THE ROLE OF AREA EXCLOSURES OF DIFFERENT AGE ON CARBON SEQUESTRATION AND BIODIVERSITY RESTORATION POTENTIAL COMPARED TO OPEN WOODY GRASSLANDS IN TSELEMTI DISTRICT OF TIGRAY REGION, ETHIOPIA

Abstract

A study was conducted in Tselemti district, which is found in the northwestern zone of Tigray Regional state, North Ethiopia. The objectives of the study were to determine vegetation structure, biodiversity restoration potentials, carbon stock potentials and physical and chemical soil properties under open grazing land, five years exclosure and ten years exclosure. Vegetation and soil parameters were collected from exclosures and open grazing lands during the flowering stages of most herbaceous species in 4th week of August to October 2015. Soil parameters were collected from three soil depths (0-10 cm, 10-20 cm and 20-30 cm) and analyzed for soil bulk density, textural classes (sand, silt and clay), organic carbon, organic matter, total nitrogen, pH, electrical conductivity, available phosphorus, cation exchange capacity and exchangeable bases (Ca++, K+, Mg++ and Na+) at Mekelle Soil Research Centre. Plant species diversity and similarity indices among management practices were computed with Shannon-Weiner Index and Sorensen coefficient of similarity index, respectively and analyzed using Paleontological Statistics (PAST). A total of 57 herbaceous species comprising of 41 non-grasses and 16 grasses and 41 woody species consisting of 16 shrubs and 25 trees were identified from the study area. Species abundance, richness, density, basal cover of herbaceous and woody species as well as total aboveground and belowground biomasses in ten years exclosure were significantly (P < 0.05) higher compared to five year exclosure and open grazing lands. Bare ground percentage, evenness and herbaceous species diversity decreases as the duration of protection increases. No significant variation (p>0.05) was observed in aboveground carbon stocks, belowground carbon stocks and soil organic carbon stocks between open grazing lands and five year exclosures while the area closed for ten years differs significantly (P < 0.01) with both the five years closed area and the open grazing land. Grasses, herbs and litter carbon stocks varied significantly (P < 0.01) with the management practices being the largest in the ten years exclosure and the lowest in the open grazing lands. Similarly, the overall total carbon stock was significantly highest (P < 0.01) for ten years exclosure (193.31 tC ha-1) followed by five years exclosure (153.98 tC ha-1) and in open grazing areas (146.6 tC ha-1). Areas closed for ten years had significantly higher (p<0.05) soil organic carbon, organic matter, total N, available P, and exchangeable K+ and Na+ compared to five years exclosure and open grazing lands. Conversely, open grazing land displayed significantly (p < 0.05) lower exchangeable Ca++ and Mg++ and cation exchange capacity than areas protected for five and ten years. Finally, the values of organic carbon, organic matter, total N, available P, exchangeable bases (Ca++, Mg++, K+ and Na+) and cation exchange capacity were significantly affected with soil depths. Therefore, this study concludes that establishment of ten year exclosures on degraded grazing lands had positive effect in restoring biodiversity losses, improving carbon sequestration potentials and soil nutrient contents. So, degraded grazing land requires management interventions aimed at improving biodiversity restoration, carbon sequestration potentials and soil nutrient contents and properties.