Biomes & Communities of the Sonoran Desert Region
World travelers can scarcely help but notice the great diversity of landscapes on this planet. That diversity is as much due to the vegetation as to the landforms. Closer observation reveals a mind boggling diversity of plants and animals in these different 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 apparent contrast, some widely distant parts of planet Earth show certain broad scale similarities. The vegetation of the Mediterranean coast of Europe looks remarkably similar 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 have the same underlying causes, namely the interactions between geography and the global climate machine which governs the biosphere on both grand and microscopic scales.
The Sonoran Desert region has a great variety 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, but to understand it we need to know something about 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 general appearance, for example, tree, shrub, annual, succulent, and so on. Such global scale habitats are called biomes, and are determined primarily by the climatic factors of temperature and rainfall. These factors are in turn determined by latitude, 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 only 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 single habitat.
All of the world's biomes occur in the Sonoran Desert region. This tremendous diversity in a fairly small area is due to two influences. For one 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 great topographical relief creates the cold, wet climates that allow northern biomes to occur farther south than they would ordinarily.
It is important to recognize that biomes and most other biological classifications are largely subjective concepts. an attempt to make sense of the nearly incomprehensible diversity of nature. In addition, their boundaries are rarely distinct. Wherever two biological communities or biomes meet there is usually a zone of intergradation which is sometimes very wide. For these reasons classifications differ among 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 simply on vegetation height and density. The biomes as defined here are so distinctive that you should be able to place any terrestrial habitat on the planet within one of them at a glance (see plate 1).
Biomes are subdivided into a hierarchy of smaller categories, defined by the particular species that inhabit them. There are many classification systems and the categories have many names. We use the general terms biotic community, biological community, or simply, community. The names used here for the communities are mostly those of Brown and Lowe (1982).
TUNDRA is the most poleward and highest-elevation biome and is characterized by extremely cold winters. The dominant plant life forms are ground-hugging woody shrubs and perennial herbs. Intense cold excludes trees and succulents and the growing season is too short for annuals.
Temperatures become warmer at lower latitudes (toward the equator) at the same elevation. But an increase in elevation at a given latitude has the same climatic effect as does traveling toward a pole: temperature decreases. So a climate that supports tundra, like that in the arctic, can be found on high mountains all the way to the equator. The other cold biomes in both hemispheres also extend toward the equator where sufficiently high elevations meet their climatic requirements. The San Francisco Peaks near Flagstaff, Arizona, rise to 12,600 feet (almost 3900 m). These are the only mountains in our region that extend above timberline (about 11,200 feet, 3400 m elevation in Arizona). There, only forty-five miles (72 km) from the saguaros of the Sonoran Desert, is a small area of alpine tundra that includes some of the same plant species that occur in the arctic tundra of Alaska (see plate 2).
CONIFEROUS FOREST (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 only a little less severe than in tundra. Tree height ranges from a few feet (a couple of meters) near the tundra boundary or at timberline to over 300 feet (90 m) 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 higher mountain ranges, mostly to the north and east of the Sonoran Desert. Our most widespread coniferous community is Petran Montane (Rocky Mountain) forest, the dominant vegetation of the cold-temperate Rocky Mountains. Its elevation increases southward into Mexico until it is pushed off the tops of the mountains by excessive aridity and warmth. In the mountains west of the Sonoran Desert are isolated islands of Sierran (as in Sierra Nevada) coniferous forest, characterized by different conifer species.
TEMPERATE DECIDUOUS FOREST is characterized by dense stands of broadleaf trees that drop their foliage in winter. The winters are milder than those of most conifer-dominated climates, though still too cold for plant growth. Summers are typically warm and humid. There are more species than in the two 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 forest and temperate deciduous forest tree types. The oaks, however, are mostly evergreen species; they are not deciduous except during severe droughts. This Madrean evergreen woodland (also called Mexican oak-pine woodland) is a warm-temperate community of the Sierra Madre Occidental. It extends as far north as central Arizona, where it is squeezed out by the cool-temperate Rocky Mountain 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 (see plate 3).
In the Sonoran Desert region tundra, coniferous forest, and temperate deciduous forest are restricted to mountains that rise well above 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 analogous to oceanic islands, the terms "mountain islands" and "sky islands" have been coined for these and similar ranges.
GRASSLAND is a semiarid biome characterized by warm, humid summers with moderate rain and cold, dry winters. (The central 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 cover over large areas. Other well-represented life forms are annuals and geophytes (herbaceous perennials such as bulbs that die to the ground each year). Populations of trees, shrubs, and succulents are kept at low levels by periodic fires during the dry season.
Most of the grasslands in the western states are intermediate between the true prairies of the American Midwest and deserts. They are called semi-desert or desert grasslands. (Again the California grasslands are an exception. They are heavily influenced by the unique California floristic province and not much by the Midwest prairies.) Compared with prairie grassland, the grasses in desert grassland are shorter, less dense, and are more frequently interspersed with desert shrubs and succulents. Desert grassland or chaparral borders the northern Sonoran Desert on the east (see plate 4).
CHAPARRAL is a semiarid biome that occurs on the west coast of every continent between about 30° and 40° North latitude. This smallest biome is unique for its Mediterranean climate: mild, moist winters and hot, dry summers. Mature chaparral consists almost solely of woody evergreen shrubs with small leathery leaves. The numerous species form impenetrable thickets from five to eight feet (1.5 to 2.5 m) tall. During the long dry summers the typically resinous foliage 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 bare ground is briefly colonized by a large number of annual species, but the land is soon reclaimed by the shrubs which sprout from seeds or root crowns. Trees and succulents are rare life forms in chaparral because they are more vulnerable to destruction by the very hot fires.
This young biome evolved from early Tertiary tropical forest during the Pliocene and Pleistocene. The uplift of the great mountain ranges of western North America blocked the summer monsoon moisture from reaching the far west, creating a summer dry season (see chapter on "Deep History of the Sonoran Desert" ).
The main area of chaparral occurs west of the coast, transverse, and peninsular ranges and is called Californian chaparral (see plate 5). Disjunct patches of chaparral occur inland of these ranges and are called interior chaparral. Interior chaparral differs in having only a few species; it is often comprised almost entirely of manzanita (two species of Arctostaphylos) and shrub live oak (Quercus dumosa). Interior chaparral also receives substantial summer rainfall, though the plants do not respond to it.
California chaparral borders the western edge 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 mountain islands.
DESERT is the driest biome, its vegetation is determined solely by the extreme aridity. Temperature and seasonality of rainfall determines the specific vegetation and fauna, but all desert vegetation looks more or less similar; most plants are widely spaced and have small or absent leaves (see plate 6). A detailed discussion of deserts follows this section.
THORNSCRUB is intermediate between the desert and tropical
forest biomes. The vegetation consists largely of short trees, ten to twenty
feet (3-6 m) tall, and shrubs, with cacti also being common 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 form. are well represented. During the dry season most perennial
plants are drought-deciduous (as opposed to plants of more temperate regions
which are cold-deciduous). In contrast, the rainy season, though short, is moderate
and dependable and the vegetation grows lush. The climate is nearly frost-free,
so temperature is not limiting; the vegetation is determined by the alternating
dry and wet seasons (see plate 7).
TROPICAL FOREST is determined by the absence of freezing temperatures and the occurrence of ample rainfall for at least part of the year. Some tropical forests have a dry season, while tropical rain forest is never stressed for water. Tropical deciduous forests have a dry season lasting from three to nine months, during which time many of the plants become deciduous. any of the tree species flower during the winter-spring dry season while leafless. In the rainy season the dense vegetation grows luxuriantly and forms closed canopies of foliage. The upper canopy ranges from fifteen to thirty feet (4.5 to 9 m) above ground in dry forests, to 150 feet (45 m) in lowland rain forests. Almost 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 8).
To the south, the Sonoran Desert merges almost imperceptibly into thornscrub in central Sonora, and thornscrub in turn merges with the northern limit of tropical deciduous forest 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.