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The Desert Shrubs – Food Chain Producers

Desert Food Chain - Part 7


While the cacti, yuccas and agaves rank as the showy botanical stars of the desert, it is the humble but tenaciously aggressive shrubs that dominate the landscape, serving up both food and shelter for animal life.  The shrubs, Radford University’s Geography Department says in its Internet site, “may be evergreen or deciduous [shedding leaves seasonally]; typically have small leaves; and frequently have spines or thorns and/or aromatic oils.”  Further, as the United States Geologic Survey said in its Internet site, “Science for a Changing World,” the “shrubs are commonly widely spaced with herbaceous [soft-stem] plants and grasses living beneath them and intershrub spaces that are barren or contain microphytic [biological] crusts composed of lichens and algae.”

Of course, we can scarcely think of the Chihuahuan Desert without seeing in our mind’s eye the sentinel-like Torrey Yucca, with its shaggy and powerful 15- to 20-foot high stem crowned with its rosette of broad-sword-like leaves and its spring cluster of creamy white flowers.  Think of the Sonoran desert, and we see the startling columnar Saguaro Cactus, sometimes with arms shaped like those of a posing body builder.  Recall the Mojave, and we see the Joshua Tree Yucca, its arms raised to beckon struggling pioneers westward, to the promised land of California.  Recall any of the three deserts and you can visualize the agave with a stemless rosette of leaves that give rise to a disproportionately towering bloom stalk, signifying the pinnacle of the plant’s life and signaling the imminence of its death. 

While we tend to see the cacti, yuccas and agaves as individuals with distinctive characters, we see the shrubs as communities of sameness, covering desert slopes, flats, dunes, drainages and playa lake shores like a tattered blanket.  Ranging in size from “dwarfs” to small trees, the shrubs, characterized by solid woody stems and branches, grow in scattered stands that reflect the distribution of the land’s sparse water and nutrients.  Some of the shrubs grow only in a home desert.  Others, for instance, the Creosotebush, have spanned all three deserts. 

Origin and Range

The desert shrubs began their drive to botanical dominance in the Southwest’s basins during the final millennia of the last ice age, about 10,000 to 12,000 years ago.  As temperatures warmed and rain and snowfall diminished, desert vegetation began to migrate northward from Mexico into the Southwestern United States and from the low-lying Yuma and Death Valley areas into higher elevations.  For instance, the Creosotebush, from its northernmost outposts along the Arizona/Sonora border began what would become a botanical takeover of large regions in all three deserts.  Faced with the steady advance of the desert shrubs, the pinyon-juniper-oak woodlands retreated from the basins into the mountain foothills, and the spruce-fir-conifer forests withdrew from the mountain foothills to the higher mountain elevations. 

The desert vegetation’s conquest has been tracked largely through the “analysis of plant and animal remains preserved in fossil packrat…middens [or nests],” according to the U. S. Geological Survey’s Land Use History of North America.  (See Chapter 9, written by Craig D. Allen, Julio L. Betancourt and Thomas W. Swetnam.)  “packrats gather nearby plant materials and accumulate them in dry caves and crevices; there, the plant and other debris [including animal remains] are cemented into large masses of crystallized urine (referred to as amberat), which can persevere for tens of thousands of years The extensive archive of sorted, identified, and dated [by the radiocarbon method] material represents the richest and best-documented source of plant remains in the world ”  As a result, scientists can use deposits from packrat middens like snapshots of our Southwestern deserts environmental chronology.

In the middle of the 19th century, the shrubs tightened their grip on the desert, perhaps thanks more to human interference than to continuing environmental change.  Shrubs have spread nearly unopposed into areas where ranchers allowed cattle, sheep and goats to overgraze and all but obliterate the native grass communities.  The shrubs thrived, less threatened, in areas where well-intentioned government agencies and commercial interests suppressed wildfires.  They prospered in areas where settlers exterminated consuming wildlife, especially the prairie dog. 

Mesquites, for example, one of the most aggressive of all the shrub plants, has grown virtually unimpeded in the absence of the prairie dogs, voracious consumers exterminated from much of the plant’s potential range.  Where they are present, prairie dogs, according to the Great Plains Restoration Council Internet site, “remove pods and seeds and nip and strip bark from young [mesquite] seedlings, which contributes to seedling mortality.”

Pointing to some of the more dramatic examples of the march of the shrubs, the USGS said in “Science for a Changing World,” the “Sonoran and Chihuahuan desert shrubs are expanding their ranges at the expense of semidesert grasslands”  As a result of severe grazing pressure [in southern Arizona], “these grasslands were greatly altered by 1900 in the United States and by 1940 in Mexico; most areas were converted to degraded forms of Sonoran Desert shrubland

“Similarly, in New Mexico and western (Trans-Pecos) Texas, researchers have documented extensive conversions of Chihuahuan semidesert grasslands to Chihuahuan Desert shrubland” 

The shrubs’ increasing domination over other plants, especially the grasses, has come to be called “desertification,” a word signifying a downward spiral in the environmental health of hot and arid lands.

Desert Adaptations

The scrub brush – typically equipped with small leaves, protective thorns, multiple branches, disagreeable smells and tastes, and extensive root systems – have succeeded in their expansion because they come superbly adapted to cope with the heat and dryness of the desert. 

Many, according to the University of Virginia’s Department of Environmental Sciences Internet site, have shiny small waxy leaves that both deflect heat and minimize transpiration (the evaporation of water vapor through the stomata, or pores, of the leaves).  Others have small leaves with light-colored densely enveloping hairs that help reflect sunlight and minimize transpiration.  Some shrubs curl their leaves or turn their leaf edges to minimize the exposure of the leaf to the direct midday sunlight.  A few secrete light-colored salt onto their leaf surfaces to further increase reflectance of sunlight.  In periods of extreme drought, some brush species sacrifice leaves or even twigs and branches to reduce exposure to the unrelenting sunlight and to reduce the plant’s need for water. 

In addition to specialized leaves, shrubs have highly adapted root systems, the primary means for absorbing water from the soil.  Small shrubs often have a dense network of shallow roots, according to Tony L. Burgess, “Desert Grassland, Mixed Shrub Savannah, Shrub Steppe, or Semidesert Scrub?” The Desert Grassland.  Classed as “intensive exploiters,” the small shrubs’ roots compete tenaciously for water near the surface.  Larger shrubs, “extensive exploiters,” may have root systems that not only extend laterally well beyond the plant canopy to intercept rainfall or snow melt near the surface, they also have roots that, given proper soil conditions, can penetrate deeply enough to reach water tens of feet below the surface.  Some authorities say that at least one species, the Creosotebush, has a root system that may secrete chemicals that discourage growth by other plants.  “Root toxins produced by the roots of Creosotebush reduce competition for water by killing plants that grow too close,” said Ann and Myron Sutton in The Life of the Desert.   In any event, a desert shrub’s root system controls its territory so efficiently that it may prevent growth by other species, effectively creating a “dead zone” around the plant. 

While the leaves deflect solar heat and reduce transpiration and the roots provide water, a desert shrub’s seeds, also adapted to the hot and arid environment, help assure the species’ future.  Typically, a desert shrub produces tough-coated seeds in great abundance, in effect, a botanical strategy based on the premise that only with a protective coating can even a small percentage of the seeds survive a long hard wait for the right site, soil conditions, seasonal timing and moisture required for germination. 

Some Typical Desert Shrub Species

Across the three deserts, shrubs grow in mosaics with highly varied populations, with the mix and relative abundances of species reflecting factors such as elevation, climate, water and nutrient availability, soil conditions, competition, predation, the immediate environment’s health, and sheer chance. 

In the Chihuahuan Desert, the signature Tarbush, a one- to seven-foot-tall perennial (a plant that lives for more than one season), grows in thickets or in mixed stands in the dry soils of valleys, mesas, flats and foothills, according to the United States Department of Agriculture, Forest Service, Fire Effects Information System Internet site.  It prospers in heavy clay loam soils.  Its dense growth of elliptical, one-inch-long leaves cloaks multiple branches and produces a tar-like odor.  Like other successful desert shrubs, the Tarbush, one of the extensive exploiters, has a root system that extends laterally beyond the plant’s canopy and may probe 15 feet or more down into the soil.

Unlike most other desert shrubs, the Tarbush produces new growth in midsummer, during the monsoon rains, and it blooms, not in the spring, but in the late fall.  It produces densely hairy seeds during the winter, releasing them for distribution by the winds of spring.  Aggressive and tenacious, the Tarbush has helped spearhead the shrubs’ invasion of the Chihuahuan Desert grasslands, and it “frustrates attempts to convert such rangeland back to grassland,” according to the FEIS Internet site.

The Catclaw Acacia, a 6- to 12-foot tall perennial with devilish spines, has staked its claim to floral fame primarily in the Sonoran Desert, although its reach extends across the desert Southwest and eastward into the Great Plains, according to the Tarleton State University Internet site.  It belongs to the legume, or pea, family.  Often growing in dense thorny thickets along washes, it has quarter-inch-long, compound leaves (multiple leaflets growing on a single stem) that it sheds for the winter.  It has a woody trunk with elaborate branching.  With thorns shaped like the claws of a cat, the shrub’s thickets can form a virtually impenetrable barrier to hikers, sometimes leaving them feeling as if they’ve been in a battle with junkyard felines.  Like other desert shrubs, the Catclaw Acacia has, according to Hillary Sellmeyer’s Native Plants Class Internet site, a root system that includes both lateral roots and a tap root.  As with all legumes, its roots have nodules that contain nitrogen-fixing bacteria.  From spring into fall, the Catclaw Acacia produces puffy and fragrant yellow flowers that attract numerous insects, especially bees.  Its pods, twisted and curled, begin to ripen in early summer, with each yielding perhaps half a dozen hard brown seeds that can survive the desert environment for years while awaiting a propitious moment for germination.  The Catclaw Acacia, a stylish but threatening shrub, may live for more than 100 years. 

The salt- and high-heat-tolerant Four-wing Saltbush, a 1- to 6-foot high perennial evergreen shrub, found its quintessential home in the Mojave Desert, growing around the margins of the inland basin’s dry mineralized lake beds.  A member of the goosefoot family – which includes weeds such as Russian thistle and pigweed and cultivated plants such as spinach and sugar beets – the Four-wing Saltbush has thin half-inch long, pale green leaves that grow either singly or clustered on heavily branched stems.  “The young stems and leaves are covered with minute white scales called scurf, which helps protect the plant against water loss,” according to DesertUSA, and the mature leaves cloak themselves with small scales.  Another of the extensive exploiters, the Four-wing Saltbush has a multiple-branched root system that may penetrate as much as 40 feet below the surface, according to Texas A&M University’s horticulture Internet site.  It produces inconspicuous yellow blooms from spring through fall.  Its seeds, enclosed in distinctive capsules with four papery wings, give the plant its name. 

The mesquites – opportunistic, aggressive and persistent perennial legumes that include the Honey Mesquite, the Velvet Mesquite and the Screwbean Mesquite – have strengthened their hold over their range over the past century and a half.  They have asserted domination over desert grasslands stripped by domestic livestock, according to the FEIS Internet site.  While all three of the mesquite species have taken up widespread residence across our Southwestern deserts, the Honey Mesquite’s home lies primarily in the Chihuahuan Desert, east of the Continental Divide; the Velvet Mesquite’s, primarily in the northern Sonoran Desert, west of the Continental Divide; and the Screwbean Mesquite, primarily in the northern Sonoran Desert up into the Mojave Desert.  The mesquites have the legume’s trademark compound leaves, with a single stem bearing narrow leaflets.  The Velvet Mesquite’s leaflets have a distinctively hairy, or “velvety,” texture.   The three mesquites, all armed with thorns, may grow as multiple-branched or single-branched stems.  They typically reach four or five feet to 10 to 15 feet in height, although they often grow substantially taller under favorable conditions.  The mesquites, especially the Honey Mesquite, have the extensive exploiters’ far-reaching root systems.  The Honey Mesquite’s roots may reach dozens of feet beyond the plant’s canopy and – famously – may burrow nearly 200 feet below the surface in a determined reach for water.  All the mesquites bloom from spring into summer, producing pale yellowish clusters of flowers called “catkins” that prove irresistible to bees and other pollinating insects.  As legumes, they all produce seed pods that are very high in protein.  The Screwbean Mesquite’s tightly wound pod serves as its distinguishing characteristic.  Various species of grazing and browsing wildlife as well as domestic livestock have contributed to the spread of the mesquites because the seeds pass through the animals’ digestive tracts, usually days after they have eaten the pods.  The mesquites have become “the most common shrub/small tree of the Desert Southwest,” according to DesertUSA. 

The Creosotebush, comparable to the mesquites in aggressiveness, tenacity and range, probably ranks as the shrub most perfectly adapted to the desert environment.  It is, says the FEIS Internet site, “a dominant or codominant member of most plant communities in the Mojave, Sonoran, and Chihuahuan deserts.  Creosotebush occurs on 35 to 46 million acres…in the Southwest…”  In the American Museum of Natural History’s Internet site, Jack C. Schultz and Ted Floyd said, in their article “Desert Survivor – North America’s Creosote Bush,” that the Creosotebush is “able to grow and reproduce even when water seems virtually absent.”  They added that, “In the driest deserts, it may be the only plant that can survive.”  The plant’s small leaves bear a resin coating that shields it from ultraviolet radiation and minimizes water loss.  Heavily branched, the Creosotebush turns the edges instead of the broad surfaces of its leaves toward the sun, further reducing the plant’s exposure to ultraviolet radiation and transpiration.  As a result, it offers thin shade.  The Creosotebush root system, according to Schultz and Floyd, extends laterally across more than 50 square yards and penetrates 15 feet below the surface.  It “monopolizes soil nutrients, starving competing grasses and other plants…”  The Creosotebush blooms in the summer, producing inconspicuous yellow solitary blooms.  It produces velvety seeds that snag in animals’ fur or tumble readily in the wind, being carried considerable distances from the parent plant to germinate.  Additionally, the Creosotebush’s lateral roots produce clones that grow in rings around the parent plant, creating immediate botanical brethren with direct connections to tissue that may date back thousands of years, to the end of the Ice Ages.  “…some…clones,” the FEIS Internet site says, “may be the earth’s oldest living organisms.”

The many dozens of Southwestern desert shrubs that dominate or co-dominate the Southwest’s micro-environments have forged a kaleidoscope of botanical alliances.  It is reflected in the diverse community of the animal life, the consumers.

The Consumers

The shrubs – dominant producers that have filled the void left by the depletion of grasses – have assumed a proportionately more prominent role in the desert food chain over time, both for invertebrates (animals without backbones, for instance, insects, worms and spiders) and for vertebrates (animals with backbones, for example, mammals, birds and reptiles).  Many of the shrubs, for instance, mesquites, acacias and saltbushes, also contributed to the food store of the Native Americans, both prehistorically and historically. 

In addition to serving as a cornerstone in the food chain, shrub plant communities have long been a vital habitat for the animal kingdom—a year-round as well as a seasonal stage for shelter, camouflage, burrows, nests and perches.  If it is the cacti, yuccas and agaves that catch our eye in the desert southwest, it is the shrubs that most of the animals would call home.

The annual forbs

by Jay W. Sharp


Part 1 Desert Food chain - Introduction
Part 2 Desert Food chain - The Producers
Part 3 Desert Food chain - The Cacti: A Thorny Feast 
Part 4 Desert Food chain - The Yuccas
Part 5 Desert Food chain - The Agave
Part 6 Desert Food chain - Desert Grasslands
Part 7 Desert Food chain - Desert Shrubs
Part 8 Desert Food chain - The annual forbs
Part 9 Desert Food chain - Mavericks of the Desert Plant
Part 10 Desert Food chain - Outlaw desert plants
Part 11 Desert Food chain - Animals: The Consumers
Part 12 Desest Food chain - The Insects
Part 13 Desest Food chain - The Ugly, the Uglier and the Ugliest

Also see: The Desert Food Chain for the young student




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