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of the 10 most botanically diverse U.S. states (no. of species of vascular plants), 7 are in the arid west. None are in the xeric northestern US. California has the most native plant species of any state in the US. 40% of all the vascular plants in CA occur in the deserts, which cover only 28% of the state's landmass.
The Arcto-Tertiary expansion of genera in the arid southwestern U.S. has led to extraoradinary plant diversity there. The only other deserts in the world with higher diversity are those found in southern Africa and Austrailia.