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Biomes and Communities of the Sonoran

Mark A. Dimmitt

orld travelers can scarcely help tropical forest to frigid subalpine meadows. The Wbut notice the great diversity of land- focus of this book is on the in scapes on this planet. That diversity is due as the heart of the region, but to understand it we much to the vegetation as to the landforms. need to know something about the other habi- Closer observation reveals a mind-boggling tats within it and on its borders. These adjacent diversity of and animals in these differ- geographical features and biological communi- ent places. Those who travel in the American ties exert profound, complex influences on the Southwest or on the western side of any conti- desert itself. nent near 30° latitude will see dramatic changes Ecologists who study nature on a global scale within distances of only a few miles. recognize about 10 basic, widespread classes of In apparent contrast, some widely distant habitats that are easily identified by their domi- parts of planet Earth show certain broad-scale nant life-forms. (Life-forms are basic catego- similarities. The vegetation of the Mediterra- ries based on general appearance—​for example, nean coast of looks remarkably similar tree, shrub, annual, succulent, and so on.) Such to the of Southern , though global-scale habitats are called , and they no two plant species occur in both places are determined primarily by the climatic factors (except some introduced weeds). Both the diver- of temperature and rainfall. These factors are in sity and the similarities have the same under- turn determined by latitude, elevation, and wind lying causes, namely the interactions between patterns. classification is based on vegeta- geography and the global climate machine tion because plants, being generally immobile, that governs the biosphere on both grand and are the most obvious and easily recognizable microscopic scales. components of a biological community. In addi- The Sonoran Desert Region has a great vari- tion, plants are more definitive of their biomes ety of species and habitats, the latter ranging because, since they are rooted in place, they must from extremely hot, arid desert to semiarid be adapted to that specific environment. Plants,

5

UC_ASDM-S2--TEXT.indd 5 9/9/15 2:11 PM therefore, are often endemic (occurring only in Tundra is the most poleward and high- the named area) to a biome or smaller commu- est-elevation biome and is characterized by nity. Biomes do contain characteristic animal extremely cold winters. The dominant plant life as well, including many endemic insects life-forms are ground-hugging woody shrubs and other invertebrates. Most vertebrates, how- and perennial herbs. Intense cold excludes trees ever, are more mobile, and rather few species are (in this biome no plant can survive exposed restricted to a single habitat. above the winter snow cover) and succulents; Almost all the world’s biomes occur in the the growing season is too short for annuals. Sonoran Desert Region. This tremendous Everywhere on Earth, temperatures at the diversity in a fairly small area is due to two same elevations become warmer as one moves influences. First, our region is on the west toward the equator (to lower latitudes). But an side of a near 30° latitude, a position increase in elevation at a given latitude has the where several biomes typically occur in close same climatic effect as traveling toward a pole: proximity (this phenomenon is explained later temperature decreases (and usu- in this chapter). Second, the great topographi- ally increases). So climates that support tundra, cal relief here creates the cold, wet climates that like those in the , can be found on the allow northern biomes to occur farther south highest mountains all the way to the equator. than they would ordinarily. The other cold biomes in both hemispheres also It is important to recognize that biomes and extend toward the equator, where sufficiently most other biological classifications are largely high elevations meet their climatic require- subjective concepts—​an attempt to make sense ments. The near Flagstaff, of the unwieldy and nearly incomprehensible , rise to 12,600 feet (almost 3900 m). diversity of nature. In addition, their boundar- These are the only mountains in our region that ies are rarely distinct. Wherever two biological extend above timberline (about 11,200 ft/3400 communities or biomes meet, there is usually m elevation in Arizona). There, only 80 miles a zone of intergradation, which is sometimes (130 km) from the Sonoran Desert, is a small very wide. For these reasons, classifications area of that includes some of the differ among classifiers. For example, some same plant species found in the Arctic tundra biologists recognize thornscrub as a separate of Alaska (see plate 2). biome while others call it an ecotone (transition Coniferous forest (also known by its Rus- zone) between desert and tropical forest. Some sian name, taiga) is dominated by cone-bearing combine tropical and temperate forests into the trees, especially pines, firs, and spruces in same biome based simply on vegetation height the . Many conifers are and density. As defined here, the biomes are so adapted to cold only a little less severe than in distinctive that you should be able to place any tundra. Tree heights range from a few feet (a terrestrial habitat on the planet within one of couple of meters) near the boundary with tun- them at a glance. dra or at timberline to over 300 feet (90 m) Biomes are subdivided into a hierarchy of in more temperate latitudes. Some coniferous smaller categories, defined by the particular extend into subtropical climates, for species that inhabit them. There are many example, in the southeastern and classification systems, and the categories have southern , . many names. We use the general terms biotic In our region, coniferous forest occurs in the community, biological community, or simply, higher mountain ranges, to the north and east community. The names used here for the com- of the Sonoran Desert. Our most widespread munities are mostly those of Brown and Lowe coniferous community is Petran Montane (1982). The biomes below are listed in latitudi- (Rocky Mountain) forest, the dominant vegeta- nal order, from pole to equator. tion of the cold-temperate . It

6 The Big Picture

UC_ASDM-S2--TEXT.indd 6 9/9/15 2:11 PM is found at increasing elevations as one moves well above the intervening basins, and in the southward into Mexico, until it is pushed off great gap between the highlands of the mas- the tops of the mountains by excessive arid- sive Rocky Mountains and the Sierra Madre ity and warmth. In the mountains west of the Occidental, they occur only on the higher Sky Sonoran Desert are isolated islands of Sierran Islands (see chapter 3). (as in Sierra ) coniferous forest, charac- Mediterranean and Shrub­ terized by different conifer species. land is a semiarid biome that occurs on the Temperate Broadleaf forest is charac- west coast of every continent between about terized by dense stands of broadleaf trees that 30° and 40° latitude. This smallest biome is drop their foliage in winter. The temperatures unique for its Mediterranean climate: mild, are milder than those of most conifer-domi- moist winters and hot, dry summers. In North nated climates, though still too cold for plants America this biome is called chaparral. Mature to grow in winter. Summers are typically warm chaparral consists almost solely of woody, ever- and humid. More species occur in this biome green shrubs with small, leathery leaves. The than in the two more poleward biomes (tundra numerous species form impenetrable thickets and coniferous forest). The herbaceous peren- from 5 to 8 feet (1.5–​2.5 m) tall. During the nial life-form is well represented here, along long, dry summers, the typically resinous foli- with trees and shrubs. Pure deciduous forest age and dry, woody stems become explosively is rare in our region; there are only scattered combustible. raze large areas to ash- aspen groves and ribbons of riparian trees. covered earth every few decades. Fires are not The foothills and lower mountain slopes harmful to this community; its species are on the eastern boundary of the Sonoran Des- well adapted to periodic blazes. Following fires ert support oak woodlands mixed with pines the bare ground is briefly colonized by a large at slightly higher elevations (see plate 3). The number of annual species, but the land is soon oaks, however, are mostly evergreen species; reclaimed by the shrubs that sprout from seeds they are only ephemerally deciduous in spring or root crowns. Trees and succulents are rare except during severe droughts. This commu- life-forms in chaparral because they are more nity, called Madrean evergreen woodland in the vulnerable to destruction by the very hot fires. Brown-Lowe-Pase classification, is a warm-tem- This young biome evolved from early Tertiary perate community of the Sierra Madre Occiden- tropical forest during the Pliocene and Pleisto- tal. It extends as far north as central Arizona, cene. The uplift of the great mountain ranges where it is squeezed out by the cool-temperate of western blocked the summer Rocky Mountain forests above it and the more moisture from reaching the far west, arid and desert below. (Though its creating a summer dry season (see chapter 9). official name is woodland, in its southern part The main area of chaparral occurs west of it’s actually a forest; i.e., the tree canopies over- the Coast, Transverse, and lap.) This is a semiarid community that expe- and is called Californian chaparral (see plate 5). riences a dry season in spring. The Madrean Disjunct patches of chaparral occur inland of evergreen woodland has been split into two these ranges and are called interior chaparral. communities: oak woodland and Mexican Interior chaparral differs in having only a few pine-oak woodland. The term “Madrean” was species; it is often made up largely of manza- dropped because, at least in the Sky Island nita and shrub live oak. Interior chaparral also Archipelago, the communities contain almost receives substantial summer rainfall, though as many Rocky Mountain as Madrean species. the plants do not respond to it—​that is, they In the Sonoran Desert Region, tundra, don’t grow or flower in response to the . coniferous forest, and temperate broadleaf Californian chaparral borders the western forest are restricted to mountains that rise edge of the Sonoran Desert in California and

Biomes and Communities of the Sonoran Desert Region 7

UC_ASDM-S2--TEXT.indd 7 9/9/15 2:11 PM the northern Peninsula, and als such as bulbs, which die to the ground each interior chaparral is scattered along the desert’s year). Populations of trees, shrubs, and succu- northeastern edge, where it meets the Mogol- lents are kept at low levels by periodic fires dur- lon Rim of Arizona. Interior chaparral also ing the dry season. occurs in isolated patches on the lower slopes Most of the in the western states of some Sky Islands. are intermediate between the true prairies of Grassland is a semiarid biome character- the Midwest and . They are called semi- ized by warm, humid summers with moderate desert or desert grasslands. (Again, the California and cold, dry winters. (The Central Valley grasslands are an exception. They are heavily of California is an exception; it is a winter-rain- influenced by the unique California floristic fall grassland at a lower-than-typical elevation.) province and not much by the Midwest prai- Grass is the dominant life-form, and scores of ries.) Compared with Plains grassland, the species form a nearly continuous cover over grasses in desert grassland are shorter and less large areas. Other well-represented life-forms dense, and the land supports more shrubs and are annuals and geophytes (herbaceous perenni- succulents (see plate 4). Desert grassland or

8 The Big Picture

UC_ASDM-S2--TEXT.indd 8 9/9/15 2:11 PM chaparral borders the northern Sonoran Desert season that lasts three to nine months, during on the east. which time many of the plants become decidu- Desert is the driest biome; its vegetation ous. In our region this biome is labeled tropi- is determined solely by the extreme aridity. cal deciduous forest. Many of the tree species Temperature and seasonality of rainfall deter- flower during the winter-spring dry season, mine the specific vegetation and fauna, but all while leafless. In the dependable rainy season desert vegetation looks more or less similar; the dense vegetation grows luxuriantly and most plants are widely spaced and have small forms closed canopies of foliage (see plate 8). or absent leaves (see plate 6). A detailed discus- The upper canopy ranges from 15 to 30 feet sion of deserts follows in this chapter. (4.5–​9 m) above ground. Almost all life-forms Tropical Savanna is a warm, semiarid are represented. Flowering epiphytes (plants biome characterized by flat land covered with that grow on other plants or rocks but are not continuous herbaceous vegetation, usually parasitic) are almost completely restricted to dominated by grasses, with some trees and tropical habitats. There are also many vines, other woody plants. It is an extensive biome in another life-form that is most common in , and it also occurs in , India, and tropical habitats. . It is usually not recognized as a At its southern limit, the Sonoran Desert biome in North America. Our region has fairly merges almost imperceptibly into thornscrub in small areas of grass with sparse trees, which we central Sonora, and thornscrub in turn merges consider to be ecotones between the grassland with the northern limit of tropical dry forest in and temperate broadleaf forest biomes. Most of the southern tip of that state. A major propor- these ecotones are warm-temperate, not tropi- tion of the Sonoran Desert’s biota evolved from cal. A small part of the Plains of Sonora subdivi- ancestors in these tropical biomes; examples sion of the Sonoran Desert is sometimes called are noted in the species accounts. a savanna biocommunity, because there are Tropical Wet Forest is determined by the open patches dominated by grasses in wet years. absence of both freezing temperatures and a Thornscrub is intermediate between the dry season; life is seldom limited by either and tropical forest biomes. The veg- temperature or scarcity of water. This biome is etation consists largely of short trees (10–​20 often called tropical rainforest. The upper tree feet/3–​6 m tall) and shrubs, with cacti also canopy can attain 150 feet (45 m) in height. The being common in the communi- epiphyte and vine life-forms are common, and ties. It is generally denser and taller than desert annuals are almost absent. This biome does vegetation, and many species are thorny (see not occur in the Sonoran Desert Region, but a plate 7). Annuals and herbaceous perennials couple of canyons in southern Sonora support are abundant, and vines—​a primarily tropical vegetation that is intermediate between dry and life-form—​are well represented. During the wet tropical forests. dry season most perennial plants are drought- deciduous (as opposed to plants of more tem- Riparian Communities perate that are cold-deciduous). In contrast, the rainy season, though short, is Riparian communities are not biomes. Though moderate and dependable and the vegetation they could be considered isolated ribbons of grows lush. The climate is nearly frost-free temperate broadleaf forest, they are better (tropical), so temperature is not limiting; the viewed as a unique habitat type. They occur vegetation is determined by the alternating dry within any biome wherever there is perennial and wet seasons. water near the surface. The term riparian spe- Tropical Dry Forest is determined by the cifically refers to the zones along the banks of absence of freezing temperatures and a dry rivers; however, it is also applied to the shore-

Biomes and Communities of the Sonoran Desert Region 9

UC_ASDM-S2--TEXT.indd 9 9/9/15 2:11 PM line communities along slow or nonflowing for dispersal of plants (seeds) and animals (see waters such as and (see plates 24 plate 9). and 25). The drier the surrounding habitat, the more Coastal Communities distinct is the . In the desert or grassland, a flowing stream supports a conspic- As with riparian communities, coastal wet- uous with forests and wildlife that would lands are distinctive habitats that are not suf- not otherwise occur in the area. The available ficiently extensive to be categorized as biomes. water also augments populations of more arid- The vegetation consists of halophytes (plants adapted species in the adjacent habitat. that are adapted to highly saline water and soil). Riparian zones are so different at different In temperate zones the halophytes are mostly latitudes and elevations that they should not be low-growing species, such as pickleweed and thought of as a single community type; they saltgrass. In the tropics there are forests of can be one of several communities with similar several unrelated large shrubs and trees col- physical characteristics, primarily their depen- lectively called mangroves. Five species of man- dence on perennial water. Montane streams groves occur in our region: red, black, white, support alders and aspens, while at lower eleva- sweet, and button. tions there are cottonwoods and sycamores. In tropical deciduous forest a riparian zone may be What Is a Desert? visually indistinguishable during the wet sea- son because the overall appearance of stream- Although many people visualize deserts as dry, bank and hillside trees is similar, though the desolate wastelands, the term actually defines a species may be different. But in the dry season wide spectrum of landscapes and plant and ani- most of the slope vegetation is deciduous, while mal population densities. The Sonoran Desert tropical riparian species are typically evergreen. does have seas of sand and expanses of desert Some ecologists broaden the concept of pavement that are nearly devoid of visible life, riparian communities to include dry washes but most of it is more reminiscent of a sparse in deserts. A wash in the Lower River woodland. Valley, with its woodland of palo verdes, iron- The common denominator of all deserts is woods, and desert willows, is clearly distinct extreme aridity—​water is freely available only from the surrounding creosote flats. These dry for short periods following rains. Desert is washes occupy less than 5 percent of the area of often defined as a place that receives less than the Arizona Upland subdivision of the Sonoran 10 inches (250 mm) of annual average rainfall, Desert, but they support 90 percent of its bird but this definition is inadequate. For example, life. This concentration of life is the result of the Pacific coast of the state of Baja California the greater availability of water, even though and the north slope of Alaska both receive less, the wash may carry surface water for only a few but those places are vegetated with chaparral hours a year. Desert drainageways should be and tundra, respectively. An accurate measure labeled dry riparian, desert riparian, or xerori- of aridity must compare rainfall (abbreviated parian to avoid confusion with wetter habitats p for precipitation) with potential water loss that have surface water all or most of the year. through evaporation and transpiration (the loss Dry riparian habitats share most of their of water from leaves). Potential evapotranspira- defining characteristics with traditional “wet” tion (abbreviated pet, the water that would be riparian habitats. They are chronically dis- lost from evaporation and transpiration if water turbed, unstable sites where water and nutri- were present to evaporate) is difficult to mea- ents are harvested and concentrated from larger sure accurately, but it is crudely estimated to be areas (watersheds). Finally, they are corridors 60 percent of pan evaporation (the water that

10 The Big Picture

UC_ASDM-S2--TEXT.indd 10 9/9/15 2:11 PM evaporates from a wide pan of water exposed basics of global climate. Atmospheric thermo- to the weather). Pan evaporation varies sever- dynamics is an extremely complicated field, but alfold within a local area depending on slope the basic rules are simple. First, hot air rises and exposure to wind, so it is applicable only to and cool air sinks. Second, rising air expands the specific site where it is measured. Tucson and cools, while sinking air compresses and receives an average of 12 inches (305 mm) of becomes warmer. Third, warmer air can hold rain a year, while the pan evaporation is about more water vapor than cooler air. These three 100 inches (254 cm). In other words, the climate natural phenomena, plus the sun’s heat, deter- of Tucson could evaporate eight times more mine where rain falls on the planet. water per year than is supplied by rain, a pan The sun shines almost vertically on the evaporation to precipitation ratio of 8:1. Using equatorial belt year-round, but it shines on the the 60 percent estimate for pet, Tucson’s pet to polar regions at a shallow angle and only dur- p ratio is 5:1; climatologists classify areas with ing each pole’s summer. There are two conse- ratios higher than 3:1 as semiarid. This mois- quences. A 1-square-meter beam of sunlight ture deficit presents a significant challenge to shines on about 1 square meter of the Earth’s the biota, but it is not large compared with that surface at the equator at noon, but it covers of hyperarid deserts such as that around Yuma, more than twice that area near the poles. The Arizona, which has an aridity index of 30:1, or sun’s light and heat are thus less concentrated the interior Desert’s 600:1. at higher latitudes. In addition, at the equator A concise, nontechnical definition of a des- the sunlight travels straight down through ert is “a place where water is severely limiting the atmosphere, but near the poles it travels to life most of the time.” (Without the word through much more air, where more of the severely the phrase defines semiarid habitats, light is reflected, absorbed, or scattered, and such as grassland, chaparral, and tropical less reaches the ground. This is why the equa- deciduous forest.) Though desert plants and tor is hot and the poles are cold. animals must cope with scarce water, the com- Because of the great quantity of heat deliv- mon perception that they are struggling to ered to the equatorial belt, it is a zone of warm, survive is grossly inaccurate. The native biota rising air. It absorbs much water vapor from is adapted to and usually thrives under these the oceans and land vegetation. As this air rises conditions and, in fact, most of the species it cools. Eventually it reaches saturation (dew- require an arid environment for survival. Look point temperature), and water vapor condenses at it this way: if a desert received much more into clouds and often falls as rain. So the equa- rain, it wouldn’t be a desert. A different, wetter torial region is both hot and wet. biome would replace it. Thus an alternative and The equatorial air rises, then spreads hori- more positive definition might be: “A desert is zontally at high elevations to the north and a biological community in which most of the south. Eventually, the now-cool air sinks and indigenous plants and animals are adapted to flows along the surface to replace the rising chronic aridity and periodic, extreme droughts, air at the equator, forming a circulation cell. and in which these conditions are necessary It tends to sink at about 30° north and 30° to maintain the community’s structure.” (The south latitude. (These two zones were called desert biome requires chronic aridity, but not the by mariners. Before motor all of its component species do.) power, sailing vessels could get becalmed in these latitudes for weeks at a time. According to one story, crews threw horses overboard to Why Are Deserts So Dry? reserve precious water for themselves; other The low rainfall typical of deserts is more eas- ships would encounter the floating carcasses.) ily understood if one knows a little about the As the air sinks it warms by compression, and

Biomes and Communities of the Sonoran Desert Region 11

UC_ASDM-S2--TEXT.indd 11 9/9/15 2:11 PM 60° 60°

30° 30°

0° 0°

30° 30°

World Deserts

50° Arid & Extremely Arid 50°

Deserts of the world (light areas). Horse latitude deserts are those on the western edges of all the near 30° north and 30° south latitude. The rest are deserts. The difference between the two types of desert can be seen in South America, where the Mountains stretch the entire length of the continent near the west coast. North of 30°, the trade winds blow from the northeast, causing a rain shadow desert on the west, or coastal, side of the Andes. South of 30°, the easterlies blow onshore and the rain shadow desert is on the eastern, inland side of the Andes. Near the 30th parallel there is no prevailing wind; the stable high pressure zone creates horse latitude deserts on both sides of the Andes.

because there is no source of evaporating water, tain. As it rises, it cools and drops most of its it becomes drier with increasing temperature. moisture on the windward slope. On the lee- Not only can sinking air not produce rain, but ward side it descends, warms, and dries. At when it reaches the ground it absorbs water latitudes that have a prevailing wind direction, from the soil and vegetation, creating even rain shadow deserts are created on the lee sides more arid conditions. of the mountains. The horse latitude zones of sinking air are Aridity is the primary attribute of deserts, not continuous belts. The combination of the but it also generates several other character- Earth’s rotation and the interaction between istics of deserts. In addition to being meager, landmasses and oceans creates stable high- desert precipitation is also highly variable and pressure zones (sinking air) over the oceans unpredictable. The more arid the desert, the west of the continents. The resulting aridity is more variable is its rainfall. The average annual reinforced by the cold ocean currents that also precipitation is a poor predictor of the rainfall occur on western coasts at this latitude; the cold in a given year. For example, Yuma, Arizona, water further inhibits the potential for rising has an average annual rainfall of 3½ inches (90 air currents that are necessary to make rain. mm), but in most years it receives less, some- Thus on the west edge of every large landmass times none at all. When the stable weather pat- there is a hyperarid area near 30° latitude called tern that enforces aridity breaks down occasion- a horse latitude desert. Despite the proximity ally, Yuma may receive two or three times its of the oceans, the high-pressure zone is so annual average, sometimes in a single storm. strong over the Atacama and Sahara Deserts Desert temperatures vary widely both daily that decades may pass without rain. and seasonally. The dry, transparent air and Deserts are also caused by rain shadow cloudless skies transmit maximal solar energy effects wherever mountains block prevailing to the ground, where much of it is absorbed winds. Wind is forced up and over the moun- and converted to heat; the temperature rises

12 The Big Picture

UC_ASDM-S2--TEXT.indd 12 9/9/15 2:13 PM dramatically. At night the same conditions per- trees grow there. The only common tree species mit most of this heat to be radiated to the sky, is the characteristic Joshua tree, an arborescent and the temperature plummets. (Water vapor, (tree-like) that forms extensive wood- either as humidity or cloud cover, reflects lands above 3000 feet (900 m) elevation. infrared radiation and slows heat loss.) Daily Though the (see temperature variation can be more than 50°F plate 12) is the southernmost North American (28°C). The same conditions create great sea- desert, it lies at a fairly high elevation and is not sonal fluctuation. High-elevation deserts that protected from Arctic air masses by any barrier, have 100°F (38°C) days in summer can experi- so hard winter freezes are common. Its vegeta- ence nights below 0°F (–18°C) in winter. tion consists of many species of low shrubs, Besides the heat it creates, the intense sun- leaf succulents, and small cacti. Trees are rare. light in arid lands is itself a challenge. The Rainfall is predominantly in the summer, but ultraviolet radiation can damage animals’ reti- in its northern reaches there is occasionally nas, cause skin cancer, and destroy vital plant enough winter rain to support massive blooms molecules, such as chlorophyll. Desert organ- of spring annuals. Despite the winter cold, the isms have evolved a variety of adaptations to Chihuahuan Desert is rich in species. avoid getting too much sun.

The Sonoran Desert The North American Deserts The Sonoran Desert as currently defined covers North America has four major deserts: the approximately 100,000 square miles (25,900 , Mojave, Chihuahuan, and ha) and includes much of the state of Sonora Sonoran Deserts. All but the Sonoran Desert (Mexico), most of the southern half of Arizona, have cold winters. Freezing temperatures are southeastern California, most of the Baja Cali- even more limiting to plant life than is aridity, fornia Peninsula, and the islands of the Gulf so colder deserts are poorer in both species and of California. Its southern third straddles 30° life-forms, especially succulents. north latitude and is a horse latitude desert; The (see plate 10) is the rest is rain shadow desert. It is lush in both the highest-elevation and northernmost of comparison to most other deserts. The visually the four North American deserts and has very dominant elements of the landscape are two cold winters. The seasonal distribution of pre- life-forms that distinguish the Sonoran Desert cipitation varies with latitude, but temperatures from the other North American deserts: legume limit the growing season to summer. Vegeta- trees and large columnar cacti. This desert also tion is dominated by a few species of low, small- supports many other life-forms, encompassing leafed shrubs; there are almost no trees or suc- a rich spectrum of more than 2000 species of culents and few annuals. The indicator plant plants, about 550 species of vertebrates, and (the most common or conspicuous one used to unknown thousands of invertebrate species. identify an area) is big sagebrush, which often The amount and seasonality of rainfall are grows in nearly pure stands over huge vistas. defining characteristics of the Sonoran Desert. (Such cold shrub-deserts in the are Much of the area has a bi-seasonal rainfall pat- called steppes.) tern. From December to March frontal storms The (see plate 11) is charac- originating in the North Pacific bring occa- terized largely by its winter rainy season. Hard sional widespread, gentle rain to the northwest- freezes are common but not as severe as in the ern two-thirds. From July to mid-September, Great Basin Desert. The perennial vegetation is the summer monsoon brings surges of wet composed mostly of low shrubs; annuals carpet tropical air and localized deluges in the form the ground in wet years. Few succulents and of violent to the southeastern

Biomes and Communities of the Sonoran Desert Region 13

UC_ASDM-S2--TEXT.indd 13 9/9/15 2:11 PM

IDAHO

WYOMING

41°

42°

NEVADA

UTAH

COLORADO

CALIFORNIA

ARIZONA NEW

MEXICO

TEXAS

B

A

J A P

C

A A G

L I U C F

O L I R F N

F I A

SONORA I N 29° O C R T E O 28° F COAHUILA B A J A C Desert Areas of North America C A A L S L Mojave Desert I I F I N O O F R A C O Sonoran Desert N L I R E A O N A Chihuahuan Desert A S U I R N A Great Basin Desert ZACATECAS 0 100 200

MILES KILOMETERS 0 100 200

North America has four major deserts: the Great Basin, Mojave, Chihuahuan, and Sonoran Deserts. All but the Sonoran Desert have cold winters.

UC_ASDM-S2--TEXT.indd 14 9/9/15 2:11 PM ­two-thirds. So distinct are the characteristics of water that cannot be replaced from the parched the two types of rainfall that Sonoran residents mineral soil. Annual rainfall in the driest sites have different Spanish terms for them—​the averages less than 3 inches (76 mm), and some winter rains are equipatas (derived from the localities have gone 36 months with no rain. Yaqui-Mayo word for rain, quepa), and the sum- Even so, life exists here, abundantly in the rare mer rains are las aguas (“the waters”). wet years. The Sonoran Desert prominently differs The terrain consists mostly of broad, flat from the other three North American deserts valleys with widely scattered, small mountain in having mild winters. Most of the area rarely ranges of almost barren rock (see plate 13). experiences frost, and the biota are partly tropi- (There are also seas of loose sand and the spec- cal in origin. Many of the perennial plants and tacular Pinacate volcanic field [see plates 14 and animals are derived from ancestors in the tropi- 29]). The valleys are dominated by low shrubs, cal thornscrub to the south, their life cycles primarily creosotebush and white bursage. attuned to the brief summer rainy season. These are the two most drought-tolerant peren- The winter rains, when ample, support great nial plants in North America, but in the driest populations of annuals (which make up nearly areas of this subdivision even they are restricted half of the species of our plants). Some of the to drainageways. Trees grow only in the larger plants and animals are opportunistic, growing washes. The mountains support a wider variety or reproducing after significant rainfall in any of shrubs and cacti, but the density is still very season. (See chapter 9 for details on the evolu- sparse. Columnar cacti, one of the indicators of tion of the Sonoran Desert.) the Sonoran Desert, are rare (virtually absent in California) and are restricted to valley floors. Annual species make up over half the flora, up Subdivisions of the to 90 percent at the driest sites; they are mostly Sonoran Desert winter-growing species and appear in large Forrest Shreve, an American botanist who numbers only in wet years. worked at the Carnegie Institution’s Desert This is the only part of the Sonoran Desert Laboratory in Tucson, Arizona, first defined that extends into California, where most resi- the Sonoran Desert in 1953. He divided it into dents call it the . North of a sag- seven subdivisions based on the diverse and ging line between the (Palm distinctive vegetation found here. One of these Springs) and Needles, California, it merges (the Foothills of Sonora) has since been reclas- almost imperceptibly into the lower Mojave sified as foothills thornscrub, a non-desert Desert. biocommunity. Arizona Upland Lower Valley This northeastern subdivision is the highest Named for its location surrounding the lower and coldest part of the Sonoran Desert. Located Colorado River in parts of four states, this is in south-central Arizona and northern Sonora, the largest, hottest, and driest subdivision. It the terrain contains numerous mountain challenges the Mojave Desert’s ranges and valleys narrower than those of the as the hottest and driest place in North Amer- Lower Colorado River Valley subdivision. Trees ica. Summer highs may exceed 120°F (49°C), are common on rocky slopes as well as drain- with surface temperatures approaching 160°F ageways, and grow on slopes above (71°C). The intense solar radiation from cloud- the cold valley floors. This community is also less skies on most days and the very low humid- called the –palo verde forest (see plate ity suck the life-sustaining water from plants, 15). It is the only subdivision that experiences

Biomes and Communities of the Sonoran Desert Region 15

UC_ASDM-S2--TEXT.indd 15 9/9/15 2:11 PM 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 different climate, topography, and vegetation.

UC_ASDM-S2--TEXT.indd 16 9/9/15 2:11 PM Five Seasons

The following descriptions apply clearly to the Tuc- flower, but the growing season for winter annuals son area, but they are fairly applicable to all the begins if there is enough rain. Late summer and Arizona Upland subdivision and to the eastern half autumn occasionally receive heavy rains from the of the Lower Colorado River Valley subdivision. remains of Pacific hurricanes (tropical storms). Summer monsoon or summer rainy season (early Winter (December and January, sometimes Feb- July to mid-September). In Tohono O’odham tradi- ruary). Mostly sunny, mild days, with intermittent tion, the year begins with the most dramatic weather storms that bring wind, rain, and cool-to-cold tem- event of the region—the often abrupt arrival of the peratures; February is often warm and dry, more summer rains (see photo on page 22). Monsoon is spring-like (see plate 17). derived from an Arabic word for “season,” and it Spring (early to late February through April). was applied to a wind that changes directions sea- Mild temperatures; little rain; often windy; one of sonally. The North American monsoon wind shift, two flowering seasons; winter annuals may start in our case typically a southerly wind in July, brings blooming in February in warm, wet years (see tropical moist air that moderates the temperatures plates 18 and 27). from June’s extremes and can generate frequent Foresummer (May and June). High tempera- thunderstorms. This is the main growing season for tures; very low humidity; no rain in most years; many of the larger shrubs and trees. (A sixth season, May is very warm and often windy; June is hot and late summer, lasting from mid-August through Sep- usually calm. The only common plants in flower tember, is sometimes recognized. This is a hot and are saguaro, prickly pear, foothill palo verde, and dry period in years when the monsoon ends early.) desert ironwood; usually only saguaro continues Autumn (October and November). Warm tem- into June. Most plants and many animals are dor- peratures; low humidity; little rain; few species in mant until the rains arrive (see plate 19).

frequent hard winter frosts, so many species of tan Phoenix, the Lower Colorado River Valley the lower-elevation and more southerly subdi- subdivision intergrades with Arizona Upland. visions cannot survive here. Nevertheless, it is Residents who have moved to this area from a rich area. The small range that is the Desert temperate climates often complain about the Museum’s home, the , has lack of seasons. Actually, Arizona Upland has about 630 taxa (species and subspecies) of five seasons, which, though more subtle than plants. This richness is partly explained by the the traditional temperate four, are distinct if two equal rainy seasons, which total 12 inches one learns what to look for. The “Five Seasons” (305 mm) per year on average. The hilly terrain sidebar above applies to the Tucson area, but it provides a multitude of microhabitats on north is fairly applicable to the rest of Arizona Upland and south slopes and deep, shaded canyons. and to the eastern half of the Lower Colorado The proximity to chaparral, woodland, and River Valley subdivision as well. The seasons grassland communities contributes still more are a little later at higher latitudes and eleva- species to the flora (see plate 16). tions, earlier at lower ones. The monsoon is Arizona Upland’s climate, vegetation den- later and more sporadic farther west; in some sity, and biodiversity resemble thornscrub at years it fails to reach the Colorado River. least as much as desert. It may join the former Foothills of Sonora subdivision and be reclassi- Plains of Sonora fied as non-desert in the future. Tucson is the only major city entirely This small region of central Sonora is a series within Arizona Upland, while in metropoli- of very broad valleys between widely separated

Biomes and Communities of the Sonoran Desert Region 17

UC_ASDM-S2--TEXT.indd 17 9/9/15 2:11 PM ranges. It supports denser vegetation than does the most bizarre plants and eerily beautiful Arizona Upland because there is more rain landscapes in the world. There are fields of (with summer rain dominant) and the soils are huge, sculpted, white granite boulders or black generally deeper and finer. It contains most of lava cliffs that shelter botanical apparitions the same species as Arizona Upland, plus some such as boojums, twisted and swollen Baja tropical elements, because frost is less frequent elephant trees, 60-foot-tall (18-m) cardóns, and less severe. There are abundant legume rock figs, and blue palm trees. In stark contrast trees, especially mesquite, and relatively few to the hilly interior of this subdivision, the columnar cacti. The few hills in this region coastal Vizcaíno Plain is a flat, cool, fog des- support islands of thornscrub. Most of this ert of shrubs barely a foot tall, with occasional subdivision has been converted to mass blooms of annual species (see plates 21 in the last few decades. If Arizona Upland is and 26). reclassified as thornscrub, the wetter Plains of Sonora subdivision would also have to be Magdalena reclassified from desert to thornscrub. Located in coastal , south of the Vizcaíno, Magdalena is similar in appear- Central Gulf Coast ance to the Vizcaíno but the species are some- The Central Gulf Coast occupies a strip along what different. Most of its meager rainfall both sides of the . Extreme comes in summer, and the aridity is modified aridity dictates the distinctive appearance of by Pacific breezes. The bleak coastal Magdalena this subdivision. It straddles the horse latitude Plain’s only conspicuous endemic plant is the belt, and desert vegetation grows right to the weird creeping devil (see plate 22), but seashore. Small shrubs are nearly absent; their inland the rocky slopes are rich and dense with shallow root systems and lack of water storage trees, succulent shrubs, and cacti. cannot sustain them through the droughts, which commonly last for several years. Large- Foothills of Sonora stem succulents, particularly the massive cardón (a giant relative of the saguaro), and This was Shreve’s seventh subdivision of the trees such as palo verde, tree ocotillo, ironwood, Sonoran Desert. It has since been reclassified elephant tree (Bursera spp.), and limberbush as foothills thornscrub community and is no dominate the vegetation; the trees are leafless longer considered part of the desert biome most of the time. The average annual rainfall because of its greater rainfall, taller trees and of less than 5 inches (125 mm) occurs mostly in cacti, and denser vegetation. summer, though not dependably enough to call Shreve’s delineation of the Sonoran Desert’s it a rainy season. A year with no rain is not rare boundary and subdivisions are the most widely (see plate 20). accepted. There are at least five other major attempts to define this area, with dramatically differing boundaries. One version excludes Vizcaíno most of the Baja California Peninsula from the The Vizcaíno subdivision is on the Pacific side Sonoran Desert. Another includes the Mojave of the Baja California Peninsula. Though rain- as part of the Sonoran Desert. (Indeed, it is dif- fall is very low, cool, humid sea breezes with ficult to distinguish the two along the currently frequent fog mitigate the aridity. Winter rain accepted boundary.) These differences of inter- predominates and averages less than 5 inches pretation reflect the great diversity of geography (125 mm). This subdivision contains some of and biota found here.

18 The Big Picture

UC_ASDM-S2--TEXT.indd 18 9/9/15 2:11 PM Additional Readings of the Arizona-Nevada Academy of Science 14, no. and References S1 (1979): 1–​16. Dunbier, R. The Sonoran Desert: Its Geography, Econ- Brown, D. E., ed. Biotic Communities: Southwestern omy, and People. Tucson: University of Arizona United States and Northwestern Mexico. Salt Press, 1968. City: University of Press, 1994. Lowe, C. H. Arizona’s Natural Environment: Land- Brown, D. E., and C. H. Lowe. “Biotic Communi- scapes and Habitats. Tucson: University of Ari- ties of the Southwest” (map). General Technical zona Press, 1964. Report RM—​78. Fort Collins, CO: U.S. Forest Shreve, F., and I. L. Wiggins. Vegetation and Flora of Service, 1982. the Sonoran Desert. Stanford, CA: Stanford Uni- Brown, D., C. Lowe, and C. P. Pase. “A Digitized versity Press, 1975. Classification System for the Biotic Communities Walter, H. Ecology of Tropical and Subtropical Vegeta- of North America, with Community (Series) and tion. New York: Van Nostrand Reinhold, 1971. Association Examples for the Southwest.” Journal

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