The Sonoran Desert written by Brian McGill

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The Sonoran Desert written by Brian McGill

The Sonoran desert covers a large area including 2/3 of Arizona and much of northwest Mexico. It is bounded on the east by the Colorado river (or the Pacific Ocean on Baja California). On the North it is bounded by the Mogollon rim and on the east it is bounded by the Continental Divide (Rockies & Sierra Madre Occidental).

For most of geologic history, the Sonoran desert was a large, flat plate that was periodically inundated by seas, leaving large sedimentary rock deposits. About 25 million years ago, the Pacific coast of North America got hooked to a new tectonic plate and started moving northwest (this new plate is found west of the San Andreas fault, the dividing line). This exerted a stretching/pulling force that caused the crust to stretch by 30-80%. The result was a series of fault-block mountains where there are numerous parallel faults running roughly north-south and occurring approximately every 50 miles in the east-west direction. On one side of these faults, an enormous uplift occurred, creating mountains. On the other side, an enormous drop occurred. These cracks also led to some vulcanism (such as what formed the Tucson Mountains). Since the pulling settled down about 8 million years ago, the formation of new mountains has stopped, and the main force has been erosion. The erosion of the mountains has filled the valleys, often to a depth of 5000+ feet (bedrock in Tucson is 7,000 feet below the sediments that fill the Tucson valley). This causes a characteristic zonation of 1) very flat basins (often called flats), 2) a slope at angle of about 30 consisting of course rocky debris that has fallen from the mountains and called a bajada, and 3) more steep mountain slopes, often rising to 8,000-9,000 feet. The bajadas and flats are carved with a network of washes that drain them. Some basins (such as the Avra valley to the west of the Tucson Mountains) have no wash draining the valley, and they can fill with water, causing a shallow lake called a playa. During the last ice age, the Sonoran desert was free of ice, but the climate was much cooler. The oak juniper woodlands that we now see at about 4,000 feet was found on the valley floors and formed a contiguous blanket of vegetation. With the retreat of the ice, this life zone moved up the mountains and was broken into a series of sky islands, and the vegetation we now recognize as desert vegetation (creosote, cacti) moved up from the south. These wetter times also left considerable water in the basin sediments. This water is called “fossil” water since it is left from the last ice age and is not being renewed. In Tucson the water filled the entire basin to within a few feet of the surface, but has now dropped 200 feet due to our wells. This drying has caused enormous cracks to open in the earth on the highway between Tucson and Phoenix.

The dryness of the Sonoran desert is caused by two factors. First it is in the rain shadow of the Sierra Nevada’s in California. Second, there is a global pattern of air that rises in the tropics (causing great uplift which causes great thunderstorms) and then heads pole-ward until it descends at about 30North and South latitude. This descending (subsiding) air becomes very dry, causing most of the world’s great deserts to occur in belts around 30North and South latitude. The climate distinguishes the Sonoran desert from the other three deserts of North America. The Great Basin Desert covering Utah, northern Arizona, parts of Nevada and up into Oregon and Idaho is a cool desert – it has freezing temperatures on a regular basis. This makes cactus less common and sage brush the dominant plant. The other two deserts are warm and occur to the west (Mojave) and east (Chihuahan) of the Sonoran. The Sonoran desert is lucky enough to experience two rainy seasons. The winter rains occur from about October-early March and come when the Jet Stream bends southward, bringing Pacific storms inland. This rain also reaches the Mojave west of us but usually drops out as it lifts over the continental divide, leaving the Chihuahan desert quite dry in the winter. Tucson gets about 6” of rain in the winter spread out over many gentle rains. The summer rains occur from July-early September and are called monsoons. They are caused when a high-pressure zone forms over the four-corner area causing clockwise air circulation, which teams up with the western edge of the westward extension of the Bermuda high to cause the winds to shift and come from the southeast, bringing moisture from the Gulf of Mexico. When this moisture encounters the thermal uplifts due to the solar heating of the desert floor and the vertical uplift given by the mountains, considerable thunderstorms occur. These thunderstorms tend to hit the Chihauhan desert in May-June and September-October. In the July-August time frame they shift west and cause the monsoons in Tucson, giving us an average of about 6” of rain for a total of about 12”. For comparison, most of the East coast gets 30-45” and the Midwest gets 20-35” (rain forests get as much as 100”).

Plants of the Sonoran Desert

This section briefly describes plans found in the flats and bajadas, not including washes.


Found in the desert

All the trees found in the desert are “bean trees” – members of the bean family, Fabaceae. They all have compound leaves, spines and bean-pod seeds. These trees are all phreatophytes – they live in the desert by having roots that tap down into permanent underground water. 40 feet is common and the record is mesquite roots found in a mine at 200 feet. Because bean trees are among the few desert plants that can fix nitrogen (which is scarce in the desert), bean pods are a nitrogen rich and popular food for almost all animals in the desert. Much of what is now Tucson used to be “mesquite bosques” – open forest woodlands. However, due to human usage, the water table has dropped so low mesquites can no longer grow and it has reverted to creosote flats. The one exception is up near the road up Mt Lemmon, where enough water still flows to keep a mesquite bosque supported.

Trees with one thorn

Trees with green stems back as far as the main trunk

These trees are palo verde’s (Spanish for green stick). They are often leafless and instead photosynthesize in the stem – a trick that is used often in Legumes. Two species are found here:

Foothill (yellow) Paloverde (Cercidium microphyllum)trunk yellowish, thorns longer, tips of twigs spiny

Blue Paloverde (Cercidium foliridum) – trunk greenish, thorns shorter, twigs normal

Trees with dark bark stems

Catclaw acacia (Acacia greggii) – thorns curved like a cats claw. There are many other species of acacia all over the world and in Texas, but only A. greggii is common here.

Trees with paired thorns

Leafs only once compound

Ironwood (Olneya tesota) – leaves a lightish silvery green. Grows only in the lower warmer areas around Tucson. Glorious pink/purple flowers visible for miles in the late spring. Wood is so dense it sinks, hence the name ironwood which is applied to many trees around the world with dense wood.

Leafs twice compound (branches once after leaving the stem before leaves begin)

Honey mesquite (Prosopis glandulosa) – leaves hairless and about 1” long

Velvet mesquite (Prosopis velutina) – leaves hairy (hence the name) and less than ½” long

Found only near wash/stream

Desert Willow (Chilopsis linearis) – to 30 feet tall with long narrow leaves and large pink trumpet-shaped flowers. Not a true willow.

Cottonwood (Populus fremontii) – the only “normal” looking tree around here with large triangular leaves. Found only along the banks of washes that run year round


The first two shrubs are evergreen. The last uses stem photosynthesis. The rest in the middle are drought deciduous (lose leaves in dry periods).

Creosote (Larrea tridentata) – 3-6 feet high with small, resinous – two-forked leaves with tiny yellow flowers and a furry seed. It is the most common plant in the flats, often called creosote flats. The creosote bush is quite regularly spaced — there is considerable debate whether this is a response to competition for sparse water or the result of a more active excretion of toxic substances to kill other plants. This plant is a serious survivor – able to handle extended droughts of a couple of years. The “smell of desert rain” is largely due to these plants. Due to toxins, only specialized animals are able to eat creosote including the jackrabbit and Croesote grasshopper. Creosote plants, due to the leaf litter below them, create tiny islands of increased nitrogen and water. A great many desert annuals prefer to grow under a creosote bush. Go check out several other species of Larrea from South America in the cement planter at the southwest corner of BSW (notice the directions of different leaves). One creosote plant may live 200 years, but the plants grow clonally through underground extensions, in an ever-enlarging ring in a manner similar to mushroom fairy rings. The oldest known clone is found in the Mohave Desert and may be 11,000 years old!

Jojoba (Simmondsia chinensis). A woody shrub 2-5 feet tall with leathery gray-green leaves that are all very distinctively oriented in the same direction. The leaves are arranged to point their edge towards the sun at the hottest part of the day. The “beans” (not truly a bean) have a wax which is used in shampoos, making this the 2nd most economically important plant in the desert (after the Washington Palm).

Triangleleaf bursage (Ambrosia deltoidea, also called burrobush or rabbitbush) – a shrub about 2 feet high with triangular leaves. This is the other common shrub found in creosote flats. It loses its leaves during drought periods. A close relative (white-leaf bursage) is found west of here in the drier deserts. A bursage may live 50 years. A great many plants also grow under bursages, including pincushion cacti and young creosotes!

Brittle-bush (Encelia farinosa) A shrub 3-5 tall. In wet periods it is found with leaves about 2” long that are a whitish-yellowish green and yellow sun-flower type flowers. In dry periods, these disappear and all that is seen is a hemispherical mound of dry dead-looking twigs mostly sticking straight up.

Ocotillo (Fouquieria splendens) From 5-20 spiny sticks about 1” in diameter sticking up 10-20 feet into the air. The Ocotillo is drought deciduous and will lose its leaves during dry periods and then regrow them as many as six times year. They have a beautiful red flower spike that attracts humming birds in the spring. Their ecology is more similar to that of the cacti than the other shrubs listed (drought buffering instead of drought resisting).

Desert broom (Baccharis sarothroides). A bush 3-8 feet tall with narrow green twigs and usually no leaves. The seeds occur on white-tassles that can look like snow blowing in the air. This is a native plant that likes to grow in disturbed sites. In the desert this often meant along washes whose banks were often torn up. Here in the city it often means empty lots, construction sites, etc.

Ratany (Krameria grayi) A bush 2-3 feet tall. The leaves are tiny and needle like. The gray/silver-green colored stems are the most obvious feature of this plant. You will study later in the semester whether Krameria is a partial parasite on other plants. Krameria is one of the very few number of plants that produce oil instead of nectar (sugar) for its pollinators (bees).

Mistletoe (Phoradendron californicum) Leafless plant with yellowish, tubular, brittle stems that grows in trees, especially mesquite. This is a different mistletoe than that made famous by Christmas tradition, but shares the trait of being a tree parasite. The berries are a critical winter food resource for Phainopepla (a black desert relative of the cardinal).


Cacti are all succulent plants with a flower containing many petals arranged in a circle around a disk containing many stamens. Cacti have CAM photosynthesis, in which they close the stomata during the day and open them during the cooler night. The CO2 is then taken up and stored in an organic acid for use during the day when light makes photosynthesis possible. This is very water efficient (90% more efficient). During very dry periods the stomata are closed all the time. Due to the CAM mechanism low levels of photosynthesis can continue. This ability to store water and CO2 allows the Cacti to keep the metabolic engine “idling” even in the worst of times. This makes them prepared to take advantage of even the briefest of rainfalls. At the first hint of water, the plant kicks into high gear shooting out many new fine roots in very shallow soil. This soaks up the water very quickly before it can evaporate or drain deeper into the soil. This makes cacti specialists in very coarse, sloped soils where water is available for only brief periods of time.

Columnar cacti

These tall cacti are the most spectacular sight of the desert. They are all taller than a human.

Saguaro (Carnegia gigantea) – the species that defines the Sonoran desert. It is 40-50 feet tall with branches occurring at about 10 feet. The flower is white, blooms at night, is pollinated by bats as well as bees, and closes forever by about 10AM. One hectare of saguaros produce about 5,000,000 seeds of which only about 0.2% germinate. Of these, about 1.5% will survive the first five years. The biggest dangers are freezing, drying out, and being eaten by insects. Growing under a small plant helps with all of these and you will notice that young saguaros are found almost exclusively under other plants (often Palo Verdes) which are called nurse plants. There is considerable debate about whether the saguaro ultimately out competes and kills the nurse plant or if it just outlives it. If the plant makes it through the first 7 years, it is in good shape. After 20-50 years the plant reaches about 1m height. After 50-70 years it reaches about 10 feet in height and begins branching, which starts to make it more vulnerable to mortality again. It will then live to be about 150-300 years old with frost and blowover being the biggest dangers.

The saguaro grows further north than all other columnar cacti due to the special arrangements of spines and many flutings on its stem (which keep it warmer). Still, the northern and altitudinal range limits of the Saguaro are set by areas where frost occurs for more than 24 hours. Some other columnar cacti that are found just south of here are the Organ Pipe (Stenocereus thurberi) with 10-20 stems growing about 10 feet high and 6” in diameter, the Senita (Lophocereus shcottii) looking somewhat like an organpipe (although thicker) and with a dense set of spines at the top of each shaft giving an appearance of hair on top, and Cardon (Pachycereus pringlei) which is similar to the Saguaro but much thicker and with a somewhat woody base.

Segmented Cacti (Opuntia)

The genus Opuntia is highly speciose (200 spp) and difficult to identify, in part because so many species hybridize with each other. The plants are all 3-8 feet tall, consisting of many jointed segments, each roughly 6-12” long. Often a joint will break off, and a new cactus will begin growing where it falls (vegetative reproduction).

Thin joints (diameter roughly same size as a pinky, spines sparse – 1 per areole)

Pencil Cholla (Opuntia arbuscula) fruits greenish-purple, spines 1” long

Christmas Cholla (Opuntia leptocaulis) fruits bright red, spines 2+” long

Flat joints (pad-like)

Beavertail cactus (Opuntia basilaris) – pad shaped with a distinct narrowing towards the bottom

Prickly pear cactus (Opuntia engelmanii) – pad circular to elliptical. A subspecies that rarely grows over a foot or two above the ground is called Sprawling prickly pear.

Thick, cylindrical joints (2”, clear trunk)

Teddy bear cholla (Opuntia biglovii) – joints without knobs, spines extremely dense and whitish-yellowish giving plant a white-yellow rather than green appearance from a distance. Clearly one-trunked

Jumping cholla (Opuntia fulgida) . – joints knobby, more spiny than those listed below, less spiny than above, trunk with several branches

Medium, cylindrical joints (1”, no clear trunk, often reddish-purple in color, no trunk, knobby)

Buckhorn Cholla (Opuntia acanthocarpa) - fruit dry and spiny, fruits drop when in flower

Staghorn Cholla (Opuntia versicolor) – fruit fleshy and spineless, fruits persist >1 year

Other Cacti

Fishhook barrel cactus (Ferocactus wislizeni) - a large (2 foot diameter) 2-4 foot tall cactus. This species does indeed have spines shaped like a fishhook. Several other barrel cacti (Ferocactus & Echinocactus) are found in other regions of the Sonoran. Barrel cacti are better than moss for a compass – they grow pointing southwest to minimize the heat they are exposed to. When they get too old they fall over. Contrary to myth, it is extraordinarily hard to get water out of a barrel cactus.

Hedgehog cacti (Echinocereus) are small 1-2” in diameter and 6”-1’ tall with several stems in a cluster. There are a number of species found around here.

Pincushion cacti (Mammilaria) These are extremely small cacti (less than 6” tall) with many dense hairs arranged around circles with no ribbing on the stems. There are a number of species found around here.


Another common strategy for surviving the dry desert is to only live for a brief period when there is some reasonable expectation of rain. This is the strategy for annual plants which live from one to many years as a seed in the underground seedbank. When they germinate they grow, flower, and reproduce in a matter of weeks or months. The annuals are divided into two time periods. The winter annuals germinate in October-November and bloom in February-March. When the rains are good in both the fall (germination time) and February (growth & flowering time), the entire desert can be covered in flowers. In a bad year, they can hardly be found. Two common winter annuals are Mexican poppy (Eschscholtzia mexicana) with bright yellow flowers about a foot high and plantain (Plantago insularis) which is a tiny plant with green flowers. Summer annuals begin germinating with the first monsoon rain and may flower as soon as a few weeks later.

Some good websites:

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