Survival of the Thriftiest
The plants of our desert Southwest have developed a variety of strategies for surviving and even thriving in hard times.
by Jay W. Sharp
Most of the modern native plant species of our Southwestern deserts — one of the more biologically challenging environments in the United States — have managed not only to subsist but to prosper, since the end of the last Ice Age, roughly 10,000 years ago. In fact, according to experts at the University of Nevada-Las Vegas, "Desert scrub communities often contain surprisingly large amounts of plant biomass, and possess remarkable diversity of plant growth forms."
These plants owe their successes as species to basic survival strategies that they have developed during their long histories in our arid climate. Reflecting the resourcefulness of nature and the persistence of life, they have managed to make a home in a land with periodic severe drought, impoverished soils, blistering summer air and ground temperatures, occasional powerful and dusty windstorms, low humidity, unpredictable and spotty rainfall, rare streams or ponds, high evaporation rates, and intense inter-species competition. Through time, the desert plants have endured even in difficult periods like our recent severe drought.
Generally, say botanical scientists, our desert plants have developed diverse and individualistic strategies for coping with desert stresses. Some of our more familiar plants typify these strategies.
The annual forbs, or the short-lived non-woody plants other than the grasses, simply refuse to deal with the desert drought. Rather, these plants — including, for a few examples, various daisies, asters, coneflowers, sunflowers and paintbrushes — have found a way to simply lie in wait for the right season and the right rainfall. Of course, this coincidence of events may not occur for years in the desert, given its erratic climatic events.
Typically, the desert forb uses its seeds as botanical currency to purchase its future in the desert. Scattered widely by wildlife, wind and even occasional flash flood waters, the seeds may lie in the desert soil for a decade or more, dormant, awaiting just the right cues from nature for germination. Some of the forbs may even deposit different sizes of seeds into their "seed-savings accounts," as if anticipating that the large seeds can respond to perfect conditions and produce vigorous sprouts and the small seeds can respond to marginal conditions and produce less water-thirsty sprouts. Either way, the species reinforces its opportunity for survival.
When the moment does come, however, the forb races to sprout, develop, flower and produce abundant seeds before inevitable drought comes again. During this time, at least one of the forbs, the Indian paintbrush, may dispatch roots to attack nearby plants' roots, helping itself to its neighbors' moisture and nutrients.
The ocotillo, a distinctive long-living, non-succulent plant, just takes time off during dry periods. It resumes its growth activity when rains return.
The ocotillo produces brilliant red flower clusters at the end of its stems. The plant can be induced to produce leaves just by wetting its stems.
Sometimes called the "devil's walking stick" or the "coachwhip," it has several long, whip-like, spiny stems that may spray upward for perhaps 20 feet or more from a root crown. Its stems have water-absorbent central tissues and essentially waterproof bark. As described by James A. MacMahon in his book Deserts, the ocotillo has one- to two-inch-long green oval leaves that appear soon after a respectable rain falls and then wither and fall off as the soil dries — a cycle that can be repeated four or five times during the warm season.
As the leaves fall off, they leave behind the plant's rigid, conical-shaped spines. "No other plant family makes spines in this way," according to Arthur C. Gibson in the Mildred E. Mathias Botanical Garden's online newsletter.
The ocotillo has a shallow radiating root system. It produces a brilliant red cluster of flowers at the ends of its stems during the spring, sometimes even when no leaves have appeared. It yields an abundance of small, flat, feathery seeds during the early summer, casting them into the desert winds.
The ocotillo's succulent stems preserve water, and its shallow root system quickly lays claim to new rainfall or snowmelt. The ocotillo's leaves, during their short lives after a rain, turn swiftly to the business of photosynthesis (the use of solar energy to synthesize the organic compounds necessary to sustain life). When the plant sheds its leaves during drought, the plant becomes essentially dormant, minimizing transpiration — that is, the evaporation of the plant's water supply — although its stems still perform photosynthesis after its leaves have fallen, even if "in a feeble manner," as H.A. Mooney and B.R. Strain put it in "Bark Photosynthesis in Ocotillo." With the next warm-season rain, the plant springs back to life, producing a new coat of leaves that quickly resume the business of vigorous photosynthesis.
Only a minuscule percentage of the ocotillo seeds that germinate during the desert's rainy season survive over the next two years. But those that do may yield plants that live for up to two centuries, according to authorities Annette Lamb and Larry Johnson.
Highly resourceful, the desert grasses — which covered much of the Southwestern landscape before they were heavily overgrazed by domestic livestock — have developed several basic strategies for surviving the arid environment, according to Thomas R. Van Devender and Mark A. Dimmit, writing for the Arizona-Sonora Desert Museum.
Like annual forbs, some grasses die back during drought or winter temperatures, then draw on seed banks to produce new plants given the right coincidence of season and rainfall. Other species die back just to their root crowns, from which they swiftly grow new stems, leaves and flowers when rainfall and acceptable temperatures return. Among still other species, new individual plants may either spring from a seed bank or may re-grow year after year from their root crowns, given the right moisture and season. Some of the larger species die back only to living nodes along their stems, then regrow their stems and produce new leaves and flowers when the time is right. Remarkably, some individual plants among the larger species of grass may live for more than a century, say Van Devender and Dimmit.
Given enough moisture, the grasses grow swiftly, producing seeds that are distributed by the animal life and the wind. Some species curl their leaves, shrinking away from the sunlight and heat. The grasses produce a dense tangle of shallow roots that compete effectively for rainwater. Before the suppression of wildfires, the grasses often capitalized on lightning-sparked desert blazes, which did less damage to the grasses than to the shrubs that would otherwise compete for resources.
Collectively, the shrubs, now the dominant plants across much of our desert landscape, follow perhaps the widest range of strategies for survival. They may come equipped with small leaves, protective thorns, multiple branches, disagreeable smells and tastes, and extensive root systems.
Some have small, shiny, waxy leaves that reflect heat and impede evaporation. Others have leaves covered with dense, light-colored hairs to reflect the intense sunlight. Others ooze light-colored salt onto the surfaces of their leaves to reflect the light. Some turn their leaf edges rather than their broad surfaces toward the sun, minimizing exposure to the intense light. Others shed leaves and even twigs — effectively pruning themselves — to reduce exposure and resource requirements during prolonged periods of drought. A few shrubs — for instance, the heavily branched and pungent-smelling creosote bush — may have roots that secrete chemicals toxic to nearby plants.
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