Woody plants in drylands: plastic responses to environmental stress

Plants in drylands are exposed to a suite of stress factors. The most obvious stress factor is drought stress induced by a strongly negative balance between precipitation and potential evapotranspiration. Drylands are increasingly being used for grazing livestock and with increasing human populations this form of land use is intensifying. Overgrazing is argued to be the major cause of desertification worldwide by affecting both plants directly and the surrounding environment. Burial, by wind-blown sand, directly damages leaves and dramatically changes the growth conditions of plants. High level of mechanical stress caused e.g. by animal trampling or drag forces imposed by the generally high wind speedpossibly contributes to reduction in growth and survival by increasing the risk of rupture of aboveground shoots. These stress factors may have strongly interactive effects. Woody plants are an important component of drylands vegetation and appear to exhibit traits that make them particularly well-adapted to these conditions. Moreover many woody species are used in combating desertification. This thesis focuses on the growth, phenotypic and mechanical characteristics and eco-physiology of woody species in Mu Us Sandland, Inner Mongolia, China, in response to different stress factors mentioned above. Unlike most previous studies on this issue, I place particular emphasis on the interactive effects of different stress factors. Particularly, the objectives of this thesis include: 1) a study of how woody species respond to local stress; 2) investigations on the interactive effects of these stress factors on plants; 3) evaluation of the role of clonal growth in alleviating the effects of these stress factors; 4) a deepening of the understanding of inter- and intra-specific variation in adaptive responses of the plants. Finally the significance of the results of this study in the context of management and restoration of vegetation in drylands ecosystems are discussed.