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Nearctic Deserts

The Nearctic deserts are formed by five lowland deserts (Chihuahuan Desert, Sonoran Desert, Mojave Desert, Great Basin shrub steppe, and Meseta Central matorral); two coastal deserts (Baja California Desert and Gulf of California xeric scrub), and four montane relict sky-islands (Western Madrean Archipelago, Eastern Madrean Archipelago, Great Basin montane forests, and Sierra de Juárez and San Pedro Mártir pine-oak forests). They cover in total 1.7 million square kilometres, of which 19 per cent is under some level of legal protection. Because of the growth of large urban conglomerates such as Phoenix in the US, their mean population density is high (44 persons per square kilometre), and their mean human footprint (21) is the second highest of the world's deserts, with footprint especially high in the Sonoran and Chihuahuan Deserts.

These deserts present a combination of sage brush (Artemisia tridentata) and creosote bush (Larrea tridentata) shrublands, halophytes (such as Atriplex confertifolia), and open grasslands in the moister parts. There are numerous cactus species except in the Great Basin shrub steppe, where freezing temperatures limit cactus growth. The giant saguaro cactus is emblematic of the Sonoran Desert; it grows very slowly but may reach a height of 15 m over its long life span of more than 200 years. The Baja California desert is home to over 3 500 species of endemic plants, almost a quarter of which are endemic, ranging from a diversity of shapes and sizes of cactus, to thickstemmed trees and shrubs in the rocky mountain soils. Likewise, among the 3 200 plant species in the Chihuahuan Desert, around 1 000 are endemic. Creosote bush, tarbush, viscid acacia, yucca, and cactus are characteristic. The Joshua tree (Yucca brevifolia, Figure 4.4) is probably the most recognizable plant of the Mojave; growing with a wide variety of cactus, creosote bush, white bursage (Ambrosia dumosa), jojoba (Simmondsia chinensis), and small trees such as paloverde (Parkinsonia microphylla) and ironwood (Olneya tesota). Grazing, hunting and salt extraction (in the Baja California desert) are significant activities in these ecoregions, which also contain several large and fast-growing cities: Las Vegas, Reno, and Salt Lake City in the Great Basin, Los Angeles sprawling into the Mojave, Phoenix and Tucson in the Sonoran Desert.


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