The Sonoran Desert at Organ Pipe National Monument, Arizona, USA.

For many people the concept of a desert means hot temperatures together with a profound shortage of water. There are, however, deserts that are cold for much if not all of the year. The Great Basin Desert in the United States and the Gobi Desert in Asia for example have bitterly cold winter seasons yet present the sparse vegetation and dry soils typical of deserts.

In Central Florida, where rainfall exceeds 1500 mm/year, there are sand hills that become very dry only days after rain because the water leaches out of the root zones very quickly. There are spiny cacti, drought tolerant shrubs together with birds and reptiles typical of "real" deserts. In the canopies of tropical American rain forests the tree branches themselves support epiphytic plants ("air plants") including cacti and bromeliads that very much resemble desert plants. Tree canopies are sunny and hot while gravity ensures that despite abundant rainfall, water is nearly always in short supply.

Within the boundaries of well recognized deserts there are wet places such as oases and river banks. Most authorities recognize a desert as a place that receives less than 300 mm of rain each year and where the evaporation rate exceeds the precipitation rate for the greater part of the year.

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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.

The area generally recognized as the Sonoran Desert includes the southwestern third of Arizona, a small area of southeastern California, most of Baja California del Norte and the western half of Sonora, Mexico.

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 California 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. The characteristic Boojum Tree plays as a succulent and a drought deciduous tree at the same time.

When I was a young boy, apparently at a very impressionable age, my mother read me a book (she read me a lot of books): Horton Hears a Who by Dr. Seuss. In a similar way to the story I've discovered that every tree, every shrub, and even the soil in the Sonoran Desert all have hidden worlds within and upon them. Worlds that only become apparent after close and quiet inspection. Now, even as I hurry along a highway past a palo verde or cactus by the side of the road, my imagination runs wild thinking about all the little creatures living mysterious lives hidden from view and neglected by everyone (almost). There are minute caterpillars munching leaves, even smaller worms tunneling within a single leaf, beetles smaller than pinheads tunneling beneath the bark of dead branches, bee pollinators provisioning nests for their babies deep within circuitous galleries. Bees, flies and butterflies providing pollination services for the open flowers and hidden deep underground the nymphal stages of cicadas, now silent and now devoted only to drawing juice from roots. Throughout the more than 1500 pages of the Sonoran Desert Naturalist I hope that you will learn that it is no fantasy at all - there really is an absolutely incredible world of intrigue and curiousness - smaller than a speck. Where there's solitude far out in the desert, away from cars and grinding business just stroll up to one of the straggling shrubs and immerse yourself and come to know some of the desert's small and not insignificant residents.