Outlaw Desert Plants
From homegrown outlaws like mesquite and creosote bush to foreigners like salt cedar, invasive species are reshaping the desert.
by Jay W. Sharp
In the best of times, drought, intense heat, variable weather and organically impoverished soils conspire to make desert food chains a game of chance, a stressful venue in which only the most biologically nimble and resourceful of the plants and animals can adapt and survive. In the worst of times, drought, intense heat, variable weather and organically impoverished soils, in collusion with "humanized" desert landscapes, changed wildfire patterns and aggressive native and non-native plant and animal species, can completely reshuffle desert food chains, sometimes inflicting severe damage on interconnecting links. But for invasive plants — botanical outlaws that have spread across the desert landscape like the Mongol hordes of Ghengis Khan — the worst of times has been the best of times.
Before the Invasion
Before settlers with European roots came with their vast livestock herds and their iron and steel plows and axes to colonize the Southwest, a "patchy vegetation of grass, shrubby trees, succulents [e.g., the cacti], rosette plants [e.g., the yuccas], and subshrubs — a distinctive species assemblage unlike that of any other North American landscape," grew in the desert basins, according to Biosphere 2 plant ecologist Tony L. Burgess.
Shrubs such as the four-wing saltbush, the catclaws, various sumacs and the desert willow grew as thickets along arroyos. Gallery forests of various cottonwood, willow and mesquite species grew in the floodplains of the deserts' meandering permanent and intermittent streams, especially those connected to the Rio Grande and Colorado River systems. The plant communities, particularly in the basins, drew much of their character and relative plant abundances from periodic fires ignited by the dry lightning storms of the deserts' monsoon seasons. The fast-burning but also fast-growing grasses recovered quickly, maintaining a dominant position. The shrubby trees, succulents, rosette plants and subshrubs grew back more slowly, restrained to the more marginal positions.
The desert grass and shrublands and the drainages supported a community of vertebrate animals that included mammals such as the bison (up to late prehistoric times), elk, mule deer, pronghorns, grizzly bear, black bear, coyote, the foxes, the big cats and many rodents. There were also numerous bird species, ranging from hummingbirds to eagles; reptiles including various lizards and turtles, a dozen different rattlers, a coral snake, and numerous non-poisonous snakes; and amphibians including frogs, toads and salamanders. The deserts' vegetation also sustained an abundant and diverse collection of invertebrate animals — for instance, protozoans (single-celled animals), spiders, centipedes, millipedes, scorpions, vinegaroons, beetles, butterflies and moths, crickets, grasshoppers, flies, ants, bees and wasps.
The Landscape Altered
With the coming of the European colonists, especially those who migrated into the deserts from the eastern half of the United States after the Mexican-American War (1846-1848), the desert grasses begin to give way before great herds of livestock. "By the late 1800s and continuing throughout the 1900s," wrote Matthew L. Brooks and David A. Pyke in their paper, "Invasive Plants and Fire in the Deserts of North America," "all desert ecosystems experienced unsustainable levels of inappropriate seasons of livestock grazing." The grasses could withstand wildfire but not total denudation.
Trees along drainage areas disappeared from the food chain and reappeared as building timbers and firewood. Desert grass and shrublands and riverine thickets and forests became fractured, isolated like islands, or vanished altogether in making way for fields and development. Some species, especially the voracious and almost innumerable prairie dogs, that competed with commercially valuable animals for plant foods became government statistics expressed in terms of body counts. The deserts' plant community, with depletion of natural fuel and the misguided fire-prevention efforts of the human community, experienced a substantial change in the frequency and patterns of wildfires.
In the newly barren, cleared and fragmented desert landscape — now forced open for botanical colonization, virtually free of prairie dogs, encouraged by altered wildfire patterns — the aggressive home-grown and newly introduced plants saw unprecedented opportunities. They seized the advantage. Well-adapted native shrubs, long held at bay by the resilient grasses and the wildfires, quickly extended their reach across the desert. Introduced grasses, weedy plants and trees swiftly conquered the desert basins and the drainages vacated by the natives.
The Homegrown Invasion
Among the most prominent of the native plants that have slipped their traditional botanical bonds and mounted a territorial conquest are the mesquites and the creosote bush, both superbly adapted to the desert environment. Together, they are the Butch Cassidy and Sundance Kid of the Southwest ecosystem.
The mesquites include the western honey mesquite, centered in the Chihuahuan Desert; the velvet mesquite, centered in the northern Sonoran Desert; and the screwbean mesquite, centered in the northern Sonoran and the Mojave Deserts. All have small compound leaves that minimize transpiration (water evaporation). They come armed with thorny stems that help protect the plant from large browsing animals. They have far-reaching root systems that spread both laterally and deeply in a resolute reach for water. (In fact, the velvet mesquite may send its roots 150 feet or more downward to reach a water table.) They have high resistance to insects and disease. They produce nutritious and palatable seeds that are consumed by the larger grazing and browsing wildlife and domestic livestock, which then serve as agents for distributing the plant across the landscape.
The creosote bush may be even more ideally adapted to the desert — indeed, the only plant that may survive and reproduce in the harshest desert conditions. It has small leaves with a resin coating that reflects hot desert sunlight and restrains transpiration. Further, the plant turns the edges rather than the flat surfaces of its leaves toward the sun, further minimizing exposure to the solar heating. Its root system, according to Jack C. Schultz and Ted Floyd in the American Museum of Natural History's website, may extend laterally across 400 to 500 square feet and downward for some 15 feet. The creosote bush seizes tyrannical control of available soil nutrients. Its roots may even contain compounds toxic to neighboring plants.
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