WOODY SPECIES DIVERSITY AND CARBON SEQUESTRATION POTENTIALS OF GRA- KAHSSU NATURAL VEGETATION IN SOUTHERN TIGRAY, ETHIOPIA

Abstract

Land degradation, which includes the degradation of vegetation cover and soil, and nutrient depletion, is a major ecological problem of Ethiopia in general and the study area in particular. Restoration of such degraded lands could improve the species composition of vegetation and ultimately enhance carbon sequestration potential of the vegetation. Therefore, this study was conducted in two land use systems, protected natural vegetation (PNV) and communal grazing land (CGL) of Gra-Kahssu natural vegetation, in Alamata district of Tigray region, northern Ethiopia. The plant community types, species diversity, regeneration status as well as above and below ground carbon stocks of the natural vegetation of the adjacent land use systems were described. In addition, the socioeconomic implications of the two land use systems was analyzed in relation to restoration of degraded lands. Collection of vegetation data was made using systematic sampling methods, laying 19 transects (10 at PNV and 9 at CGL) and 62 quadrats (35 at PNV and 27 at CGL). Heights (>1.5m), diameters (>2.5cm) and numbers of woody species were recorded each with 20 m x 20 m for trees and 5m x 5m for shrubs. In addition, 1 m x 1m sub-quadrats were laid within the main quadrat to sample herb and litter biomass as well as soil sample. Soil was collected from three soil depth (0-10, 10-20 and 20-30 cm) and analyzed at Mekele soil laboratory research center. The socioeconomic survey was carried out using a semi-structured questionnaire, focus group discussion and key informants. In the two land use systems, species richness, plant abundance, basal area, species diversity and Important Value Index of woody plants were calculated. For both land use systems, above and below ground carbon stock's potential of woody species were estimated using allometric equations. Finally, socioeconomic data were analysed using descriptive statistics. A total of 64 species of vascular plants were identified belonging to 52 genera and 37 families from PNV. On the other hand, 43 plant species belonging to 25 families and 34 genera were identified from CGL. Fabaceae was the most species-rich family comprising 18.75% and 27.91% of the total plant species identified in PNV and CGL, respectively. The mean densities of tree and shrub species in the PNV (6537.14) were higher than the adjacent CGL (6379.1). The Shannon diversity index (H') value was 2.1 and 1.8 in the PNV and CGL, respectively. Both diameter and height class distribution of the population structure of the PNV reflected reversed J-shape; however, the xix CGL showed a bell-shaped and interrupted inverted J shape, respectively. The regeneration status of the PNV and CGL were considered as good (sapling (47.62%) > seedling (35.69%) >matures (17.34%)) and fair (mature (58.59%) > sapling (25.31%) >seedling (20.32%)), respectively. The similarities of woody species in terms of species richness, genera and families of the two land use systems were about 40.22%, 39.43% and 42.59%, respectively. The mean above ground carbon stock of the PNV (21.05 ton/ha) was higher than CGL (15.31ton/ha). The average values of soil organic carbon for PNV was 16.60 and 13.76 ton/ha for CGL. The total carbon stock estimated in the study area was 50.74ton/ha and 37.11ton/ha for PNV and CGL, respectively. The majority of the local community has a positive attitude towards the establishment of PNV with restricted resource access and believed that they have gained benefits from it, especially the availability of fodder for their livestock. Most of the species present in these two land use systems were used for fuel, fodder, making agricultural tools, house building, and medicinal purposes. However, as they stated, the resource potential of CGL was declining from time to time, which may not support them longer for their livestock and other uses. Whereas, the resource potential of PNV was improved. The major output of this research showed that, floristic composition, diversity, density and above and below ground carbon stock in the PNV are improving due to a rehabilitation program with limited access of the surpluses resource to the local community. This is indicative of the role of established such PNV as the best practice of restoration programs through exclusion of livestock free grazing and human interference.