Impact of land use on vegetation resources with emphasis on woody vegetation in the semi-arid area of Abala district, North Afar, Ethiopia
This study aimed at investigating changes in vegetation resources with emphasis on the woody component in relation to land use changes (i.e. settlements, cultivation and grazing). It also made an attempt to understand the perception of local communities as regards these changes in their land-based resources. The study area was characterised by a semi-arid type of climate, where agropastoral production system was the predominant land use practice. The methods employed were interpretation of aerial photographs taken at two points in time i.e. 1964 and 1994, field vegetation survey and analysis, interviews using a structured questionnaire, and informal and formal discussions. The results of aerial photograph analyses indicate that conspicuous changes occurred in land use and vegetation types (physiognomically) over the 30-year period between 1964 and 1994. Changes in land use have apparently caused changes in vegetation structure. These changes have resulted from introduction of settlements and subsequent practices of flood recession cultivation. Clearance of vegetation for cultivation in the flood plains was found to be the major cause of depletion of vegetation resources and overgrazing of the uncultivated areas thereby leading to environmental deterioration. The size of land under cultivation increased from 0.27% (97.5 ha) to 7.34% (2,605 ha) of the total area. Settlement increased from 0.01% (5 ha) to 0.51% (180 ha) and vegetation cover decreased from 87.88% (31,252.5 ha) to 75.52% (26,787.5 ha). Field vegetation survey results showed that there was still a moderate woody vegetation cover dominated by Acacia etbaica wooded bushland in the uncultivated plain areas, the hills and ridges. However, herbaceous cover was very poor and the understorey vegetation was mainly dominated by Euphorbia and Aloe species. Woody vegetation identified and recorded included Acacia, Grewia, Balanites, Salvadora, Cordia, Commiphora and other related genera. A locational comparison of six selected woody species i.e. Acacia etbaica, Acacia mellifera, Acacia nubica, Acacia tortilis, Grewia erythrea and Salvadora persica showed no significant difference (P>0.05) among three study sites (Abala, Shugala and Murga) for species composition, percent crown cover, density and diversity. Comparison vi among distances (in a 10 kilometre transect ran from each of the three selected land uses) within each site for percent cover and density also showed no significant difference (P>O.05). Comparison among species for each site and among distances for each species, however, showed a significant difference (P<O.05). Acacia etbaica had higher species composition, percent cover and density than other species in all the three study sites. From the socio-economic survey, it was possible to retrieve information on characteristics of the indigenous Mars and settled Tigrians, woody vegetation use and perceptions of environmental degradation. These communities are already aware that settlement and cultivation in the area hampered the pastoral form of life style and thus forced them to shift to agricultural practices. They acknowledged the vital role of trees and shrubs since they provide a range of products and services in their daily life. In sum, declining vegetation cover, formation of big gullies, declining water availability, reduction of wildlife numbers and species diversity are the outcomes of recent settlement, cultivation and recurrent drought in the area. Based on the findings of this study, recommended actions to mitigate and prevent negative environmental impacts include encouraging participation of local communities in the management of natural resources so as to utilise their indigenous technical knowledge, introducing soil and water conservation measures, protecting the existing grazinglands from encroaching cultivation, protecting and regenerating important woody species, and selective clearing and establishment of grazing reserves. The study also recommends further research on long-term vegetation monitoring, soil and vegetation relationships, ethnobotany/ethnoveterinary practices, assessment of herbaceous and woody vegetation biomass, and evaluation of the nutritive value of browse species and crop residues as dry season feed for livestock.