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Sonoran Desert Biotic Province (from Brown, 1994) Arizona contains two of the five subdivisions recognized as constituting the Sonoran Desert –the Lower Colorado River Valley subdivision and the Arizona Upland subdivision. The other three lie to the south in Mexico. The flora (plant-life) of the Sonoran Desert has its ancestors in the subtropical forests that lie to the south in Mexico and beyond. The modern Sonoran Desert formed roughly 8000 years ago as climate became drier. The bimodal rainfall pattern within the Sonoran Desert has resulted in a greater structural diversity than in the other deserts of North America. While the other North American deserts are dominated by lowgrowing shrubs the Sonoran Desert has a large arboreal (tree) component, large cacti, shrubs, and numerous succulent plants. Lower Colorado River Valley Subdivision (lower Sonoran Desert) This is the largest and most arid subdivision of the Sonoran Desert. Rainfall averages less than 8” of rain per year. It is differentiated from the Arizona Upland Subdivision due to its warmer winters and drier summers. The most common plant community (Series) in the Lower Colorado River Valley Subdivision of the Sonoran Desert is the creosote-bush - white bursage Series (Photo at left). These two plants (below) either together or alone occur over thousands of acres in the broad valleys of the basin and range. These two plants are remarkable in their ability to survive under extremely dry, hot conditions. creosote white bursage Plant growth is both open and simple, reflecting the intense competition between plants for water resources (right). The desert is so dry that vegetation is often restricted to drainage-ways (wash obligates), where sand may hold moisture from previous runoff events (see below). Where plants do manage to grow at a distance from these drainages they are often smaller than the same plant would be if growing along a wash (wash facultatives). Small trees are common along the larger drainage-ways, particularly trees of the pea/bean family (Leguminaceae). Common small trees include: mesquite; palo verde; ironwood; and smoketree. These trees have tiny leaves (microphyllous) and have chlorophyll in or beneath their bark to help them photosynthesize. These small trees also may grow at a distance from washes, except smoketree, but will be even shorter and smaller. Other plants grow only in these larger desert wash habitats due to their higher water requirements (wash obligates). These species include Desert Willow (a tree); chuparosa; desert honeysuckle; and canyon ragweed (all shrubs). Other shrubs are found growing in the drier tiny shallow water-courses: catclaw acacia; burro-brush; Anderson thorn-bush; and desert broom. The most arid parts of this subdivision are extensive areas of desert pavement, gently sloping surfaces covered by tightly fitting pebbles. Desert pavement supports only a sparse seasonal cover of tiny ephemeral plants. Again however, tiny drainages crossing through desert pavement areas will support small populations of perennial plants demanding more water: saguaro; ocotillo; brittle-bush; palo verde, ironwood; bursage; and creosote-bush. Other plant communities take over for Creosote - White bursage where soil becomes either sandier or finer. Arizona Upland Subdivision (Upper Sonoran Desert) Forms a narrow curving area to the north, northeast, and east of the Lower Colorado River Valley Subdivision (see map). This subdivision is slightly cooler and wetter than the LCRV Subdivision. Rainfall ranges from 8 inches to 16 inches per year. *Phoenix gets 7 inches per year. Most of this subdivision lies on slopes, broken ground, and on sloping plains dissected by streams while very little lies on flat alluvial basin floors of the basin and range, as does the LCRV Subdivision. This plant community appears as a scrubland or low woodland of leguminous (pea/bean family) trees with open spaces filled with low shrubs and perennial succulents. The trees are the same species as found in the LCRV Subdivision farther west except here they are not confined to the drainage-ways because there is greater rainfall. The trees include Palo Verdes; Mesquites; Ironwoods; Acacias. The vegetation of this part of the Sonoran Desert is so luxurious that some ecologists do not even consider it a desert, but instead as a sub-tropical thorn-scrub. Cacti are a large part of the vegetation in this community. Cacti are classified in terms of their life form as being stem succulents. The following cacti are either restricted to this subdivision or are best represented in this community: Buckhorn Cholla Cane Cholla Staghorn Cholla Chain Fruit Cholla Teddy Bear Cholla Desert Christmas Cactus Pencil Cholla Klein Cholla Devil's Club Ground Cholla Saguaro Organ Pipe Senita Night-blooming Cereus Fishhook Pincushion Thornber Pincushion Fish-hook Barrel Cactus Compass Barrel Cactus The most common plant community (Series) within the Arizona Upland Subdivision of the Sonoran Desert is the Paloverde-Cacti-Mixed Scrub Series (at left). This community is dominated by palo verde trees, with tall columnar saguaro often reaching above the trees. Often the ironwood tree is found in a secondary role to the palo verde. Beneath the trees and in the open spaces between the trees can be found the following plants: trees forbs small shrubs hackberry desert zinnia triangle-leaf bursage smaller acacia little-leaved ratany palo verde brittle-bush ironwood feather duster cacti see above large shrubs creosotebush (but in much greater numbers than in the LCRVS limber bush ocotillo jojoba The creosote-bush-white bursage Series of the Lower Colorado River Valley Subdivision and the palo verde-cacti-mixed scrub Series of the Arizona Upland Subdivision come into contact with each other throughout the basin and range of Arizona. The area in which they come into contact is called the ecotone. Within the ecotone a mix of species of both communities is found as one community transitions to the next. This ecotone is found along the margins of the desert valleys (basins). The creosote-bushwhite bursage Series dominates the warmer drier valleys while the Arizona Upland Series dominates the pediments, bajadas and higher edges of the basins. Animals Mammals found in the Sonoran Desert as well as other N. American deserts include: California Leaf-nosed bat; Coyote; Ring-tailed Cat; Black-tailed Jack-rabbit; Desert Cottontail; Merriam's Kangaroo Rat; White-throated Woodrat; Desert Pocket Mouse; and Desert Kangaroo Rat. Mammals found only in the Sonoran Desert include: Bailey's Pocket Mouse; Roundtailed Ground Squirrel; and Arizona Cactus Mouse. The young age of the Sonoran Desert is also reflected in its birdlife. The region is not ancient enough for birds to have evolved that are restricted to the Sonoran Desert. Common birds include: Road runner; Mourning Dove; Lesser Nighthawk; Verdin; Cactus Wren; Black-tailed Gnatcatcher; Phainopepla; and Black-throated Sparrow Birds that are found mainly in the Sonoran Desert include: Gambel's Quail; Costa's Hummingbird; Gilded Flicker; and Gila Woodpecker. Reptiles found in the Sonoran Desert as well as the Mojave Desert include; Chuckwalla; Desert Tortoise; Desert Iguana; Spotted Leafnosed Snake; Rosy Boa; and Western Shovelnose Snake. The Sonoran Desert shares the Zebra-tailed Lizard with the Great Basin Desert, the Mohave Rattlesnake with the Chihuahuan Desert, and the Banded Gecko; Desert Spiny Lizard; Patch-nosed Snake; Glossy Snake; Western Ground Snake; and the Western Diamondback Snake with other dry regions of the southwest.