Vegetation

With the uplift of the Sierra Madre Occidental in the late Oligocene-early Miocene, tropical deciduous forest was isolated in northwestern Mexico. Thornscrub and then the Sonoran Desert formed during a drying trend in the middle Miocene. The opening of the Gulf of California at the same time dramatically influenced the vegetation and flora of the region. Today the transition from the New World Tropics to the Northern Temperate Zone is in Sonora with the northernmost tropical deciduous forest in the Sierra San Javier (28°N).

The New World tropics reaches its northern limit in Sonora, not as often stated at the Tropic of Cancer (23.37°N) just north of Mazatlán, Sinaloa. The northernmost tropical deciduous forest (TDF) is in Sonora (28.6˚N), 680 kilometers north-northwest of Mazatlán and 300 km south of the Arizona border.