4. Natural Vegetation of Arizona

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4.1. Introduction

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Arizona has 3,666 species of native and naturalized plants in 1,003 genera and 145 families (Lehr and Pinkava, 1980). This rich variety of flora is equalled by few other regions of the United States. The composites, the grasses, and the legumes constitute the three largest families. In 1969, Texas and California were the only states with more than the 381 grass species recorded for Arizona. The cactus family, especially well-represented in Arizona, has 74 native species. Arizona also is rich in members of the fern family and fern allies. Nearly 100 species are known.

At Yuma, in the southwest corner of Arizona, the elevation is about 30 m (100 ft) above sea level, while slightly more than 400 km (250 mi) northeast, the San Francisco Peaks rise to more than 3,820 m (12,600 ft). The ecologist recognizes a rule-of-thumb that states that every 300 m (1,000 ft) in rise in altitude is equivalent to moving about 480 km (300 mi) distance north. Thus, while Yuma and the San Francisco Peaks are only some 400 km (250 mi) apart geographically, they are some 6,000 km (3,750 mi) apart ecologically. Yuma has a subtropical climate, but on top of the San Francisco Peaks the climate is similar to that of northern Canada and Alaska.

Distribution of plant life is dependent on a number of interacting environmental factors that dictate the type of vegetation established. Temperature, precipitation, slope, soil, animals and man all are part of the complex of interacting factors. Some of these factors in the Southwest are extreme and often are decisive in determining the type of vegetation that grows in a given area.

Forests generally grow where the soil is moist year-round, but especially when it is moist during the growing season. Grasslands, woodlands and chaparral occupy areas where an extreme dry period occurs during the year, or where the soil does not remain moist because of limited precipitation. Deserts, regions of sparse vegetation, are in areas with warm temperatures and precipitation so limited that the soil usually is not moistened beyond a depth of several centimeters (a few inches).

Quite often deserts are thought of as areas without vegetation. If this were true, deserts would be rare because even under the most extreme climatic conditions some plant life can exist. Shifting sand dunes, the popular conception of a desert, are deserts of extreme barrenness because of the shifting sands rather than lack of moisture. The Southwest ‘‘deserts’’ are better thought of as semideserts or semiarid areas. Vegetation on Arizona deserts is sparse compared with vegetation in the eastern United States, but Arizona deserts rarely are devoid of all vegetation.

Some of Arizona's flora comes from other regions to the north, east, south and west. Regions to the north contribute species to two main vegetation associations, the coniferous forests and the cold desert. The coniferous forest region in Arizona culminates along the high Mogollon Rim, the escarpment forming the southern limit of the Colorado Plateau. This great belt of coniferous forest stretches along the rim from the New Mexico boundary to the vicinity of Williams. Less extensive forests grow on the higher regions north and south of the Mogollon Rim country.

In addition to the southern extensions of the northern coniferous forest flora there is also an extension of the Great Basin Desert flora, mostly from Utah. This cold, winter desert

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follows the low-lying land of the eroded river valleys in northern Arizona. The Great Basin Desert landscape is outstanding along the northern reach of the Colorado River and the Little Colorado River, its main tributary in Arizona.

Grasslands in northern Arizona are closely related to the Great Plains. This vegetation is usually thought of as being the western extension of the short-grass plains that occupy a region just east of the Rocky Mountains.

Both desert and grassland vegetation extend into southeastern Arizona from Chihuahua. Some authorities believe that the small area of southeastern Arizona labeled on some maps as the northwestern-most extension of the great Chihuahuan Desert actually is a transition zone between the Chihuahuan and Sonoran deserts. Others believe that it is true Chihuahuan Desert. Nonetheless, whether it is Chihuahuan or transition, the desert scrub in that part of Arizona is similar to that in northern Mexico. The semidesert grasslands of northeastern Sonora and western Chihuahua along the east side of the Sierra Madre Occidental extend into southeastern Arizona.

Desert vegetation in Arizona is floristically related to areas south and west. Sonoran Desert vegetation occupies the southwestern quarter of the state and extends southward into Sonora and Baja California. The Mohave Desert of California extends from the west into northwestern Arizona up to the western base of the Hualapai Mountains in Mohave County and into the lower Grand Canyon. The interior chaparral, which occurs in central Arizona below the forests and above the deserts, has floristic affinities with the California chaparral to the west.

Arizona is well-endowed with national parks and monuments (See Figure 18). Two monuments were established exclusively because of the vegetation they contain. Saguaro National Monument near Tucson has many species of desert vegetation including a large stand of the giant saguaro cactus. Organ Pipe Cactus National Monument in southwestern Pima County along the Mexican border also has a large and varied array of desert vegetation including the only abundant stand of organ pipe and senita cacti in the United States.

Grand Canyon National Park and Chiricahua National Monument (in Cochise County) were established because of topography, but each has vegetation of considerable interest. Several national forests cover extensive areas of Arizona.

Cities, counties, the state and other organizations also have established parks and recreation areas that exhibit natural vegetation. Tucson Mountain Park has abundant and diverse desert vegetation. The Arizona-Sonora Desert Museum near Tucson has assembled a large collection of plants labeled in detail, while the Desert Botanical Garden at Papago Park near Phoenix has a large collection of labeled cacti. Native desert plants are the focus at the Tucson Botanical Garden. The Boyce Thompson Southwestern Arboretum, a research and teaching arboretum that has many public events and tours, is about 96 km (60 mi) east of Phoenix near Superior. The arboretum exhibits about 1,500 plant species belonging to 600 genera in 125 families. These plants are not only from the arid and semiarid regions of the U.S. Southwest, but from similar regions around the world.

4.2. Bioclimatic Classification of Vegetation

Because of its vegetal diversity, Arizona is and has been of particular interest to plant ecologists, as evidenced by some of the earliest research on bioclimatic classification of vegetation (Merriam, 1890) in the United States. Merriam ( 1890) studied the San Francisco Peaks and surrounding north-central Arizona to develop his classical concept of vertical life zones. Figure 33 is a diagram of the vertical zonation of these life zones in the San Francisco Peaks. Figure 34 is a similar representation of the Pinaleno and Santa Catalina mountains in southeastern Arizona. Although the Merriam life zone concept has shortcomings, as reviewed and discussed by Lowe ( 1972), it was expanded upon by other early investigators and has been used considerably. It is particularly useful in the western United States by providing a direct method for describing vegetation distribution.

More recently, Shreve ( 1942) named and defined nine principal types of vegetation based on altitude. Nichol ( 1937, 1952) described and mapped the natural vegetation of Arizona while Kuchler ( 1964) prepared a map and accompanying manual of the potential natural vegetation of the conterminous United States that is well known and often referenced. A 1:500,000-scale vegetation map of Arizona was prepared by Brown ( 1973) that was accompanied by a manual published by Lowe and Brown ( 1973) that described and illustrated Arizona's natural vegetative communities (Plate 11). Brown, Lowe and Pase ( 1977) published a map of biotic communities of the Southwest. This map has a scale of 1:1,000,000, the same scale as the Arizona General Soil Map. It includes all of Arizona and New Mexico, and parts of Southern California, southern Nevada, southern Utah, southern Colorado, West Texas and northern Mexico. A recent book (Brown, 1982) describes in detail the vegetation types shown on this map.

FIGURE 33. Altitudinal Distribution of the Major Vegetation Zones of the San Francisco Peaks in Northern Arizona (after C. H. Merriam, 1890)

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The following descriptions of the Arizona natural vegetation zones are based mostly on those of Lowe and Brown ( 1973), although supplemented by additional references.

4.2.1. Alpine Tundra

The alpine tundra zone in Arizona is restricted to about 650 ha (1,600 ac) on the San Francisco Peaks above the timberline at elevations exceeding 3,340 m (11,000 ft) on Humphreys, Agassiz and Fremont peaks. Two plant associations were described, the alpine rock field and the alpine meadow (Little, 1941), but the zone has no forest overstory species. The alpine rock field association is the more extensive areally, covering most of the peaks above the timberline, but has much less dense cover and occupies a more unstable and less weathered substrate than the alpine meadow association. It is characterized by lichens and mosses on rock outcrops and in crevices, and by vascular plants scattered among boulders if soil is sufficient. Normally, the alpine rock field association merges into and is succeeded by the alpine meadow association, the species of which start as pioneers on rocky slopes and at the base of rock slides, then spread vegetatively to form mats.

These vegetative communities represent a relict alpine tundra flora definitely related to the high peaks tundra of the Rocky Mountains north and northeast (Moore, 1965). Twenty of about 50 species of the alpine tundra flora on San Francisco Peaks are arctic-alpine disjuncts that also live in arctic tundra zones. Fifteen of the 20 are circumpolar, growing in Arctic Eurasia as well as Arctic North America. Moore ( 1965) believed that at least 90 percent of the alpine tundra vascular species on San Francisco Peaks migrated from the north during Pleistocene time, possibly as recently as 65,000 to 75,000 years ago, coinciding with the last period of glaciation described by Sharp ( 1942). Updike and Péwé ( 1976) provided evidence of more recent glaciation. However, this last glaciation in the San Francisco Peaks was fairly limited in areal extent. Moore ( 1965) suggested further that relict alpine tundra on San Francisco Peaks has been losing its true alpine tundra character for at least 10,000 years.

FIGURE 34. Altitudinal Distribution of the Major Vegetation Zones of the Santa Catalina Mountains in Southern Arizona (after C. H. Lowe, 1972)

The climate is characterized by relatively high mean annual precipitation that ranges from 890 to 1,020 mm (35 to 40 in), most of which is snow, and cold temperatures. This zone probably has a pergellic soil temperature regime, a mean annual soil temperature of less than 0 C (32 F) at a depth of 50 cm (20 in) or the soil-bedrock interface. The soils are shallow and rocky. The area is included in mapping unit FH2, Sponseller-Ess-Gordo Association.

4.2.2. Spruce-Alpine Fir Forest

Spruce-alpine fir forests cover about 97,130 ha (240,000 ac) on and around the summits of the highest mountains, including San Francisco Peaks and the Chuska, White, Pinaleno and Chiricahua mountains, and on the large summit area of the Kaibab Plateau. These Rocky Mountain forests reach their southernmost extension in Arizona and New Mexico (Dye and Moir, 1977). Spruce-alpine fir forests generally lie between 2,430 to 2,730 m (8,000 to 9,000 ft) and extend to the mountain summits, except for San Francisco Peaks where the upper limit is approximately 3,490 m (11,500 ft). The mean annual precipitation ranges from 760 to 1,140 mm (30 to 45 in), much of it as snow, and exceeds mean annual potential evapotranspiration (Beschta, 1976).

Seven coniferous and one deciduous species variously mixed characterize these forests. The principal boreal conifers are Engelmann spruce, blue spruce, corkbark fir, white fir, Douglas fir, bristlecone pine and limber pine. Quaking aspen is the dominant deciduous species, both intermixed with various coniferous species and in pure stands. Dense overstories common to these forests severely limit or prevent growth of herbaceous vegetation. Quaking aspen is considered to be a seral species that invades an area following a disturbance such as fire.

Moir and Ludwig ( 1979) have classified the Lowe and Brown ( 1973) spruce-alpine fir forests into eight spruce-fir and 11 mixed conifer habitat types based on the concept of Daubenmire and Daubenmire ( 1968). The dominant climax species within the spruce-fir habitats are either Engelmann spruce or corkbark fir. Climax dominants or codominants in the mixed conifer habitats include white fir, blue spruce and Douglas fir. Kuchler's ( 1964) southwestern spruce-fir forest and spruce-fir-Douglas fir forest zones are included within the spruce-alpine fir forest zone on the vegetation map (Plate 11).

The spruce-alpine fir forests are primarily in Plate 1 mapping units FH2, Sponsellor-Ess-Gordo Association, and FH4, Soldier-Lithic Cryoborolls Association.

4.2.3. Montane Conifer Forest

Most of the montane conifer zone vegetation grows along the southern rim of the Colorado Plateau in central Arizona for nearly 360 km (225 mi) as an unbroken ponderosa pine forest. In southern Arizona, the montane conifer forest grows

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primarily on the larger mountains as ‘‘islands.’’ Characteristically this conifer association grows at elevations between 1,820 to 2,430 m (6,000 to 8,000 ft), but may be as low as 1,670 m (5,500 ft) on north-facing slopes or as high as 2,730 m (9,000 ft) on south-facing slopes and in the southern mountains. This association covers about 2,023,500 ha (5 million ac) in Arizona.

The montane conifer forest grows in a climate where moisture is relatively limited. The average annual precipitation is 510 to 760 mm (20 to 30 in) and is rarely lower than 460 m (18 in) or higher than 840 mm (33 in) (Beschta, 1976).

For the most part, ponderosa pine dominates the montane conifer forest, particularly in the warmer and drier areas. On north-facing slopes and at higher elevations ponderosa pine and Douglas fir and white fir grow in varying mixes, but the firs dominate north-facing slopes at the highest elevations. Other tree species include limber pine, southwestern white pine, Gambel oak, silver-leaf oak, madrone, locust, bigtooth maple and quaking aspen. Many stands of ponderosa pine are relatively open or park-like, in contrast to the closed-canopied spruce-alpine fir forest. This permits the growth of grasses, forbs, shrubs and broadleaf trees as understory. Sometimes the understory may be multilevel.

The montane conifer forests coincide approximately with Kuchler's ( 1964) Arizona pine forest.

The montane conifer forests grow in soils in parts of all FH, Frigid Subhumid, mapping units on Plate 1.

4.2.4. Riparian Deciduous Woodland

Riparian vegetation occurs in or near drainageways and floodplains and is characterized by plant species different from immediately surrounding nonriparian vegetative species (Lowe, 1972). Although riparian communities comprise a limited geographic area in total, they are significant because of their landscape importance and recreational use, and their value as wildlife habitats. Estimates of the size of this zone in Arizona range from 113,320 to 129,500 ha (280,000 to 320,000 ac).

Three elevational divisions of riparian vegetation in Arizona are recognized (Campbell, 1970). The communities are those below 1,060 m (3,500 ft), those between 1,060 and 2,120 m (3,500 and 7,000 ft) and those between 2,120 and 3,030 m (7,000 and 10,000 ft) above sea level. Below 1,060 m (3,500 ft) many ephemeral streams have broad alluvial floodplains that support high densities of deep-rooted trees and shrubs, including mesquite, acacia, saltcedar, paloverde, cottonwood, willow, sycamore and other species (Figure 35). Between 1,060 and 2,120 m (3,500 and 7,000 ft) the riparian vegetation consists of the greatest number of plant species and has the greatest canopy cover. Typical vegetation at these elevations is cottonwood, willow, sycamore, ash and walnut with three or four species often together. Above 2,120 m (7,000 ft) willow, chokecherry, boxelder, Rocky Mountain maple and various conifer tree species are dominant along stream channels. Climates of the riparian associations vary greatly because of large differences of elevation, latitude and distribution of mountains and highlands.

FIGURE 35. Low Elevation Riparian Woodland (after C. H. Lowe, 1972)

Vegetation native to many riparian areas in Arizona has changed markedly during the past 100 years, primarily the result of changes brought about by immigrants. Riparian woodlands have dwindled as water tables fall, and flood control and various other water management activities expand. And in some areas stands of mesquite, cottonwood and willow have been nearly replaced by saltcedar (Hasse, 1972).

Riparian deciduous woodland communities grow in the soils of some of the floodplain mapping units such as HA1 and TS2 (Plate 1), Torrifluvents associations, and along stream channels in many other mapping units.

4.2.5. Pinyon-Juniper Woodland

The pinyon-juniper woodland zone encompasses more than 5,665,800 ha (14 million ac) and is adjacent to and surrounds montane conifer and spruce-alpine fir forests in Arizona. Pinyon-juniper woodlands are mostly in the northern half of Arizona at elevations between 1,370 and 2,280 m (4,500 and 7,500 ft). In southern Arizona these woodlands merge with the chaparral zone.

Pinyon-juniper woodlands are in semiarid to dry, subhumid climatic regions that have relatively broad fluctuations of precipitation, temperature, evapotranspiration and

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wind. Annual precipitation varies from 300 to about 530 mm (12 to about 21 in). Generally mean annual air temperatures are about 3.3 C (6 F) above the adjacent, higher montane conifer forests and about 2.8 C (5 F) below the lower vegetation zones (Beschta, 1976).

Three juniper and two pinyon species are the dominant trees in this zone. North of the Mogollon Rim, Utah and one-seed juniper are intermixed with pinyon and to the south alligator juniper grows. Common pinyon (Pinus edulis) is the characteristic species throughout nearly the entire zone. Singleleaf pinyon grows locally intermixed with Utah juniper, mostly in northwestern Arizona. Junipers commonly are dominant below 1,820 [...] 1,970 m (6,000 to 6,500 ft), while pinyon attain greatest size and density above 1,970 m (6,500 ft) (Lowe and Brown, 1973). Grassland, chaparral or desert scrub may form an understory beneath and between woodland trees, depending on the area.

Pinyon-juniper woodlands are mostly in the MS, Mesic Subhumid Soils, mapping units on Plate 1.

4.2.6. Encinal and Mexican Oak-Pine Woodland

These woodlands, for the most part, are in southeastern Arizona. Their greatest development is on foothills and lower slopes of the larger mountains at elevations between 1,210 and 2,120 m (4,000 and 7,000 ft). These communities in Arizona represent their northern extension from the Sierra Madre region in northwestern Mexico and cover about 870,270 ha (2,150,400 ac).

Encinal and Mexican oak-pine woodlands receive mean annual precipitation of between 300 and 610 mm (12 and 24 in). The woodland habitat has warmer winter temperatures than the equivalent precipitation zone farther north that supports pinyon-juniper communities. Also, more precipitation is associated with the monsoon season during July through September.

The encinal communities are composed of evergreen oaks or of mixtures of oak, juniper and Mexican pinyon. The dominant oak species are Emory, Arizona and Mexican blue oak. Mexican oak-pine woodland communities also contain evergreen species including Chihuahua and Apache pine, Mexican pinyon and Arizona madrone. Silverleaf oak is the dominant oak species and Emory and Arizona oak also are present. The Mexican oak-pine woodland is above the encinal on mountain gradients, although not as widely distributed as the encinal woodland.

Kuchler's ( 1964) oak-juniper woodland corresponds approximately with the encinal and Mexican oak-pine woodland vegetation class.

These vegetation communities are in the Plate 1 MH, Mesic Subhumid Soils, mapping units in southeastern Arizona.

4.2.7. Interior Chaparral

The chaparral vegetation zone is mostly on rough, discontinuous mountainous terrain south of the Mogollon Rim, generally extending in a discontinuous band across the state from Seligman in the northwest to Safford in the southeast. Chaparral grows at elevations ranging from 910 to 2,430 m (3,000 to 8,000 ft) below woodland or coniferous forest and above grassland or desert scrub. Estimates of the area covered by chaparral in Arizona vary from 1,214,000 ha (3 million ac) to 2,428,200 ha (6 million ac), although the former and more recent value is thought to be the more accurate (Carmichael et al, 1978). The broad variance of estimates is caused by lack of common classification criteria.

Mean annual precipitation in the chaparral communities ranges from 380 mm (15 in) to more than 640 mm (25 in). The climate is further characterized by a cool, wet period from November until March, followed by a warm, dry period until the summer rains begin in July.

Chaparral consists of deep-rooted evergreen shrubs and trees that have broad, sclerophyllous leaves. They develop best on deep soils or on deeply weathered rock mantles. Although 50 or more shrub species are in the chaparral vegetation zone in Arizona, generally fewer than 15 are important in terms of density (Charmichael et al, 1978). Shrub canopy cover may vary from less than 40 percent on dry sites to more than 80 percent on the wetter sites. Annual and perennial grasses and forbs may grow where the overstory canopy is only moderately dense or is open.

Chaparral grows mostly below the Mogollon Rim in central Arizona within the soils of Plate 1 MH, Mesic Subhumid Soils, mapping units.

4.2.8. Grasslands

About 30 years ago, grasslands covered nearly 25 percent of the state, an estimated 7,369,000 ha (18,210,000 ac). But invasion by plant species from other proximate vegetation associations may have reduced the area covered by grasslands. Arizona has three types of grassland: mountain meadow, plains and desert.

Mountain meadow grasslands are scattered throughout the spruce-fir and montane forests at elevations ranging from 2,280 to 3,030 m (7,500 to 10,000 ft). Unlike plains and desert grasslands, mountain meadow grasslands receive relatively high average annual precipitation, 510 to 890 mm (20 to 35 in). The most extensive areas are in the White Mountains and on the Kaibab Plateau. These grasslands also occur in some higher, isolated ranges, such as the Pinaleno and Chiricahua mountains in southeastern Arizona.

Many grass species grow in the meadows. Grasslands in drier, warmer montane coniferous forest zones contain species such as Arizona fescue, pine dropseed and blue grama. Boreal grasses, such as mountain timothy, meadow hairgrass and mountain bluegrass, tend to dominate the higher, wetter, colder spruce-fir forests. In some wetter areas, including those with high water tables, forbs, mountain clover, wild daisy, mountain dandelion, sedges and a number of other species may outnumber grasses by as much as 10 to 1.

Mountain meadow grasslands dominate the soils of Plate 1 mapping unit FH 8, Gordo-Tatiyee Association, and are in soils that are important inclusions of mapping unit FH 4, Soldier-Lithic Cryoborolls Association.

The plains grassland extends southwest into Arizona where it grows mostly in the northeast as nearly uninterrupted

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ground cover between 1,520 and 2,120 m (5,000 and 7,000 ft). The desert grassland is principally in southeastern Arizona between about 910 to 1,520 m (3,000 to 5,000 ft) as Chihuahuan Desert Grassland. It occurs also in the northwestern quarter of the state, where it extends to the eastern edge of the Mohave, southern edge of the Great Basin and northern edge of the Sonoran deserts.

The plains grassland zone receives about 430 mm (17 in) mean annual precipitation with extremes of 250 and 510 mm (10 and 20 in), while the desert grasslands are the most arid of all North American grasslands, receiving only 200 to 380 mm (8 to 15 in) precipitation.

Plains grasslands in Arizona consist primarily of short grama grass species, blue, black and sideoats, and shrubs are absent or nearly absent. Desert grasslands are essentially pure stands of grass in some places, in other places, an open savanna with grasses beneath oaks or mesquites is common, and in still other places, the grasses are interspersed with a variety of low-growing trees or shrubs. Threeawn and tobosa species together with grama grasses dominate desert grasslands.

Changes in desert grassland vegetation during the last 100 years were two principal kinds: invasion by woody species, and change in the mix and density of nonwoody species (Humphrey, 1958). The first of these changes is well- documented and includes photographic evidence (Hastings and Turner, 1965; Martin and Turner, 1977). One or more of five explanations for these changes were suggested by various researchers, as reviewed by Humphrey ( 1958), and include

  • grazing by domestic livestock,
  • interspecies competition or lack of it,
  • effects of rodents,
  • changes in climate, and
  • suppression of natural grassland fires.

Although the most popular explanation is overgrazing by livestock, Humphrey ( 1958) suggested that suppression of natural fires may be the most important factor. Before immigrant settlement of the Southwest, fires were frequent and widespread and may have restricted shrub invasion. Plains grasslands characteristically are associated with soils in the Plate 1 MS, Mesic Semiarid Soils, mapping units. Desert grasslands grow mostly on more humid soils in southeastern and northwestern Arizona within the TS, Thermic Semiarid Soils, mapping units of Plate 1.

4.2.9. Desert Scrub

Desert scrub vegetation occupies about 14,447,800 ha (35.7 million ac) in Arizona. Making a clear distinction between desert scrub and grassland vegetation zones, however, often is difficult because of the invasion of the grasslands by desert scrub vegetation. For the purposes of this discussion, desert scrub vegetation has been broken into four classifications: Great Basin, Mohave, Sonoran and Chihuahuan (Plate 12).

Great Basin desert scrub is limited mostly to elevations between 910 and 1,970 m (3,000 and 6,500 ft) north of the Colorado and Little Colorado rivers where average annual precipitation is between about 180 to 300 mm (7 and 12 in) and is more evenly distributed throughout the year than in the other Arizona desert regions. The Great Basin Desert is dominated mostly by shrubs of relatively low stature. Pure, unbroken stands of big sagebrush are commonly associated with the Great Basin Desert. Blackbrush and shadscale are characteristic and rabbitbrush, horsebrush, winterfat and Mormon-tea are important shrub species. Much of the Great Basin desert scrub in Utah and Nevada is salt tolerant since numerous salt-affected soils are associated with the several large, internal drainage basins in those states. This situation occurs on a much smaller scale in Great Basin areas in northern Arizona.

Great Basin desert scrub grows mostly in soils in the Plate 1 MA, Mesic Arid Soils, mapping unit, and to a lesser extent in the Grand Canyon, Plate 1 mapping unit TA1, Torriorthents-Camborthids-Rock Outcrop Association.

Mohave desert scrub extends into northwestern Arizona, mostly Mohave County, from southeast-central California and southern Nevada. Mohave desert scrub in Arizona grows usually between about 300 and 1,210 m (1,000 and 4,000 ft) elevation. Average annual precipitation is 130 to 280 mm (5 to 11 in), most of which falls in the winter. Mohave sage and woolly-fruited bursage are near-endemic shrubby species. Although the Mohave Desert is shrub-dominated and lacks giant cacti and many subtropical desert tree species, several large plants such as the Joshua tree and Mohave yucca are endemic, and catclaw and mesquite grow along washes. Creosotebush and/or white bursage often dominate extensive areas. Some species, such as blackbrush and winterfat, associated with the Great Basin Desert, grow in the northern Mohave Desert.

The Mohave desert scrub grows in soils in several Plate 1 HA, Hyperthermic Arid Soils, and TA, Thermic Arid Soils, mapping units in northwestern Arizona, mostly near the Colorado River.

The Sonoran Desert, a large region with more than two-thirds lying in northwestern Mexico (Sonora, Baja California), contains several subdivisions. The northernmost two are in Arizona: the Arizona Upland in southern Arizona and the Lower Colorado desert subdivision in southwestern Arizona (Shreve and Wiggins, 1964).

The Arizona Upland subdivision has the most structurally diverse vegetation in the United States. It includes one of the most famous species of succulents, the giant saguaro cactus (Lowe and Brown, 1973). Other important species are organpipe, ocotillo and cholla cacti, foothill and blue paloverde, ironwood, mesquite and creosotebush.

This Sonoran subdivision is mostly between 150 and 1,210 m (500 and 4,000 ft) where average annual precipitation ranges from about 130 to 150 mm (5 to 6 in) in Yuma County to about 280 to 330 mm (11 to 13 in) on some southeastern mountain ranges. Although winter and summer precipitation amounts are approximately equal, the proportion of summer rain does increase across the state in an easterly direction.

The Lower Colorado subdivision is the largest of the Sonoran Desert. In Arizona it encompasses the lower drainages of the Colorado and Gila rivers. Elevation ranges from about 30 m (100 ft) near the Colorado River to 910 m (3,000 ft) in the eastern valleys. It is one of the most arid regions in the

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Sonoran Desert, with average annual precipitation that varies from 80 to 250 mm (3 to 10 in). Precipitation increases from west to east and with elevation.

Vegetation of the Lower Colorado subdivision is dominated by creosotebush and its major associate, white bursage. Low, open stands, of which these two shrub species constitute at least 90 percent, often cover thousands of hectares on the intermontane plains. Smaller areas that have low, undrained and salt-affected soils commonly are dominated by saltbush, desert-thorn and sometimes mesquite.

In addition to perennial vegetation, the Sonoran Desert also has annual species, sometimes referred to as ‘‘ephemerals’’ since they grow only after brief moist periods and are short-lived. The biseasonal precipitation distribution in the Sonoran Desert, especially in the Arizona Upland subdivision, produces two distinct floras of ephemerals (Lowe, 1959). One is derived from the ancient Madro-Tertiary flora to the south, being naturally attuned to summer moisture and hot temperatures. The other is derived from the Arcto-Tertiary flora of the north, being naturally attuned to winter and early spring moisture and cool temperatures. Shreve and Wiggins ( 1951) concluded that this pattern of summer and winter ephemerals, so clearly related to the seasonal distribution of precipitation, represents a geographic and climatic segregation that has endured since early Pliocene time.

The Arizona Upland subdivision desert scrub grows in the higher elevation Plate 1 HA, Hyperthermic Arid Soils, mapping units and the lower elevation TS, Thermic Semiarid Soils, mapping units in southern Arizona. Lower Colorado subdivision desert scrub grows mostly in valley areas of the Plate 1 HA, Hyperthermic Arid Soils, mapping units.

The Chihuahuan Desert covers most of north-central Mexico. The question of this desert extending across West Texas and southern New Mexico into Arizona is a source of conflicting opinions since in Arizona it represents more a zone of transition between the Chihuahuan and Sonoran deserts than true Chihuahuan Desert (Schmidt, 1979). The region in the southeastern corner of Arizona, mostly in Cochise County, referred to as the Chihuahuan Desert is at elevations between 970 and 1,520 m (3,200 and 5,000 ft) but mostly above 1,060 m (3,500 ft). Although the precipitation in this area ranges from 200 to 360 mm (8 to 14 in), most of the area receives less than 250 mm (10 in). More than half the precipitation falls July through September.

Creosotebush, tarbush, whitethorn and sandpaperbush are among the important shrub species in the Chihuahuan Desert in Arizona. These species frequently grow in essentially pure stands, often on limestone, or in complex community mixtures with mariola, ocotillo, allthorn, shrubby senna, whitebrush, desert zinnia and little coldenia. The Chihuahuan desert scrub also is characterized by numerous herbaceous root perennials, and by numerous species of small cacti.

Chihuahuan desert scrub in Arizona is mostly in the Duncan, San Simon and San Pedro valleys where it grows in the soils of several of the Plate 1 TS, Thermic Semiarid Soils, mapping units.

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, Cynonomys

Prairie dog, Cynonomys gunnisoni, at mouth of burrow. (Photo courtesy of Arizona-Sonora Desert Museum)

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