, N, P, K, Ca, Mg and K content of soil in the different degradation areas could be observed. There was however an increasing trend for OC and N content with distance from the watering point. Sodium concentration and pH increased significantly in areas close to the watering point. Cation exchange capacity content of the soil was variable and no clear trend could be established. Significantly higher TEB and ESP contents were observed in the SD area. In general, the differences in plant biomass production and basal cover, botanical composition, forage quality and soil status over the degradation gradient clearly implicates the negative impact of unrestricted grazing pressure on the rangeland around the watering points. In the rangeland restoration trial, establishment of three local and three exotic grass species in the SD area was investigated. Treatments applied included application of inorganic fertilizer, dry dung organic manure and grass mulch. The mulch treatments caused a significant yield increase for all the sown species. Among all the species, Ischaemum afrum and Tragus berteronianus performed better and produced significantly higher dry matter yields than Enteropogon rupestris, Chloris gayana and Panicum coloratum. In general the study indicated the importance of mulching when planning to restore degraded rangeland under arid environmental conditions.