Dryland Ecosystems

Drylands occupy approximately 40% of the Earth’s land surface and have low inputs of mean annual precipitation (P) relative to mean annual potential evapotranspirational (ET) losses (Millennium Ecosystem Assessment 2005). The United Nations Educational, Scientific and Cultural Organization (UNESCO 1979) proposed the following classification scheme for drylands: hyper-arid zone (P/ET \u3c0.03), arid zone (P/ET 0.03–0.20), semi-arid zone (P/ET 0.20–0.05) and subhumid zone (P/ET 050–0.75). The majority of studies summarised in this chapter were conducted in arid and semi-arid zones with mean annual precipitation ≤300 mm