Environmental and human determinates of vegetation distribution in the Hadhramaut region, Republic of Yemen
Al Khulaidi, Abdul Wali Ahmed
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The principal objectives of the research are to analyse the distribution and dynamics of plants of the Hadhramaut region and to evaluate the role of the physical parameters and human action on their distribution, survival and conservation. The study area is located in Hadhramaut Governorate and lies in the eastern part of the Republic of Yemen. This is a remote and inaccessible region; however, there has been rapid development in recent years with the discovery of oil, which has had a significant effect on the vegetation and landscape. The Hadhramaut region represents an important area of eastern Yemen, linking eastern and western phyto-geographical units, representing a key transition zone between northeast Africa and Southeast Asia. Previous studies in the study area have only dealt with individual species and there has been no complete botanical survey. Recent floristic studies are turning up new species with many endemic and near endemic plant species. The Hadhramaut region is a desert region, dissected by deep valleys where agriculture is possible and the main towns are surrounded by rocky, dry limestone plateaus. The northern section passes into the deserts of the Rub ‘al Khali or Empty Quarter. Hadhramaut has a long history of human occupation with ancient civilisations well reflected in the archaeological records. Archaeological sites suggest that agriculture, with a related development of irrigation technology, was more widespread during a period when rainfall was more abundant. Initially, a reconnaissance survey of the whole Hadhramaut Governorate was undertaken, leading to the selection the Wadi Hadhramaut for detailed study. Within this study area, three sites were selected for intensive survey. These sites were considered representative of the major landforms and vegetation of the area and reflect the principal patterns of land use. The three sites represent tracts of land that were either unaffected, undergoing change or already altered as a result of oil-related development. Transects were designed to cross each site, from the valley bottoms to the plateau surfaces, passing across the representative landforms and vegetation. Surveys were made of the vegetation associations, their structure and biodiversity, as well as their relationship with environment and human impact. Two preliminary transects were made across the entire region, from the southern coast to the plateau in the northwest and from east to west, in order to place the study area in a regional context. The research is the first detailed vegetation survey in the Hadhramaut region and has revealed relevant data that can be used for further studies in similar habitats or for further management and conservation activities. In the study area, major vegetation associations, their composition and biodiversity were identified and in addition, vegetation and land use maps were generated including local endemic, near-endemic and rare plant species. About 469 plant species have been identified from the Hadhramaut region. There are 107 taxa which are endemic and near-endemic; 68 of these are endemic to Yemen, of these 41 are confined to Hadhramaut region. A total of 134 species belonging to 42 families (about 30% of flora of Hadhramaut region) were recorded in the study area and, of these, seven species are endemic to Yemen (four of them endemic to Hadhramaut region). The study revealed 15 vegetation associations and thirty sociological species groups. The main wadis are covered by desert alluvial shrubland comprising Fagonia indica, Tephrosia apollinea, Cymbopogon schoenanthus, Boerhavia elegans and Dichanthium insculptum with scattered trees of Acacia campoptila. In contrast, much of the fertile lands of the main wadis, such as the bottom of the rocky slopes, are intensively cultivated with palm trees and other annual crops, notably sorghum and wheat. The rocky slopes facing the main wadis and the plateau surface are covered by stony and gravelly desert vegetation dominated by herbaceous plants, namely Stipagrostis hirtigluma, Farsetia linearis, Aristida triticoides, Fagonia paulayana, Boerhavia elegans and Dichanthium insculptum. Within the plateau there are some sloping sites and secondary wadis which support dense vegetation. The vegetation here comprises shrubland or grassland dominated by Jatropha spinosa with Zygophyllum decumbens, Commiphora foliacea, Commiphora kua, Maerua crassifolia. Dichanthium insculptum, Stipagrostis hirtigluma and Farsetia linearis. The research in the Hadhramaut region has revealed the importance of this region in terms of plant biodiversity, and particularly of endemic, rare and near-endemic species, which urgently require further management and conservation activities.