The Plant's Role in the Desert Food Chain - Part 2
In any biologically distinctive region, or biome, on earth, it is the plants – the producers – that capture the energy – the food chain’s “fuel” – from the sun through the process called “photosynthesis.” Plants use the sun’s energy, with water and carbon dioxide, to produce glucose, a basic component of the food chain. They use the glucose to produce carbohydrates, proteins and fats required for reproduction and growth, drawing nourishment from soil nutrients. As producers, plants create organic matter that serves, in effect, as storehouses of solar energy and as a food source for animalsthe consumers.
The community of desert plants includes both “perennials,” i.e., the plants that, by definition, live for more than one growing season, and “annuals,” or, perhaps more accurately, “ephemerals,” i.e., plants that bloom (provided they receive well-timed rain) and die within a single warm or cold season. The perennials (not all authorities agree on the term) include plants such as the columnar cacti, the prickly pears, the chollas, the yuccas, the agaves, the sotols, the ocotillo and the omnipresent creosote bush. The ephemerals include, for a few examples, various poppies, daisies, penstemons, lupines, milkweeds, mallows and the sacred datura.
The desert plant community, as marked by the diversity and distinguishing characteristics of the various species, has been shaped, of course, by our arid and erratic climate and nutrient-starved soils.
Our desert rains source of most of the water that the plants require for photosynthesis total no more than a few inches in a typical year. They vary widely in terms of frequency, intensity, duration and locale, taking the form of what biologists sometimes call “pulses” both a temporal and spatial patchwork of rainfall and the rain waters tend to evaporate quickly, especially during summer heat and winds.
Reflecting seasonal shifts in prevailing winds, the rains usually arrive primarily in the late summer in the Chihuahuan Desert; in the late summer and in the winter in the eastern Sonoran Desert, or the Arizona Upland; in the winter in the western Sonoran Desert, or the Lower Colorado Valley; and in the winter in the Mojave Desert. The highest total precipitation, roughly 10 inches in a typical year, falls in the Arizona Upland. The lowest, often less than two inches, falls in the Mojave Desert’s Death Valley.
The rains frequently come in the form of scattered and sometimes violent thunderstorms, which may inundate one area and scarcely dampen the soil in a neighboring area. After a torrential rainfall, much of the runoff may escape in a furious rush down arroyos and rivers, flowing too swiftly to soak into the soil, doing little to replenish reservoirs of groundwater.
Much of the rain that does fall in the desert evaporates rapidly under intense sunlight, high air temperatures and persistent winds. In a kind of vaporous, tantalizing dance, summer rainfall sometimes evaporates in mid-air, far above the desert floor and its thirsty plants.
The localized and variable rains make weather forecasting a problematic business in the deserts of the Southwest. According to a wistful reference posted on the Phoenix National Weather Service Forecast Office Internet site, the Indians used to say that rain will come within seven days after the locusts begin to sing at night. Perhaps the Weather Service feels that is as good a way as any to forecast the constantly changeable prospects for rain in the desert.
While uncertainty of precipitation and high rates of evaporation often deprive the desert plants of needed water, the high soil temperatures of summer stress the root systems of many plants. With air temperatures throughout the Southwestern deserts frequently climbing to more than 100 degrees Fahrenheit, the soil temperatures in the Lower Colorado Valley and Death Valley, with the hottest summers in the Southwest, may reach 180 to 190 degrees, within 32 to 22 degrees of the boiling temperature of water, far above the ideal range for plant roots.
In addition to scarce water and high temperatures, desert soils, typically rocky with a high content of alkali and salt, raise another environmental obstacle for plants. In a kind of vicious cycle, poorly developed desert soils inhibit productivity of the plants on one hand, and the low productivity inhibits microorganism development of the soil on the other hand. The soils, deprived of organic matter, cannot hold water, which evaporates, drawing dissolved salts to the surface, impoverishing the soil still more. In effect, the desert is a dynamic mosaic of micro-climates, a risky venue for the community of plants, and the problem is compounded because various species have widely varying resource requirements, environmental responses, life histories, forms and structures.
The timing of rainfall and seasonal change holds crucial importance for plant development and growth. For instance, rains falling “...at the wrong time of year or wrong temperature would not bring on germination [sprouting from a seed] or physiological activity [plant growth and development],” according to the article “Resource pulses, species interactions, and diversity maintenance in arid and semi-arid environments,” published online by Peter Chesson and eight other biological scientists in April of 2004. “...an annual must receive the right germination cues to respond to rainfall by germinating, and a dormant perennial must receive the right cues to trigger bud-break or the development of actively-growing tissue.”
For another example, periods of limited rainfall may be sufficient for desert evergreens (plant that do not lose their leaves) to continue with some physiological activity, but scarcity of water may cause some deciduous desert plants to shed their leaves, sometimes inhibiting their ability to capitalize on rain when it does fall. For still another example, the combination of little rainfall in the spring and rapidly climbing soil temperatures through the summer can especially stress “summer active” plants, whose root systems tend to prosper most at around 85 to 90 degrees, according to Matthew W. Fidelibus and David A. Bainbridge in their paper “Microcatchment water harvesting for desertrevegetation,” prepared for the California Department of Transportation in July 1994.
In summary, as Chesson and his associates put it: “Arid environments, in which rain occurs sporadically, leading to pulse-interpulse cycles of resource availability and so pulsed opportunities for plant growth and reproduction, provide serious challenges to plants and other organisms.” How then do the plants of the desert survive? “These challenges,” said Chesson, et al, “have caused the evolution of many different adaptations to take advantage of opportunities as they arise and for persistence through harsh times.”
The Game of Survival
The survival of the community of plants in the desert revolves around resources, diversification and adaptation.
First, plants have found ways to capitalize on the limited water and nutrients. According to James A. MacMahon’s Deserts (one of The Audubon Society Nature Guides), perennials grow in a pattern that mirrors the distribution of resources in the soil. Creosote bushes, classically, grow at a more or less equidistant spacing in those soils where water and nutrients are uniformly distributed; they use the span and depth of their root systems to guard their territory. On the other hand, they may gather in clusters in soils where resources are concentrated. The creosote bushes may also share their territory with other species provided their neighbors have different rooting strategies. They often share space with ephemerals, which grow in the shelter of the perennial’s mothering shade.
Second, the community of desert plants draws strength and viability from the very diversity of its residents; some species will nearly always be physiologically suited to capitalize on a rainfall during any season. The desert plants’ diversity appears to have developed over time as species evolved in different ways, reflecting their individual responses to the desert environment. In fact, as Chesson and his associates said, “Pulsed resource availability offers many opportunities for...differentiation in species’ behaviors, and our review suggests that such differences between species in patterns of resource use, growth, reproduction, and establishment are [universal] in arid ecosystems.”
Third, over time, both the perennials and the ephemerals have developed numerous and often complex strategies and adaptations for surviving in the desert.
Among the perennials, the cacti (called “succulents,” which means “soft and juicy”) serve as examples of plants that MacMahon calls “drought-avoiding water-savers.” The cacti, which include the columnar species, the prickly pears, the chollas, the barrell, the hedgehog, the devil claw, the pincushion and numerous others, have developed highly modified drought-resistant stems and leaves. The stems the fleshy portion of the plants comprise water- and food-storing tissues covered with a waxy skin that seals in the plant’s moisture. The skin is thicker and therefore more protective on the side of the plant with the fullest exposure to the sun’s rays. The leaves modified over time into spines offer minimal exposure to solar radiation, provide insulation for the stems and discourage feeding by some animals.
The cacti have wide-spreading and shallow root systems that conduct water from the soil into the stems quickly, even after a light rainfall. Unlike most other plants, the cacti close their pores, on the surfaces of the stems and leaves, during the heat of the day. This restricts moisture loss, but it also inhibits photosynthesis, so the cacti, like several other desert succulents, have adapted a specialized chemical system called “crassulacean acid metabolism,” or CAM, that allows it to complete the vital process. Because of the adaptations, the cacti rank at the top of any list of the most drought-resistant plants on earth.
In contrast to the drought-avoiding water savers, the “water-spenders” such as the creosote bush and the mesquites follow a different basic strategy for surviving in the desert. Most notably, they send out extensive root systems to draw water that is beyond the reach of many competing plants, and, wastefully, it would seem, they “spend” their water by the process of transpiration, or evaporation through the plant tissues. The creosote bush, perhaps the most successful of the desert shrubs, produces a root system that extends deep and spreads widely. The various mesquites put down especially deep roots, reaching 60 to 80 feet or more below the surface to reach water. Typically, the weight of the water-spenders’ root systems may exceed that of their “shoot” material (that part of the plant above ground) by several times.
Some perennials, for instance, the Ocotillo, follow still another strategy for surviving in the desert. During a dry period, they simply enter a state of dormancy, a kind of botanical “sleep,” halting their growth and shedding their leaves while they wait for rain to come. When the rains do arrive at last, the Ocotillos quickly sprout new, wax-coated leaves and produce festive new red blooms.
Following still another alternative for survival, perennials produce seeds that germinate when rains fall at the right season and right temperature, yielding new generations of the plants to replace those that die.
By contrast, the ephemerals’ central strategy for survival rests, not so much on plant structural adaptations and environmental responses, but rather on prolific seed production. An ephemeral plant will simply shrivel and die when the rains fail during the species’ growing season, but earlier, successful generations have, over the years, cast a reservoir of seeds into the desert soils. Some of those seeds will germinate when the right combination of rain, season and temperature occurs. Others will remain in reserve, ready to germinate on cue should drought or a catastrophe (like a fire, for instance) wipe out a growing generation.
Generally, two broad groups of ephemerals grow in our deserts, a summer group and a winter group. It is primarily summer ephemerals that grow in the Chihuahuan Desert, capitalizing on the late summer rains; summer and winter ephemerals that grow in the Arizona Upland, taking advantage of the summer and winter rainy seasons; and the winter ephemerals that grow in the Lower Colorado Valley and the Mojave, profiting from the winter rains.
According to MacMahon, the summer ephemerals “carry on photosynthesis more effectively at high temperatures and high intensities of light, use water more efficiently, and produce more material than the [winter ephemerals] under the same extreme conditions.” In ways only partially understood by botanists, the summer and winter ephemerals calibrate their seed germination to the seasons, guarding against premature and inopportune growth and development.
“If you examine desert soils closely,” DesertUSA said in an article, “Desert Plant Adaptations,” “you will dispel forever any notion you might have of the desert as a barren environment, for you will likely find dozens of both [ephemeral] and perennial seeds in every handful of desert soil. In the Sonoran Desert [for example], seed densities average between 5000 and 10,000 per square meter.”
Producers and Consumers
The desert’s mosaic of constantly varying micro-climates has apparently given rise a diversity of plants producers with a wide array of strategies for surviving the desert, an effect that has been relayed to the animal populationthe consumers in the food chain. Chesson and his associates said that, “...the fauna of arid environments is diverse too, and pulsed resource availability may also play an important role in this diversity. Granivorous [grain or seed-eating] rodents are particularly well studied in North American deserts..., where many studies show that rodent populations tend to increase after pulses of seed production. The responses of rodent populations to precipitation pulses are similarly diverse to those of plant populations, both seasonally and on longer, multi-year timescales...”
The plants and desert animals interact in other ways as wella cooperative effort to assure that the animals’ desert banquet table remains set. According to MacMahon, “...there are a host of plant-pollinator interactions. The forty-odd species of the genus Yucca, for example are all pollinated by three moth species...
The desert willow can be pollinated by any number of bee species, whose common characteristic seems to be their large size. ...dozens of bees and beetles are known to pollinate cacti... Some visitors are simply nectar thieves.
Vertebrates also pollinate some desert plants. Bats are known to pollinate agaves and saguaros and hummingbirds pollinate red-flowered species such as ocotillo.
Despite the many positive associations that exist between plants and animals, their roles are usually that of food and feeder, or producer and consumer, the featured players in the desert food chain.
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 Desert Food Chain - The Insects
Part 13 Desert Food Chain - The Ugly, the Uglier and the Ugliest
Sources you may find interesting include various articles in DesertUSA’s Internet site; James A. MacMahon’s Deserts, part of the Audubon Society Nature Guide series; Ann and Myron Sutton’s The Life of the Desert, part of the McGraw-Hill Book Company Our Living World of Nature series; Peter Chesson’s and associates’ “Resource pulses, species interactions, and diversity maintenance in arid and semi-arid environments,” Oecologia Internet site; Natt N. Dodge’s Flowers of the Southwest; and Matthew W. Fidelibus’ and David A. Bainbridge’s “Microcatchment water harvesting for desertrevegetation,” a research paper prepared for the California Department of Transportation in July 1994
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