From www.desertmuseum.org, the website of the Arizona-Sonora Desert Museum:
"Tucson is the only major city located in Arizona Upland."
"This northeastern section, mostly in south-central Arizona and northern Sonora, is the highest and coldest subdivision of the Sonoran Desert. The terrain contains numerous mountain ranges, and the valleys are narrower than in the Lower Colorado River Valley subdivision. Trees are common on rocky slopes as well as drainages, and saguaros are found everywhere but on the valley floors. This community is also called the saguaro-palo verde forest. It is the only subdivision that experiences frequent hard winter frosts, so many species of the lower elevation and more southerly subdivisions cannot survive here. Nevertheless it is a rich area. The small range that is the Desert Museum's home, the Tucson Mountains, has a flora of more than 630 taxa."
"An ever-increasing number of biologists is concluding that the Arizona Upland's climate, vegetation density, and biodiversity resemble thornscrub more than desert. Don't be surprised if this subdivision is reclassified as thornscrub in the future."
For more information about the Sonoran Desert visit: www.desertmuseum.org/desert/sonora.php
"Sonoran Desert - Sonoran Desert Region." Arizona-Sonora Desert Museum. http://www.desertmuseum.org/desert/sonora.php#azu (accessed October 21, 2011).