Deserts occur where the driest climates on earth prevail. If the arid zone is con-tained within the 200-mm isohyet, then the warm deserts can be identified as arid regions in the subtropics and tropics having mean monthly temperatures that exceed 20 °C in the hottest month and 9 °C in the coldest month, hence locations that experience few, if any, and very brief subzero episodes per year. In these habitats, potential evapotranspiration (PE) is exceedingly high (greater than 2800 mm yr−1 from a Class A evaporation pan), the index of aridity, as defined by Oberlander (1979), typically is greater than 85 with a year-round soil moisture deficit or with a moisture index of Thornthwaite (1948) less than −40, and the Bowen ratio (annual sums of sensible heat flux/latent heat flux) is 3-7 (Kessler 1985). Aboveground net primary productivity is low, and in extreme deserts, where rainfall may not occur for 12 or more consecutive months, vegetation is sparse or completely absent. Warm deserts as defined here fairly closely fit Zonobiome III (Arid), excluding most Zonoecotone II-III transitions, of Walter and Breckle (1984), and occupy more than one-tenth of the continental landmass (McGinnies 1979). Most narrow, maritime fog deserts also qualify as warm deserts (Amiran and Wilson 1973), although fog deserts tend to have narrower daily and annual monthly ranges of temperature (Goudie and Wilkinson 1977; Evenari et al. 1985, 1986). Both lowland inland and coastal warm deserts are inhabited by similar plant life forms, products of convergent evolution in response to the drought-prone climates.