of the Colorado River. Desert Holly (Atriplex hymenelytra) can tolerate even the hottest and driest sites. Diversity and taxa 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. Flora of the Central Gulf or California coast Flora of the Magdalena area Flora of the Vizcaino area Lichens Dimelaena thysanota is a crustose lichen found on rock faces. Perennials Plains of Sonora flora Shrubs Trees Blue paloverde trees (Parkinsonia florida), with smooth blue-green photosynthetic bark, and yellow paloverde (Parkinsonia microphylla) are in the a legume family (Fabaceae). There are extensive stands of velvet mesquite (Prosopis velutina) in the valleys of the Arizona Upland, where colder air collects. References Sonoran Sonoran Sonoran .