Flora of the Sonoran Desert includes six subdivisions based on vegetation types. Two are north of the boundary between the United States and Mexico, and four are south of the boundary. The flora of the Colorado Desert are influenced by the environment of the very dry and hot lower areas of the Colorado River valley, which may be barren, treeless, and generally have no large cacti. Flora of the Arizona Upland are comparatively lush, with trees and large columnar cacti that can withstand winter frosts. South of the border subdivisions are characterized by plants that cannot withstand frost.
The Colorado Desert is the low area of the Colorado River Valley and surrounding hills and mountains. Plants must survive its hot and dry conditions. Temperatures can be greater than. Annual precipitation is sometimes less than. Density of vegetation gradually diminishes moving from the Arizona Upland into the Colorado Desert proper. It is common to find areas where a layer of pebbles is left after wind and water removes the lighter gravels and sand, and the pebbles are cemented together with desert varnish, causing landforms called desert pavement that are impenetrable to many species. Unlike in the Arizona Upland where trees are common, they are mostly absent, except along watercourses and areas where water collects after rain. Common plants are the burrobush shrub, giving a grayish cast to the viewscape, and creosotebush. Also common, but more widely spaced, is ocotillo. In areas of intermittent water steams, ironwood, yellow paloverde, and Smoketree can be found. The Saguaro cactus is still present, but less frequently than in the Arizona uplands, and is smaller in this ecotone. It is almost entirely absent on the California side of the Colorado River. Desert Holly can tolerate even the hottest and driest sites.
Ecotone of Colorado Desert and Arizona Upland
The area where two vegetation types border, overlap, and transition is called an ecotone. The ecotone between the hotter, drier Colorado Desert and that of the relatively cooler and wetter Arizona Upland occurs from Parker, Arizona southeast to near Phoenix, then south to Organ Pipe Cactus National Monument. Density of vegetation gradually diminishes moving from the Arizona Upland into the Colorado Desert proper. can be found with stems climbing through shrubs and other vegetation, producing "spectacular" floral displays on supporting vegetation.
Blue paloverde trees, with smooth blue-green photosynthetic bark, and yellow paloverde are in the a legume family. There are extensive stands of velvet mesquite in the valleys of the Arizona Upland, where colder air collects.
Shrubs
Cacti
Perennials
Annuals
There are two seasons with significant precipitation, in the winter-spring months, and in the summer monsoon months. Annual plants mostly germinating during winter-spring precipitation tend to have closer affinities to flora of the Mojave Desert.
