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Climate and vegetation types within
the Sonoran Desert vary considerably across the region and with
elevation in the many mountain ranges. In general, the amount and
predictability of winter rainfall is higher in the west and lowest in
the southeast. Summer wet season is more generous and lengthy in the
southeast and shortest to the west. As one proceeds south within the
Sonoran Desert the winter season is less severe, where more
subtropical and tropical elements appear.
Upland Sonoran Desert (orange on map)
occurs in southwestern Arizona and is characterized by a balanced
distribution of winter vs. summer rainfall. Winter frosts are common,
but not severe. Succulent cacti, highly drought tolerant shrubs, and
thorny shrubs are equally common. Towards the north, especially on
mountain slopes, the vegetation merges towards chaparral type
vegetation which is characterized by dense thickets of evergreen
shrubs that are also fire adapted. Then towards the northwest the
Sonoran merges with the Mohave Desert where summer rainfall is usually
scarce.
The driest and hottest region is the
Colorado Desert (yellow on map) located in the lower Colorado River
valley. Annual rainfall can be less than 50 mm while summer
temperatures commonly approach 50° C. Areas of sand dunes occur
in this area, and the Gran Desierto, located in Mexico to the east of
the Colorado River presents the most inhospitable (yet also beautiful)
terrain. Vegetation consists mostly of highly drought tolerant shrubs
with few succulent cacti.
The Sonoran Desert in Sonora, Mexico has
a longer, wetter summer rainy season with a drier winter (brown on
map). Drought deciduous trees and shrubs become increasingly common
towards the south, many of which are quite thorny. Succulent cacti are
abundant and diverse. The Sonoran Desert rather gradually merges into
the more southerly Tropical Dry Thorn Forest of Southern Sonora and
Sinaloa.
The Sonoran Desert on Baja is often
referred to as the Vizcaino Desert (red on map). Cool moist weather
comes in winter, the summers are not quite as hot due to the cooling
effect of the Pacific Ocean, and occasionally summer brings good
amounts of rainfall. Moisture from fog and dew can be considerable in
some locations allowing epiphytes (air plants) to grow suspended from
typical desert plants such as the ocotillo. Succulents including
cacti, agaves and yuccas are abundant and extraordinarily diverse.
Sonora
Desert page
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The great diversity of the Sonoran Desert covers approximately 120,000 square miles and enriches the landscape with large areas of arid and semi-arid desert terrain (shown in red on map). Some 2500 species of plants are found here. The Sonoran Desert straddles the frost line where upper slopes of mountain ranges are densely forested with fir and pine trees. The majority of the desert however, is frost-free, thus providing excellent growing conditions for two forms of vegetation; legume trees and columnar cacti. Offshore islands located in the Gulf of California are also included as part of the Sonoran Desert. A variety of sea birds flock to the shores.
The idea of a desert for many people conjures an image of shifting sand dunes or barren rocks devoid of vegetation and obvious animal life or habitation. Such areas do exist within the Sonoran Desert but for the most part this is a lush desert that receives 120 to 300 mm of precipitation each year. This is also a hot desert with daytime temperatures reaching or exceeding 40° C for much of the summer season, May to September, while hard freezes are uncommon during the mild winters. A very unique feature of the Sonoran Desert is that the rain arrives during two wet seasons, one generally December through March, the other July through Sept. The lack of hard freezes and moisture distributed through the year has promoted the abundance of succulents that can store water for later use.