X Slider Background Settings
Sonoran Desert Flora
The Sonoran Desert has the greatest structural diversity and most well-known plant life of any desert worldwide. It contains over 2000 species of plants; this high number of species (species richness) is a result of warmer winters and 2 rainfall periods (Bimodal - summer and winter). The number of species and the distribution is dependent on the location. Desert ecologist Forest Shreve best explained the diversity and designated 7 subdivisions.
On this page we discuss the The Upper Sonoran or Arizona Upland, where many of the large columnar cacti the Sonoran Desert is known for are located; the tree density is greater than any other desert. Giant columnar cacti such as Saguaro (Carnegiea gigantea) and Organ Pipe (Stenocereus thurberi), and many species of Cholla (Opuntia spp) are common. The most common trees are the leguminous trees (3) such as Mesquite (Prosopsis spp.), Ironwood (Olneya tesota), and Acacias (Acacia spp.) that grow in low valleys and hillsides, especially along washes. Much of the Upper Sonoran is covered by what is known as Palo Verde Saguaro complex or forest as they are the 2 most dominant species. This area is the best watered and least desert like of any of the NA deserts (1).
This subdivision is found in the Northeastern section of the Sonoran Desert and contains the highest elevations and coldest temperatures of any of Shreve's subdivisions. More than 90% of this area is made up of slopes and broken terrain with numerous washes. Elevations range from 960 feet (300 m) to over 3000 (1000 m). More rain falls in the higher altitudes and ranges from 7 to 16 inches (200-425 mm).
Cacti are so numerous that Shreve termed it the “stem succulent desert”, with 11 larger species of cacti confined to or most common in this subdivision. Tree species are similar to those described in the Lower Sonoran, with the addition of the common Foothills Palo Verde (Parkinsonia microphylla) and Crucifixion thorn (Canotia holocantha) in higher altitudes. Ecotones (an area where 2 different vegetation types meet and coexist) can be identified as one climbs from the lower valley elevations up slope and or travelling from SW Arizona East. Creosote bush density begins to decrease, and white bursage begins to be replaced by triangle leaf bursage (Ambrosia deltoidea). The conspicuous Ocotillo (Fouquiera splendens) and saguaros increase and brittlebush (Encelia farinosa) becomes common on slopes.
As the elevation continues to increase plants such as Jojoba (Simmondsia chinensis), Feather Fairy Duster (Calliandra eriophylla), Desert Buckwheat (Erigonum fasiculatum) increase as the desert begins to blend with either semi-desert grassland or interior chaparral. Three series of vegetation have been identified in Upper Sonoran including Saguaro-Palo Verde; Jojoba-mixed scrub, and Creosotebush-crucifixion thorn (1).
If you flew low, the valleys in the Sonoran Desert would look like thousands of acres of primarily Creosote bush and Burro bush (Ambrosia dumosa), occasionally separated by darker green ribbons of trees and other species not as arid adapted growing along sand drainages known as washes. Blue Palo Verde (Parkinsonia florida), Mesquite, Ironwood, along with shrubs like chuparosa (Justica californica), and sweetbush (Bebbia juncea), line these sand drainages and provide shaded microclimates (principally a temperature reduction due to shade and increased humidity) for other plant species, particularly annual plants along with important cover and food for many desert animals. The greatest contrast in vegetation occurs is when the desert approaches one of the few rivers. Today, you could expect to see a large band of mesquite, an often-dominant invasive species known as salt cedar (Tamarix spp) and possibly a taller gallery forest of Cottonwoods (Populus fremontii) and Willow trees (Salix spp) adjacent to the water. The vegetation associated along waterways, either a river or a wash is known as riparian vegetation and is extremely important for desert animal species. Today you might find a band of different vegetation growing for 50 to 100 m on either side of the river which provides an important microclimate, nonetheless 200 years ago these ribbons were be over a mile wide!
Over 500 vertebrate species consider the Sonoran Desert home. Considering the 3 North America's warm deserts, the Sonoran, the Mohave, and the Chihuahuan recently evolved in its current distribution of North America, many Sonoran desert vertebrates are found in one or both of the other deserts. Often, biologists rate the importance of a biotic community based on the number of endemic species, but in such new communities this would be unreliable. In addition, some generalist species, such as the Coyote (Canis latrans), are not only found in all 3 deserts but several other communities as well. In the following paragraphs, I will attempt to cover representative species of all 3 vertebrate groups with some brief mention of those with adaptations to the extreme climates found in the Sonoran Desert.