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Tham khảo tài liệu 'sustainable natural resources management part 8', khoa học tự nhiên, công nghệ sinh học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Sustainable Use of Natural Resources of Dryland Regions in Controlling of Environmental Degradation and Desertification 87 2.3 Plants Most plants possess adaptations that enable them to successfully reproduce sustain their growth and survive in some of the harshest environments in the world. Some plants have evolved special root systems while others have unique leaf characteristics that allow them to withstand prolonged periods of drought. Other plants lose their leaves when soil moisture conditions become too dry to support their survival. Ephemeral annuals succulent perennials and non-succulent perennials are found throughout dryland environments Middleton Thomas 1997 Mainguet 1999 Altschul 2008 . Ephemeral annuals appear after rains complete their life cycle in a short growing season and at times form dense stands to provide limited forage for livestock. Succulent perennials store water through the enlargement of parenchymal tissue and their low rates of transpiration. Cacti are typical of the succulent perennials. Nonsucculent perennials that withstand the stress of dryland environments are the majority of plants in these regions. Three forms of non-succulent perennials are found. Evergreen plants are active biologically throughout the year drought-deciduous plants are dormant in the dry season and cold-deciduous plants are dormant in the cold season. 2.4 People Almost 40 percent of the world s people live in the dryland regions of the world Altschul 2008 . These people are grouped broadly into nomadic semi-nomadic transhumant and sedentary populations Child et al. 1987 Squires Sidahmed 1998 Tunstall et al. 2002 . Nomadic people are pastoral groups that depend mostly on livestock for subsistence with small-scale rain-fed farming when possible as a supplement. Nomads migrate in patterns determined by available forage conditions water availability and access rules worked out with other pastoral or sedentary groups of people. Semi-nomadic people are also found in