What Do Deserts Have in Common Arts and Deciduous Fore

Biomes & Communities of the Sonoran Desert Region

Mark A. Dimmitt

Earth travelers can scarcely help simply notice the groovy diversity of landscapes on this planet. That multifariousness is as much due to the vegetation as to the landforms. Closer ascertainment reveals a mind extraordinary diversity of plants and animals in these unlike places. People who travel in the American Soutwest or on the western side of any other continent near 30° latitude will see dramatic changes within distances of only a few miles.

In credible contrast, some widely distant parts of planet Earth bear witness certain broad scale similarities. The vegetation of the Mediterranean declension of Europe looks remarkably like to the chaparral of Southern California, though no two plant species occur in both places (except some introduced weeds). Both the diversity and the similarities take the same underlying causes, namely the interactions between geography and the global climate motorcar which governs the biosphere on both chiliad and microscopic scales.

The Sonoran Desert region has a swell multifariousness of both species and habitats, the latter ranging from extremely hot, arid desert to semiarid tropical forest to frigid subalpine meadows. Our focus is on the Sonoran Desert in the heart of the region, just to empathise information technology nosotros need to know something nearly the other habitats that border it. These adjacent geographical features and biological communities exert profound, complex influences on the desert itself.

Ecologists who study nature on a global scale recognize a few basic, widespread classes of habitats that are easily identified by their dominant plant life forms, which are basic categories based on full general advent, for example, tree, shrub, almanac, succulent, and and then on. Such global scale habitats are called biomes, and are determined primarily by the climatic factors of temperature and rainfall. These factors are in plow adamant by breadth, elevation, and wind patterns. Biome classification is based on vegetation because plants, being generally immobile, are the most obvious and easily recognizable components of a biological community. In addition, plants are more definitive of their biomes because, since they are rooted in place, they must be adapted to that specific environment. Plants, therefore, are often endemic (occurring merely in the named area) to a biome or smaller community. Biomes do contain characteristic animal life as well, including many endemic insects and other invertebrates. Most vertebrates, however, are more mobile and rather few species are restricted to a unmarried habitat.

All of the world'south biomes occur in the Sonoran Desert region. This tremendous diversity in a fairly small surface area is due to ii influences. For 1 thing, this region is on the west side of a continent near 30° North latitude, a position where several biomes typically occur in close proximity (a phenomenon explained later in this chapter). Secondly, our swell topographical relief creates the cold, wet climates that permit northern biomes to occur further south than they would unremarkably.

It is important to recognize that biomes and near other biological classifications are largely subjective concepts. an try to brand sense of the nearly incomprehensible multifariousness of nature. In add-on, their boundaries are rarely singled-out. Wherever two biological communities or biomes meet at that place is commonly a zone of intergradation which is sometimes very wide. For these reasons classifications differ amid classifiers. For example, some biologists recognize thornscrub as a separate biome while others call it an ecotone (transition zone) between desert and tropical forest. Some combine tropical and temperate forests into the same biome based only on vegetation pinnacle and density. The biomes equally defined hither are then distinctive that you should be able to place any terrestrial habitat on the planet inside one of them at a glance (see plate 1).

Biomes are subdivided into a hierarchy of smaller categories, divers by the detail species that inhabit them. At that place are many classification systems and the categories take many names. Nosotros utilise the general terms biotic community, biological customs, or simply, community. The names used here for the communities are mostly those of Brownish and Lowe (1982).

Tundra

Tundra is the most poleward and highest-elevation biome and is characterized by extremely cold winters. The ascendant institute life forms are ground-hugging woody shrubs and perennial herbs. Intense cold excludes trees and succulents and the growing season is likewise short for annuals.

Temperatures get warmer at lower latitudes (toward the equator) at the aforementioned height. Simply an increment in tiptop at a given latitude has the same climatic issue every bit does traveling toward a pole: temperature decreases. Then a climate that supports tundra, like that in the arctic, can be institute on high mountains all the manner to the equator. The other cold biomes in both hemispheres besides extend toward the equator where sufficiently high elevations run into their climatic requirements. The San Francisco Peaks near Flagstaff, Arizona, rise to 12,600 feet (almost 3900 m). These are the merely mountains in our region that extend above timberline (virtually 11,200 anxiety, 3400 m elevation in Arizona). There, only forty-five miles (72 km) from the saguaros of the Sonoran Desert, is a small-scale area of tall tundra that includes some of the aforementioned institute species that occur in the chill tundra of Alaska (run across plate 2).

Coniferous

Coniferous Woods (also known by its Russian name, taiga) is dominated by cone-bearing trees, especially pines, firs, and spruces in the northern hemisphere. Many conifers are adapted to cold simply a niggling less severe than in tundra. Tree height ranges from a few anxiety (a couple of meters) near the tundra purlieus or at timberline to over 300 feet (xc g) in more temperate latitudes. Some coniferous woodlands extend into subtropical climates, for example, in the southeastern United States.

In our region coniferous forest occurs in the college mountain ranges, more often than not to the north and east of the Sonoran Desert. Our nigh widespread coniferous community is Petran Montane (Rocky Mountain) forest, the dominant vegetation of the cold-temperate Rocky Mountains. Its peak increases southward into Mexico until it is pushed off the tops of the mountains past excessive aridity and warmth. In the mountains west of the Sonoran Desert are isolated islands of Sierran (as in Sierra Nevada) coniferous woods, characterized past dissimilar conifer species.

Temperate Deciduous Forest

Temperate deciduous forest is characterized past dumbo stands of broadleaf copse that driblet their foliage in winter. The winters are milder than those of virtually conifer-dominated climates, though even so besides cold for plant growth. Summers are typically warm and humid. There are more species than in the ii more poleward biomes. The herbaceous perennial life form is well-represented along with the trees and shrubs. Although pure temperate deciduous forest is rare in our region, it is represented by scattered aspen groves and ribbons of riparian trees.

The foothills and lower mountain slopes east of the Sonoran Desert are wooded with oaks and pines, a mixture of coniferous woods and temperate deciduous forest tree types. The oaks, however, are by and large evergreen species; they are not deciduous except during severe droughts. This Madrean evergreen woodland (too called Mexican oak-pine woodland) is a warm-temperate customs of the Sierra Madre Occidental. It extends every bit far northward as central Arizona, where it is squeezed out by the cool-temperate Rocky Mount forests above it and the more arid grassland and desert below. (Though its official name is woodland, in its southern part it's actually a forest; i.e., the tree canopies overlap.) This is a semiarid community which experiences a dry season in spring (meet plate 3).

In the Sonoran Desert region tundra, coniferous woods, and temperate deciduous forest are restricted to mountains that ascension well to a higher place the intervening basins. In southeastern Arizona and northeastern Sonora there is a gap between the massive Rocky Mountains and the Sierra Madre Occidental. The mountain ranges in this gap are distinct entities separated by intervening valleys. The cool, moist communities on their upper elevations are isolated from one another by "seas" of hot, arid habitat. Because this isolation is coordinating to oceanic islands, the terms "mountain islands" and "sky islands" take been coined for these and similar ranges.

Grassland

Grassland is a semiarid biome characterized by warm, boiling summers with moderate rain and cold, dry winters. (The primal valley of California is an exception; it is a winter-rainfall grassland at a lower than typical elevation.) Grass is the dominant life form; scores of species form a nearly continuous encompass over big areas. Other well-represented life forms are annuals and geophytes (herbaceous perennials such as bulbs that die to the ground each yr). Populations of trees, shrubs, and succulents are kept at depression levels by periodic fires during the dry flavour.

Well-nigh of the grasslands in the western states are intermediate between the truthful prairies of the American Midwest and deserts. They are called semi-desert or desert grasslands. (Over again the California grasslands are an exception. They are heavily influenced past the unique California floristic province and not much past the Midwest prairies.) Compared with prairie grassland, the grasses in desert grassland are shorter, less dumbo, and are more oftentimes interspersed with desert shrubs and succulents. Desert grassland or chaparral borders the northern Sonoran Desert on the east (see plate 4).

Chaparral

Chaparral is a semiarid biome that occurs on the due west coast of every continent between about thirty° and 40° Due north latitude. This smallest biome is unique for its Mediterranean climate: mild, moist winters and hot, dry out summers. Mature chaparral consists almost solely of woody evergreen shrubs with pocket-sized leathery leaves. The numerous species class impenetrable thickets from five to viii anxiety (1.5 to 2.5 m) alpine. During the long dry summers the typically resinous leafage and dry woody stems become explosively combustible.

Wildfires raze large areas to ash-covered earth every few decades. Fires are not harmful to this community; they are in fact necessary for maintaining its vigor. Following fires the blank ground is briefly colonized past a large number of annual species, but the land is soon reclaimed by the shrubs which sprout from seeds or root crowns. Copse and succulents are rare life forms in chaparral considering they are more vulnerable to devastation past the very hot fires.

This young biome evolved from early Third tropical forest during the Pliocene and Pleistocene. The uplift of the groovy mountain ranges of western North America blocked the summer monsoon wet from reaching the far west, creating a summertime dry flavor (see affiliate on "Deep History of the Sonoran Desert").

The primary area of chaparral occurs west of the coast, transverse, and peninsular ranges and is called Californian chaparral (see plate five). Disjunct patches of chaparral occur inland of these ranges and are chosen interior chaparral. Interior chaparral differs in having only a few species; information technology is ofttimes comprised almost entirely of manzanita (two species of Arctostaphylos) and shrub alive oak (Quercus dumosa). Interior chaparral also receives substantial summer rainfall, though the plants do non respond to it.

California chaparral borders the western border of the Sonoran Desert in California and northern Baja California, and interior chaparral is scattered along the desert's northeastern edge where it meets the Mogollon Rim of Arizona. Interior chaparral also occurs in isolated patches on the lower slopes of some mount islands.

Desert

Desert is the driest biome, its vegetation is determined solely by the extreme dehydration. Temperature and seasonality of rainfall determines the specific vegetation and fauna, just all desert vegetation looks more than or less similar; virtually plants are widely spaced and have minor or absent leaves (see plate 6). A detailed discussion of deserts follows this section.

Thornscrub

Thornscrub is intermediate between the desert and tropical forest biomes. The vegetation consists largely of short copse, ten to twenty feet (3-6 chiliad) tall, and shrubs, with cacti also existence mutual in the "New World" communities. It is generally more dense and taller than desert vegetation, and many species are thorny. Annuals and herbaceous perennials are abundant, and vines. a primarily tropical life course. are well represented. During the dry out flavor most perennial plants are drought-deciduous (as opposed to plants of more than temperate regions which are cold-deciduous). In contrast, the rainy flavor, though short, is moderate and dependable and the vegetation grows lush. The climate is nearly frost-free, so temperature is non limiting; the vegetation is determined past the alternating dry and wet seasons (see plate seven).

Tropical Woods

Tropical Forest is determined by the absenteeism of freezing temperatures and the occurrence of aplenty rainfall for at least office of the twelvemonth. Some tropical forests take a dry season, while tropical pelting forest is never stressed for water. Tropical deciduous forests take a dry season lasting from three to nine months, during which time many of the plants go deciduous. Many of the tree species blossom during the winter-jump dry flavor while leafless. In the rainy season the dumbo vegetation grows luxuriantly and forms closed canopies of foliage. The upper canopy ranges from xv to thirty feet (4.5 to 9 m) to a higher place ground in dry forests, to 150 anxiety (45 one thousand) in lowland rain forests. Nearly all life forms are represented, though annuals are nearly absent from rainforest. Flowering epiphytes (plants that grow on other plants or rocks but are not parasitic) are almost completely restricted to tropical habitats, and are a major component of wet tropical floras (see plate viii).

To the south, the Sonoran Desert merges nigh imperceptibly into thornscrub in key Sonora, and thornscrub in turn merges with the northern limit of tropical deciduous wood in the southern tip of that state. A major proportion of the Sonoran Desert's biota evolved from ancestors in these tropical biomes; examples are noted in the species accounts.

Riparian Communities

Riparian communities are not biomes. Though they could be considered isolated ribbons of deciduous wood, they are better viewed every bit a unique habitat type. They occur within any biome wherever there is perennial water nearly the surface. The term riparian specifi-cally refers to the zones along the banks of rivers; all the same, it is too applied to the shoreline communities forth slow or nonflowing waters such as marshes and lakes.

The drier the surrounding habitat, the more distinct is the riparian zone. In the desert or grassland a flowing stream supports a conspicuous oasis with forests and wildlife that would not otherwise occur in the area. The available water also augments populations of more arid-adapted species in the side by side habitat.

Biomes and Communities

	Tundra	Coniferous Forest	Deciduous Woods	Grassland	Desert	Thornscrub	Tropical Forest
Trees	None	Some	Many	Few	None to Some	Many	Many
Woody Shrubs	Few	Some	Many	Some	Many	Many	Many
Subshrubs	Few?	Few	Some	Some	Many	Many	Some
Annuals	None	None	Some	Many	Many	Some	Some
Herbacious Perennials	Some	Some	Many	Many	Many	Some	Some
Grasses	Few	Some	Many	Many	Some	Some	Some
Succulents	None	Few	Few	Some	None to Many	Many	Some
Vines	None	Few	Few	Few	None to Some	Many	Many
Flowering Epiphytes	None	None	None	None	None	Some	Some to Many

Riparian zones are and so dissimilar at different latitudes and elevations that they should exist idea of as several communities with similar physical characteristics, primarily their dependence on perennial water. Montane streams support alder and aspen, while at lower elevations at that place are cottonwoods and sycamores. In tropical deciduous forests a riparian zone may be visually duplicate during the moisture season because the overall appearance of stream-bank and hillside trees is similar, though the species may be different. Simply in the dry season nearly of the slope vegetation is deciduous, while tropical riparian species are typically evergreen.

Some ecologists augment the concept of riparian communities to include the banks of dry washes in deserts. A launder in the Lower Colorado River Valley with its woodland of palo verdes (Cercidium spp.), ironwoods (Olneya tesota), and desert willows (Chilopsis linearis) is clearly distinct from the surrounding creosote bush flats. These dry washes occupy less than v per centum of the area of this subdivision of the Sonoran Desert, but support ninety percentage of its bird life. This concentration of life is the result of the greater availability of h2o, even though the launder may conduct surface water for only a few hours a year. Desert drainageways should be labeled "dry riparian" or "desert riparian" to avoid confusion with wetter habitats that take surface water all or most of the year.

Dry out riparian habitats share most of their defining characteristics with traditional "wet" riparian habitats. They are chronically disturbed, unstable sites where h2o and nutrients are harvested and full-bodied from larger areas (watersheds). Finally, they are corridors for dispersal of plants (seeds) and animals (meet plate 9).

What Is a Desert?

Although many people visualize deserts as dry, desolate wastelands, the term actually defines a wide spectrum of landscapes and plant and animal population densities. The Sonoran Desert does accept seas of sand and expanses of desert pavement that are about devoid of visible life, simply virtually of information technology is more reminiscent of a thin woodland savanna.

The common denominator of all deserts is extreme dehydration — water is freely bachelor only for short periods post-obit rains. Desert is often defined as a place that receives less than x inches (250 mm) of annual average rainfall, just this definition is inadequate. For example, the Pacific coast of northern Baja California and the due north slope of Alaska both receive less, only those places are vegetated with chaparral and tundra, respectively. An accurate measure of dehydration must compare rainfall (abbreviated P for precipitation) with potential water loss through evaporation and transpiration (the loss of water from leaves). Potential evapo-transpiration (abbreviated pet, the water that would be lost from evaporation and transpiration if h2o were nowadays to evaporate) is difficult to mensurate accurately, just is crudely estimated to be sixty pct of pan evaporation (the h2o that evaporates from a wide pan of h2o exposed to the weather condition). Pan evaporation varies severalfold within a local surface area depending on slope and exposure to current of air, and then information technology is applicable merely to the specific site where it is measured. Tucson receives an average of twelve inches (305 mm) of rain a year, while the pan evaporation is about 100 inches (254 cm). In other words, the climate of Tucson could evaporate eight times more water per year than is supplied by pelting, a pan evaporation to precipitation ratio of 8:one. Using the sixty percent estimate for PET, Tucson'due south PET/p ratio is 4.3; climatologists classify areas with ratios higher than 3.0 as semiarid. This moisture deficit presents a significant challenge to the biota, just is non large compared to that of hyperarid deserts such as that around Yuma, Arizona which has a PET/p ratio of 30, or the interior Sahara Desert's 600.

A concise nontechnical definition of a desert is "a place where water is severely limiting to life well-nigh of the time." (Without the word "severely" the phrase defines semiarid habitats such as grassland, chaparral, and tropical deciduous wood.) Though desert plants and animals must cope with scarce water, the common perception that they are struggling to survive is grossly inaccurate. The native biota are adapted to and usually thrive under these weather condition and, in fact, most of them crave an arid environment for survival. Look at it this mode: if a desert received much more than rain, information technology wouldn't exist a desert. A dissimilar, wetter, biome would replace it. Thus an alternative and more positive definition might be: "A desert is a biological community in which most of the ethnic plants and animals are adapted to chronic aridity and periodic, extreme droughts, and in which these weather condition are necessary to maintain the community's structure." (The desert biome requires chronic aridity, simply not all of its component species do.)

Why Are Deserts so Dry?

The low rainfall typical of deserts is more easily understood if ane knows a little about the basics of global climate. Atmospheric thermodynamics is an extremely complicated field, but the basic rules are simple. Offset, hot air rises and cool air sinks. Second, rising air expands and cools, while sinking air compresses and becomes warmer. Third, warmer air can hold more h2o vapor than cooler air. These three natural phenomena plus the sun'southward heat decide where rain falls on the planet.

The sunday shines almost vertically on the equatorial chugalug twelvemonth round, but information technology shines on the polar regions at a shallow angle and only in the summer of each respective pole. There are two consequences. A beam of sunlight ten square anxiety (1 k²) shines on about ten square feet of World's surface at the equator at apex, but it covers more than than twice that expanse near the poles. The sun's light and heat are thus less concentrated at higher latitudes. In addition, at the equator the sunlight travels directly down through the atmosphere, only near the poles it travels through much more than air where more of the calorie-free is reflected, absorbed, or scattered and less reaches the ground. This is why the equator is hot and the poles are cold.

Because of the not bad quantity of oestrus delivered to the equatorial belt, information technology is a zone of warm, rise air. Information technology absorbs much water vapor from the oceans and land vegetation. As this air rises information technology cools. Eventually it reaches saturation (dewpoint temperature) and water vapor condenses into clouds and oftentimes falls as pelting. So the equatorial region is both hot and wet.

The equatorial air rises, then spreads horizontally at loftier elevations to the north and south. Eventually the now absurd air sinks and flows forth the surface to supersede the rising air at the equator, forming a circulation prison cell. It tends to sink at virtually 30° north and 30° south breadth. (These ii zones were called the "horse latitudes" by mariners. Earlier motorpower, sailing vessels could get becalmed in these latitudes for weeks at a time. To reserve precious h2o for themselves, the crew threw horses and other livestock overboard; other ships would come across the floating carcasses.) As the air sinks information technology warms by compression, and because in that location is no source of evaporating h2o, it becomes drier with increasing temperature. Not just can sinking air non produce rain, but when it reaches the ground it absorbs water from the soil and vegetation, creating even more barren conditions.

The horse breadth zones of sinking air are non continuous belts. The combination of the Earth's rotation and the interaction between country masses and oceans creates stable loftier pressure zones (sinking air) over the oceans west of the continents. The resulting aridity is reinforced by the cold ocean currents that also occur on western coasts at this latitude; the cold h2o farther inhibits the potential for rising air currents that are necessary to make rain. Thus on the west edge of every big land mass in that location is a hyperarid area near thirty° latitude called a equus caballus latitude desert. Despite the proximity of the oceans, the high pressure zone is and so stiff over the Atacama and Sahara deserts that decades may laissez passer without rain.

Deserts of the world (white areas). Horse breadth deserts are those on the western edges of all the continents virtually 30° north and southward latitude. The rest are rain shadow deserts. The departure betwixt the ii types can be seen in South America, where the Andes Mountains stretch the entire length of the continent near the due west coast. Due north of 30° the trade winds accident from the northeast, causing a rain shadow desert on the west or coastal side of the Andes. Southward of thirty° the easterlies blow onshore and the rain shadow desert is on the eastern, inland, side of the Andes. Virtually the 30th parallel there is no prevailing wind; the stable high pressure zone creates equus caballus latitude deserts on both sides of the Andes Mountains.

Deserts are also acquired by rain shadow effects wherever there are mountains and prevailing winds. Where wind encounters a mount, it is forced up and over. Equally information technology rises, it cools and drops most of its moisture on the windward slope. On the leeward side it descends, warms, and dries. At latitudes that accept a prevailing wind direction, rain shadow deserts are created on mountains' lee sides.

Aridity is the primary attribute of deserts, simply information technology also generates several other characteristics of deserts. In improver to being meager, desert precipitation is also highly variable and unpredictable. The more arid the desert, the more variable is its rainfall. The boilerplate annual precipitation is a poor predictor of the rainfall in a given year. For instance, Yuma, Arizona has an average annual rainfall of iii and one-half inches (90 mm), but in nigh years it receives less, sometimes none at all. When the stable weather design that enforces aridity breaks downward occasionally, Yuma may receive two or three times its annual average, sometimes in a unmarried storm.

Desert temperatures vary widely both daily and seasonally. The dry, transparent air and clement skies transmit maximal solar energy to the ground where much of it is absorbed and converted to oestrus; the temperature rises dramatically. At night the same conditions permit most of this heat to be radiated to the heaven, and the temperature plummets. (Water vapor, either as humidity or cloud cover, reflects infra-red estrus and slows heat loss.) Daily temperature variation can exist more than than 50°F (28°C). The same conditions create not bad seasonal fluctuation. Loftier-pinnacle deserts that accept 100°F (38°C) days in summertime can feel nights below 0°F (-xviii°C) in winter. Besides the heat it creates, the intense sunlight in arid lands is itself a challenge. The ultraviolet radiation can damage animals. retinas, cause peel cancer, and destroy vital plant molecules such equally chlorophyll. Desert organisms have evolved a multifariousness of adaptations to avoid getting besides much sun.

The North American Deserts

N America has 4 major deserts: Bang-up Basin, Mohave, Chihuahuan and Sonoran. All but the Sonoran Desert have common cold winters. Freezing temperatures are even more limiting to plant life than is aridity, so colder deserts are poorer in both species and life forms, especially succulents.

The four Northward American deserts

The Great Basin Desert (plate x) is both the highest-elevation and northernmost of the four and has very common cold winters. The seasonal distribution of precipitation varies with breadth, just temperatures limit the growing season to the summer. Vegetation is dominated by a few species of low, small-leafed shrubs; there are almost no copse or succulents and non many annuals. The indicator plant (the most common or conspicuous 1 used to identify an expanse) is large sagebrush (Artemisia tridentata), which oft grows in near pure stands over huge vistas. (Such cold shrub/deserts in the "Old World" are called steppes.)

The Mohave Desert (plate eleven) is characterized largely by its winter rainy season. Hard freezes are common but not as astringent equally in the Cracking Basin Desert. The perennial vegetation is composed mostly of low shrubs; annuals carpeting the basis in wet years. There are many species of these ii life forms, just few succulents and trees grow there. The simply common tree species is the characteristic joshua tree (Yucca brevifolia), an arborescent (treelike) yucca that forms all-encompassing woodlands above 3000 feet (900 m) elevation.

Though the Chihuahuan Desert (plate 12) is the southernmost, it lies at a fairly high elevation and is not protected by any bulwark from arctic air masses, so hard wintertime freezes are common. Its vegetation consists of many species of low shrubs, leaf succulents, and pocket-size cacti. Trees are rare. Rainfall is predominantly in the summertime, simply in the northern stop at that place is occasionally enough wintertime rain to support massive blooms of spring annuals. The Chihuahuan Desert is unexpectedly rich in species despite the winter common cold.

The Sonoran Desert

The Sonoran Desert every bit currently defined covers approximately 100,000 square miles (260,000 sq. km) and includes much of the state of Sonora, Mexico, most of the southern half of Arizona, southeastern California, well-nigh of the Baja California peninsula, and the islands of the Gulf of California. Its southern 3rd straddles thirty° north latitude and is a horse latitude desert; the rest is rain shadow desert. Information technology is lush in comparison to well-nigh other deserts. The visually ascendant elements of the landscape are two life forms that distinguish the Sonoran Desert from the other North American deserts: legume trees and large columnar cacti. This desert also supports many other life forms, encompassing a rich spectrum of some 2000 species of plants, 550 species of vertebrates, and unknown thousands of invertebrate species.

The amount and seasonality of rainfall are defining characteristics of the Sonoran Desert. Much of the area has a bi-seasonal rainfall blueprint, though fifty-fifty during the rainy seasons most days are sunny. From December to March frontal storms originating in the Northward Pacific occasionally bring widespread, gentle rain to the northwestern two-thirds. From July to mid-September, the summertime monsoon brings surges of wet tropical air and localized deluges in the form of tearing thunderstorms to the southeastern two-thirds. So singled-out are the characters of the two types of rainfall that Sonoran residents have different Spanish terms for them. the winter rains are equipatas (derived from the Yaqui-Mayo word for rain, quepa), the summer rains are las aguas ("the waters" in Castilian).

The Sonoran Desert prominently differs from the other three North American deserts in having balmy winters. Virtually of the area rarely experiences frost, and the biota are partly tropical in origin. Many of the perennial plants and animals are derived from ancestors in the tropical thornscrub to the southward, their life cycles attuned to the cursory summer rainy flavor. The winter rains, when ample, support groovy populations of annuals (which make up nigh half of the species of our plants). Some of the plants and animals are opportunistic, growing or reproducing after significant rainfall in any season (see the chapter "Deep History of the Sonoran Desert" for more details on its development).

Subdivisions of the Sonoran Desert

Forrest Shreve was the first person to define the Sonoran Desert past dividing information technology into 7 subdivisions, based on the diverse and distinctive vegetation establish here. One of Shreve's subdivisions (the Foothills of Sonora) has since been reclassified as foothills thornscrub, a non-desert biome.

Lower Colorado River Valley

Named for its location surrounding the lower Colorado River in parts of four states, this is the largest, hottest, and driest subdivision. It challenges the Mohave Desert'southward Decease Valley equally the hottest and driest place in North America. Summertime highs may exceed 120°F (49°C), with surface temperatures budgeted 180°F (82°C). The intense solar radiation from cloudless skies on almost days and the very low humidity suck the life-sustaining water from plants, water that cannot be replaced from the parched mineral soil. Annual rainfall in the driest sites averages less than 3 inches (76 mm), and some localities accept gone xxx-vi months with no rain. Even so, life exists here, abundantly in the rare moisture years.

The terrain consists mostly of wide, flat valleys with widely-scattered, small mountain ranges of almost barren rock. There are too seas of loose sand and the spectacular Pinacate volcanic field (encounter plate xiv). The valleys are dominated by low shrubs, primarily creosote bush-league (Fifty. tridentata) and white bursage (Ambrosia dumosa). These are the two most drought-tolerant perennial plants in N America, but in the driest areas of this subdivision fifty-fifty they are restricted to drainageways. Trees grow but along the larger washes. The mountains support a wider variety of shrubs and cacti, only the density is withal very thin. Columnar cacti, one of the indicators of the Sonoran Desert, are rare (nigh absent in California) and are restricted to valley floors. Almanac species incorporate over half the flora, up to 90 percent at the driest sites; they are mostly winter growing species and appear in large numbers only in wet years (encounter plate 13).

This is the only part of the Sonoran Desert that extends into California, where most residents phone call it the Colorado Desert. Due north of a sagging line between the Coachella Valley (Palm Springs) and Needles, California, it merges nigh imperceptibly into the lower Mohave Desert.

Subdivisions of the Sonoran Desert. The six subdivisions reflect the biological diversity of this large desert and the fact that it has been intensively studied. Each subdivision has a dissimilar climate, topography, and vegetation.

Arizona Upland

This northeastern subdivision is the highest and coldest function of the Sonoran Desert. Located in south-cardinal Arizona and northern Sonora, the terrain contains numerous mountain ranges, and valleys narrower than those of the Lower Colorado River Valley subdivision. Copse are common on rocky slopes as well as drainageways, and saguaros grow on slopes above the cold valley floors. This community is as well chosen the "saguaro-palo verde woods". Information technology is the simply subdivision that experiences frequent hard winter frosts, so many species of the lower superlative and more than southerly subdivisions cannot survive here. Nevertheless it is a rich area. The small-scale range that is the Desert Museum'south home, the Tucson Mountains, has about 630 taxonomically distinct kinds of plants. This richness is partly explained by the two equal rainy seasons which full twelve inches (305 mm) per year on average. The hilly terrain provides a multitude of microhabitats on n and due south slopes and deep, shaded canyons. The proximity to chaparral, woodland, and grassland communities contributes however more species to the flora (see plate xv).

Biologists are increasingly concluding that the Arizona Upland's climate, vegetation density, and biodiversity resemble thornscrub more than than desert. Don't exist surprised if this subdivision is reclassified in the most futurity.

Tucson is the only major urban center located in Arizona Upland, although much of metro Phoenix's parks and state higher up 2000 feet (600 chiliad) in pinnacle share its characteristics. Residents who have moved to this area from temperate climates oft complain about the lack of seasons. Actually Arizona Upland has five seasons which, though more than subtle than the traditional temperate four, are distinct if one learns what to expect for.

The following description is for Tucson, but is fairly applicable to the remainder of Arizona Upland and to the eastern one-half of the Lower Colorado River Valley subdivision equally well. The seasons are a footling later at higher latitudes and elevations, earlier at lower ones. The monsoon is later and more sporadic farther w; in some years it fails to attain the Colorado River.

Summer Monsoon, or summer rainy season (early July to mid-September)

In local native tradition, the yr begins with the most dramatic weather event of the region — the often precipitous inflow of the summer rains (plate xvi). A tropical air mass brings humidity and moderates the temperatures from June'due south extremes; frequent thunderstorms occur; this is the main growing flavor for many of the larger shrubs and trees. (Monsoon is derived from an Standard arabic word for "season", and was applied to a air current that changes directions seasonally. Be aware that it does not refer to rain or storms per se, but rather to the shift of wind direction which brings moist air that tin generate storms — in our case, a southerly wind in July. The give-and-take is often misused, even past some weather reporters.)

A sixth season, late summertime, lasting from mid Baronial through September, is sometimes added; this is a hot and dry out flow after the monsoon ends — nonexistent in some years.

Autumn (October & November)

Warm temperatures; low humidity; niggling rain; few species in flower, merely the growing season for winter annuals begins if at that place is plenty rain. Belatedly summer and autumn occasionally receive heavy rains from the remains of Pacific hurricanes (tropical storms).

Wintertime (December & Jan; sometimes February)

By and large sunny, mild days, with intermittent storms that bring wind, rain, and cool-to-cold temperatures; February often warm and dry, more spring-similar (see plate 17).

Spring (early to late February through April)

Balmy temperatures; piddling rain; often windy; ane of two flowering seasons; winter annuals may showtime blooming in February in warm, wet years (see plate xviii).

Foresummer (May & June)

Loftier temperatures; very low humidity; no rain in well-nigh years; May is very warm and often windy; June is hot and usually calm. There is little biological activity except for the flowering and fruiting of saguaro and desert ironwood. Near plants and many animals are dormant until the rains arrive (come across plate nineteen).

Plains of Sonora

This minor region of central Sonora is a series of very broad valleys between widely separated ranges. It supports denser vegetation than does Arizona Upland because in that location is more than rain (with summertime pelting dominant) and the soils are more often than not deeper and finer. It contains virtually of the same species equally Arizona Upland, plus some tropical elements, considering frost is less frequent and less severe. There are abundant legume trees, especially mesquite, and relatively few columnar cacti. The few hills in this region support islands of thornscrub. Most of this subdivision has been converted to agriculture in the last few decades.

If Arizona Upland is reclassified as thornscrub, the wetter Plains of Sonora subdivision would also have to exist reclassified from desert to thornscrub.

Central Gulf Coast

The Fundamental Gulf Declension occupies a strip along both sides of the Gulf of California. Extreme aridity dictates the distinctive appearance of this subdivision. Information technology straddles the equus caballus latitude belt, and desert vegetation grows right to the seashore. Small shrubs are nigh absent; their shallow root systems and lack of water storage cannot sustain them through the droughts which commonly last for several years. Dominating the vegetation are large stalk-succulents, particularly the massive cardón (Pachycereus pringlei, a giant relative of the saguaro), and trees such equally palo verde, tree ocotillo (Fouquieria diguetii and F. macdougalii), ironwood, elephant tree (Bursera spp.), and limberbush (Jatropha spp.); the trees are leafless most of the time. The boilerplate annual rainfall of less than five inches (125 mm) occurs more often than not in summer, though not dependably enough to call information technology a rainy season. A year with no pelting is not rare (see plate twenty).

Vizcaino

The Vizcaino subdivision is on the Pacific side of the Baja California peninsula. Though rainfall is very depression, absurd, boiling body of water breezes with frequent fog ameliorate the aridity. Wintertime rain predominates and averages less than five inches (125 mm). This subdivision contains some of the most bizarre plants and eerily beautiful landscapes in the earth. There are fields of huge, sculpted white granite boulders or black lava cliffs that shelter botanical apparitions such equally boojums (Fouquieria columnaris), twisted and swollen Baja elephant trees (Pachycormus discolor), sixty-human foot (18 m) tall cardones, strangler figs (Ficus petiolaris ssp. palmeri) that grow on rocks, and blue palm copse (Brahea armata). In stark contrast, the coastal Vizcaino Plain is a flat, cool, fog desert of shrubs barely a human foot tall, with occasional mass blooms of annual species (see plates 21 and 26).

Magdalena

Located in coastal Baja California southward of the Vizcaino, Magdalena is similar in appearance to the Vizcaino but the species are somewhat different. Most of its meager rainfall comes in summer and the aridity is modified by Pacific breezes. The dour coastal Magdalena Plain's but conspicuous endemic plant is the weird creeping devil cactus (Stenocereus eruca), just inland the rocky slopes are rich and dumbo with trees, succulent shrubs, and cacti (see plate 22).

Foothills of Sonora

This was Shreve's 7th subdivision of the Sonoran Desert. It has since been reclassified as foothills thornscrub community and is no longer considered part of the desert biome because of its greater rainfall, taller trees and cacti, and denser vegetation.

Shreve'southward depiction of the Sonoran Desert's boundary and subdivisions are the virtually widely accepted. There are at least five other major attempts to ascertain this surface area with dramatically differing boundaries. I version excludes most of Baja California from the Sonoran Desert. Another includes the Mohave equally role of the Sonoran Desert. (Indeed, it is difficult to distinguish the two along the currently accepted purlieus.) These differences of interpretation reflect the great diversity of geography and biota found hither.

The discussion is not still over. Time volition determine whether Arizona Upland and the Plains of Sonora will remain parts of the Sonoran Desert or be reclassified equally thornscrub. Any they are called, all of these regions are fascinating places for nature lovers, whether they are classified as scientists or tourists.

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Source: https://www.desertmuseum.org/books/nhsd_biomes.php

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