Also known as Sinaloan thorn-scrub, this community covers large areas in central, southern, and eastern Sonora. From west to east Sonoran Desert gives way to thorn-scrub in the Sonoran Sky Island and Sierra Madre foothills and valleys. Sometimes Sonoran Desert in canyons and slopes gradates into thorn-scrub in structure as well as species composition. Thorn-scrub in turn gradates into its even closer relative, tropical deciduous forest, as you move southward or into sheltered canyons. In southern Sonora monsoons are heavier and more reliable and frost diminishes, especially in protected canyons, basically causing thorn-scrub to grow into tall tropical deciduous forest, supporting increasingly tropical species assemblages.
Thorn-scrub is adapted to mild or frost-free, dry winters. Many thorn-scrub species are drought deciduous and lose their leaves in the fall or winter and regain them with the return of the monsoons in late June and early July. Indeed a dry, grey thorn-scrub scene can transform into a buzzing, green rainforest within a few short weeks once monsoon rains begin.
Common species in thorn-scrub include Lysiloma watsonii, Lysiloma microphylla, Acacia cochliacantha, Stenocereus thurberi, Bursera fagaroides, Bursera lancifolia, Jatropha cordata, Fouquieria macdougalii, Ceiba acuminata, Croton spp, among innumerable others.