Adaptations of desert plants
Adaptations of desert plants are the morphological and behavioral responses of flora to maximize their chances of survival in arid regions. The impression that the desert environment is hostile is strictly an outsider’s viewpoint. Adaptation enables indigenous organisms not merely to survive here, but to thrive. Furthermore, specialized adaptations often result in a requirement for the seasonal drought and heat. For example, the saguaro, well adapted to its subtropical desert habitat, cannot survive in a rainforest or in any other biome (Terrestrial biome), not even a cold desert. In these other places it would rot, freeze, or be shaded out by faster growing plants.
Aridity is the major—and almost the only—environmental factor that defines a desert, and it is this functional water deficit that serves as the primary limitation to which desert organisms must adapt. Desert plants survive the long rainless periods with three main adaptive strategies: succulence, drought tolerance, and drought evasion. Each of these is a different but effective suite of adaptations for prospering under conditions that would kill plants from other regions. Plants evincing adaptation to arid conditions are termed xerophytes.
Contents
Succulence
As a group, succulents are the most picturesque desert plants. They capture our attention because they look nothing like the more familiar plants of the temperate zone where most people live. Their vernacular names suggest how they command our attention: elephant tree, boojum, jumping cholla, creeping devil, and shindagger. Spanish names translate into equally colorful terms such as dragon’s blood, child-killer, and old man’s head. Even some scientific names are inspired by the plant's characteristics: Ferocactus (as in ferocious), Opuntia molesta (the molesting-spined cactus), O. invicta (the invincible point), and Agave jaiboli (as in a highball cocktail, because liquor is made from it).
Succulent plants store water in fleshy leaves, stems or roots in compounds or cells from which it is not easily lost. All cacti are succulents, as are such non-cactus desert dwellers as agaves, aloes, elephant trees and many euphorbias. Several other adaptations are essential for the water-storing habit to be effective.
Getting water
Succulents must be able to absorb large quantities of water in short periods, and they must do so under unfavorable conditions. Because roots take up water by passive diffusion, succulents can absorb water only from soil that is wetter than their own moist interiors. Desert soils seldom get this wet and don’t retain surplus moisture for long. Desert rains are often light and brief, barely wetting the top few inches (centimeters) of soil, which may dry out after just a day or two of summer heat. To cope with these conditions, nearly all succulents have extensive, shallow root systems. A giant saguaro’s root system lies immediately beneath the soil surface and radiates as far as the plant is tall. The roots of a two-foot-tall cholla in an extremely arid site may be nine meters long. Most succulents, in fact, rarely have roots more than ten centimeters below the surface, and the water-absorbing feeder roots are typically within the upper one half inch (1.3 cm). Agaves are an exception in lacking extensive root systems; most of the roots do not extend much beyond the spread of the leaf rosette. Instead, the leaves of these plants channel rain to the plant's base.
Conserving water
A succulent must be able to guard its water hoard in a desiccating environment and use it as efficiently as possible. The stems and leaves of most species have waxy cuticles that render them nearly waterproof when the stomates are closed. Water is further conserved by reduced surface areas; most succulents have few leaves (agaves), no leaves (most cacti), or leaves that are deciduous in dry seasons (elephant tree Spp., boojums Columnaris). The water is also bound in extracellular mucilages and inulins—compounds that hold tightly onto the water.
Many succulents possess a water-efficient variant of photosynthesis called CAM, an acronym for Crassulacean Acid Metabolism. The first word refers to the stonecrop family (Crassulaceae) in which the phenomenon was first discovered. (Dudleya is in this family, as are hen-and-chickens and jade plant.) CAM plants open their stomates for gas exchange at night and store carbon dioxide in the form of an organic acid. During the day the stomates are closed and the plants are nearly completely sealed against water loss; photosynthesis is conducted using the stored carbon dioxide. At night the temperatures are lower and humidity higher than during the day, so less water is lost through transpiration. Plants using CAM lose about one-tenth as much water per unit of carbohydrate synthesized as do those using standard C3 photosynthesis. But there is a trade-off: the overall rate of photosynthesis is slower, so CAM plants grow more slowly than most C3 plants. (An additional limitation is the reduced photosynthetic surface area of most succulents compared with “ordinary” plants.)
The equilibrium between gaseous carbon dioxide and the organic acid is dependent on temperature. Acid formation (carbon dioxide storage) is favored at cool temperatures; higher temperatures stimulate release of carbon dioxide from the acid. Thus CAM works most efficiently in climates that have a large daily temperature range, such as arid lands. Cool nights allow much carbon dioxide to be stored as acid, and the warm days cause most of the carbon dioxide to be released for photosynthesis. (A note of interest: A plant in CAM mode will store enough acid to impart a sour taste in early morning; the flavor becomes bland by afternoon when the acid is used up. But don’t taste indiscriminately—many succulents are poisonous!)
Many succulents possess CAM, as do semisucculents such as some yuccas, epiphytic (growing on trees or rocks) orchids, and xerophytic (arid-adapted) bromeliads. Exceptions are stem succulents with deciduous, non-succulent leaves, such as elephant trees ([spp.]), limberbushes (Jatropha spp.), and desert roses (Adenium spp.). Succulents from hot, humid climates that lack substantial daily temperature fluctuations also usually do not use CAM. Some succulents, such as Agave deserti, can switch from CAM to C3 photosynthesis when water is abundant, allowing faster growth. Over five percent of all plant species spread among thirty or more plant families are known to use CAM.
Another crucial attribute of CAM plants is their idling metabolism during droughts. When CAM plants become water-stressed, the stomates remain closed both day and night and the fine (water-permeable) roots are sloughed off. The plant’s stored water is essentially sealed inside and gas exchange greatly decreases. However, a low level of respiration (oxidation of carbohydrate into water, carbon dioxide and energy) is carried out within the still-moist tissues. The carbon dioxide released by respiration is recycled into the photosynthetic pathway to make more carbohydrate, and the oxygen released by photosynthesis is recycled for respiration. Thus the plant never goes completely dormant but is metabolizing slowly—idling. (This sounds like perpetual motion, but it isn’t. The recycling isn’t 100 percent efficient, so the plant will eventually exhaust its resources.) Just as an idling engine can rev up to full speed more quickly than a cold one, an idling CAM plant can resume full growth in twenty-four to forty-eight hours after a rain. Agaves can sprout visible new roots just five hours after a rain, whereas it may take a couple of weeks for a dormant nonsucculent shrub to resume full metabolic activity. Therefore, succulents can take rapid and maximum advantage of the soil moisture from a summer rain before it quickly evaporates. The combination of shallow roots and the CAM-idling which allows rapid response enables succulents to benefit from rain even in amounts less than G inch (6 mm).
Protection
Stored water in an arid environment requires protection from thirsty animals. Most succulent plants are spiny, bitter, or toxic, and often all three. Some unarmed, nontoxic species are restricted to inaccessible locations. Smooth prickly pear (Opuntia phaeacantha var. laevis) and live-forever (Dudleya spp.) grow on vertical cliffs or within the canopies of armored plants. Still others rely on camouflage; Arizona night-blooming cereus (Peniocereus greggii) closely resembles the dry stems of the shrubs in which it grows.
These adaptations are all deterrents that are never completely effective. Evolution is a continuous process in which some animals develop new inheritable behaviors to avoid spines or new metabolic pathways to neutralize the toxins of certain species. In response the plants are continually improving their defenses. For example, packrats can handle even the spiniest chollas and rarely get stuck. They also eat prickly pear for water and manage to excrete the oxalates which could clog the kidneys of some other animals. Toxin-tolerant insects often incorporate their host plant’s toxins into their own tissues for protection against their predators.
Drought Tolerance
Drought-tolerant plants often appear to be dead or dying during the dry seasons. They’re just bundles of dry sticks with brown or absent foliage, reinforcing the myth that desert organisms are engaged in a perpetual struggle for survival. They’re simply waiting for rain in their own way, and are usually not suffering or dying any more than a napping dog is near death .
Drought tolerance or drought dormancy refers to desert plants’ ability to withstand desiccation. A tomato plant will wilt and die within days after its soil dries out. But many nonsucculent desert plants survive months or even years with no rain. During the dry season the stems of brittlebush and bursage are so dehydrated that they can be used as kindling wood, yet they are alive. Drought-tolerant plants often shed leaves during dry periods and enter a deep dormancy analogous to torpor (a drastic lowering of metabolism) in animals. Dropping leaves reduces the surface area of the plant and thus reduces transpiration. Some plants that usually retain their leaves through droughts have resinous or waxy coatings that retard water loss (creosote bush, for example).
The roots of desert shrubs and trees are more extensive than are those of plants of the same size in wetter climates. They extend laterally two to three times the diameter of the canopy. Most also exploit the soil at greater depths than the roots of succulents. The large expanses of exposed ground between plants in deserts are usually not empty. Dig a hole almost anywhere except in active sand dunes or the most barren desert pavement and you are likely to find roots.
Rooting depth controls opportunities for growth cycles. In contrast to the succulents’ shallow-rooted, rapid-response strategy, a substantial rain is required to wet the deeper root zone of shrubs and trees. A half-inch is the minimum for even the smaller shrubs—more for larger, deeper-rooted plants. It takes a couple of weeks for dormant shrubs such as brittlebush (Encelia farinosa) and creosote bush (Larrea tridentata) to produce new roots and leaves and resume full metabolic activity after a soaking rain. The tradeoff between this strategy and that of succulents is that once the deeper soil is wetted, it stays moist much longer than the surface layer; the deeper moisture sustains growth of shrubs and trees for several weeks.
Mesquite trees (Prosopis spp.) are renowned for having extremely deep roots, the champion reaching nearly 200 feet. But these riparian specimens are not drought-tolerating trees—their roots are in the water table. Most large floodplain mesquites die if the water table drops below forty feet, and mesquites growing away from waterways remain short and shrubby. No desert plant is known to use very deep roots as a primary strategy for survival. In fact, the root systems of most trees —including mesquites—are mostly confined to the upper three feet of soil. Few rains penetrate deeper than this, and at greater depths there is little oxygen to support root respiration.
In contrast to succulents that can take up water only from nearly saturated soil, drought tolerant plants can absorb water from much drier soil. A creosote bush can obtain water from soil that feels dust-dry to the touch. Similarly these plants can continue to photosynthesize with low leaf-moisture contents that would be fatal to most plants.
Some plants in this adaptive group are notoriously difficult to cultivate, especially in containers. It seems paradoxical that desert ferns and creosote bushes, among the most drought-tolerant of desert plants, can be kept alive in containers only if they are never allowed to dry out. The reason is that these plants can survive drought only if they dry out slowly and have time to make gradual physiological adjustments. If a potted plant misses a watering, the small soil volume dries out too rapidly to allow the plant to prepare for dormancy, so it dies. Researchers showed that some spike mosses (Selaginella spp.) must dehydrate over a five to seven day period. If they dry more rapidly they lack time to adjust, and if drying takes longer than a week they exhaust their energy reserves and starve to death. (Selaginella lepidophylla from the Chihuahuan Desert is widely sold as a novelty under the name “resurrection fern”. Rehydration and resumption of active life takes only a few hours.)
Drought Evasion
Interstate 40 from Barstow to Needles, California traverses some of the emptiest land in the West. It dashes as straight as it can through 130 miles (200 km) of dry valleys that are almost devoid of human settlements. The vegetation is simple, mostly widely-scattered creosote bushes. It’s difficult to tell if you’re driving through the Mojave or Sonoran desert. The small, rocky mountain ranges interrupting the valleys beckon to true desert lovers, but the drive is just plain bleak to most people. The exits on this freeway average ten miles (16 km) apart and connect to two-lane roads that shoot straight over the distant horizon with no visible destinations. You rarely see a vehicle on any of them.
Frequent travelers on this freeway become accustomed to its monotony until they think they know what to expect. The creosote bush may turn green if there’s been a rain; ocotillo always flowers in April; most of the time it’s just brown gravel and brown bushes. Then one spring travelers were astonished to discover the ground between the bushes literally carpeted with flowers. It happened in March 1998, when for three weeks the freeway bisected a nearly unbroken blanket of desert sunflowers forty miles long and ten miles wide. At every exit-to-nowhere several cars and trucks were pulled off and people wandered through the two-foot-deep sea of yellow. Those with long memories may have recalled that the same thing happened in 1978. Perhaps they wondered where these flowers came from, and where they were during the intervening twenty years.
Those desert sunflowers (Geraea canescens) were annual wildflowers, plants that escape unfavorable conditions by “not existing” during such periods. Annuals complete their life cycles during brief wet seasons, then die after channeling all of their life energy into producing seeds. Seeds are dormant propagules with almost no metabolism and great resistance to environmental extremes. (A propagule is any part of a plant that can separate from the parent and grow into a new plant, for example, a seed, an agave aerial plantlet, a cholla joint.) Seeds wait out adverse environmental conditions, sometimes for decades, and will germinate and grow only when specific requirements are met.
Wildflower spectacles like the one described above are rare events. Mass germination and prolific growth depend on rains that are both earlier and more plentiful than normal. The dazzling displays featured in photographic journals and on postcards occur about once a decade in a given place. In the six decades between 1940 and 1998 there have been only four documented drop-everything-and-go-see-it displays in southern Arizona: 1941, 1978, 1979, and 1998. During that period only the displays of 1978 and 1998 were widespread throughout both the Sonoran and Mohave deserts.
Desert annuals in the Sonoran Desert can be divided into three groups, based on time of germination and flowering. Winter-spring species are by far the most numerous. The showy wildflowers that attract human attention will germinate only during a narrow window of opportunity in the fall or winter, after summer heat has waned and before winter cold arrives. In most of the Sonoran Desert this temperature window seems to occur between early October and early December for most species. During this window there must be a soaking rain of at least one inch (2.5 cm) to induce mass germination. This combination of requirements is survival insurance: an inch of rain in the mild weather of fall will provide enough soil moisture that the resulting seedlings will probably mature and produce seeds even if almost no more rain falls in that season. (Remember that one of the characteristics of deserts is low and undependable rainfall.) If the subsequent rainfall is sparse, the plants remain small and may produce only a single flower and a few seeds, but this is enough to ensure a future generation. There is still further insurance: even under the best conditions not all of the seeds in the soil will germinate; some remain dormant. For example, a percentage of any year’s crop of desert lupine seeds will not germinate until they are ten years old. The mechanisms that regulate this delayed germination are not well understood.
The seedlings produce rosettes of leaves during the mild fall weather, grow more slowly through the winter (staying warm in the daytime by remaining flat against the ground), and bolt into flower in the spring. Since the plants are inconspicuous until they begin the spring bolt, many people mistakenly think that spring rains produce desert wildflower displays.
There is a smaller group of annual species that grow only in response to summer rains. Annual devil’s claw (Proboscidea parviflora) and Arizona poppy (Kallstroemia grandiflora) are among the few showy ones.
A third group consists of a few opportunistic species which will germinate in response to rain at almost any season. Most of these lack showy flowers and are known only to botanists, but desert marigold (Baileya multiradiata) is a conspicuous exception; it is actually not an annual, but rather a short-lived perennial in most of its range. A few species of buckwheats (Eriogonum) germinate in fall or winter and flower the following summer.
The annual habit is a very successful strategy for warm-arid climates. There are no annual plants in the polar regions or the wet tropics. In the polar zones the growing season is too short to complete a life cycle. In both habitats the intense competition for suitable growing sites favors longevity. (Once you’ve got it, you should hang onto it.) Annuals become common only in communities that have dry seasons, where the perennials are widely spaced because they must command a large soil area to survive the drier years. In the occasional wetter years, both open space and moisture are available to be exploited by plants that can do so rapidly. The more arid the habitat, the greater the proportion of annual species in North America. (The percentage decreases in the extremely arid parts of the Saharan-Arabian region.) Half of the Sonoran Desert flora are comprised of annual species. In the driest habitats, such as the sandy flats near Yuma, Arizona, up to ninety percent of the plants are annuals.
Winter annuals provide most of the color for our famous wildflower shows. Woody perennials and succulents can be individually beautiful, but their adaptive strategies require them to be widely-spaced, so they usually don’t create masses of color. A couple of exceptions are brittlebush when it occurs in pure stands, and extensive woodlands of foothill palo verde (Cercidium microphyllum). The most common of the showy winter annuals that contribute to these displays in southern Arizona are Mexican gold poppy (Eschscholtzia mexicana), lupine (Lupinus sparsiflorus), and owl-clover (Castilleja exserta, formerly Orthocarpus purpurascens).
One of the contributing factors to the great number of annual species is niche separation. (A niche is an organism’s ecological role; for example, sand verbena is a butterfly-pollinated winter annual of sandy soils.) Most species have definite preferences for particular soil textures, and perhaps soil chemistry as well. For example, in the Pinacate region of northwestern Sonora there are places where gravels of volcanic cinder are dissected by drainage channels or wind deposits of fine silt. In wet years Nama demissum (purple mat) grows abundantly on the gravel and the related Nama hispidum (sand bells) on the silt. I have seen the two species within inches of each other where these soil types meet, but not one plant of either species could be found on the other soil. There are specialists in loose sand such as dune evening primrose (Oenothera deltoides) and sand verbena (Abronia villosa), and others are restricted to rocky soils, such as most caterpillar weeds (Phacelia spp.). This phenomenon of occupying different physical locations is spatial niche separation.
Another diversity (Species diversity)-promoting phenomenon is temporal niche separation: the mix of species at the same location changes from year to year. Seeds of the various species have different germination requirements. The time of the season (which influences temperature) and quantity of the first germination-triggering rain determines which species will dominate, or even be present at all in that year. Of the three most common annuals of southern Arizona listed above, any one may occur in a nearly pure stand on a given hillside in different years, and occasionally all three are nearly equally abundant. This interpretation of the cause of these year-to-year variations is a hypothesis based on decades of empirical observation. Much more research is needed to discover the ecological requirements of most species of desert annuals. And of course the Sonoran Desert’s two rainy seasons provide two major temporal niches. Summer and winter annuals almost never overlap.
The dramatic wildflower shows are only a small part of the ecological story of desert annuals. For each conspicuous species there are dozens of others that either have less colorful flowers or don’t grow in large numbers. Every time the desert has a wet fall or winter it will turn green with annuals, but it will not always be ablaze with other colors. One of the most common winter annuals is desert plantain (Plantago insularis). It usually grows only a few inches tall and bears spikes of tiny greenish flowers, but billions of plants cover many square miles in good years. The tiny seeds are covered with a soluble fiber which forms a sticky mucilage when wet by rain; this aids germination by retaining water around the seed and sticking it to the ground. A related species from India is the commercial source of psyllium fiber (Metamucil® for example). The buckwheat family (Polygonaceae) is well-represented. There are more than a score of skeleton weeds (Eriogonum spp.) and half as many spiny buckwheats (Chorizanthe spp.), most of which go unnoticed except by botanists (see species accounts). Fiddlenecks (Amsinckia spp., Boraginaceae) may grow in solid masses over many acres, but the tiny yellow flowers don’t significantly modify the dominant green of the foliage. These more modest species produce more biomass than the showy wildflowers in most years, and thus form the foundation of a great food pyramid.
Some perennials also evade drought much as annuals do, by having underground parts that send up stems, leaves, and flowers only during wet years. Coyote gourd (Cucurbita digitata) and perennial devil’s claw (Proboscidea althaeifolia) have fleshy roots that remain dormant in dry years. Desert larkspur (Delphinium parryi) is a perennial that has woody rootstocks but also sprouts only in wetter years. Desert mariposa (Calochortus kennedyi) and desert lily (Hesperocallis undulata) have bulbs that may remain dormant for several years until a deep soaking rain awakens them.
Our desert wildflower displays are in jeopardy from invasive exotic plants. Species such as Russian thistle (Salsola tragus, also called S. kali), mustards (especially Brassica tournefortii), filaree (Erodium cicutarium), and Lehmann’s lovegrass (Eragrostis lehmannii) are more aggressive than most of the native annuals and are crowding them out in many areas where they have become established. Some are still increasing their geographic ranges with every wet winter. Disturbed sites such as sand dunes, washes (naturally disturbed by wind and water, respectively), roadsides, and livestock-grazed lands are particularly vulnerable to invasion by these aliens.
Combined Drought Adaptations
These three basic drought-coping strategies—succulence, drought tolerance, and drought avoidance—are not exclusive categories. Ocotillo behaves as if it were a CAM-succulent, drought deciduous shrub, but it is neither CAM nor succulent (see details in the species accounts). The genus Portulaca contains species that are succulent annuals. The seeds may wait for a wet spell to germinate, but the resulting plants can tolerate a moderate drought. The semisucculent yuccas have some water storage capacity, but rely on deep roots to obtain most of their water. Mesquite trees are often phreatophytes (plants with their roots in the water table), but some species can also grow as stunted shrubs on drier sites where ground water is beyond their reach.
Adaptations to Other Desert Conditions
Water scarcity is the most important—but not the only—environmental challenge to desert organisms. The aridity allows the sun to shine unfiltered through the clear atmosphere continuously from sunrise to sunset. This intense solar radiation produces very high summer temperatures which are lethal to nonadapted plants. At night much of the accumulated heat radiates through the same clear atmosphere and the temperature drops dramatically. Daily fluctuations of 40°F (22°C) are not uncommon when the humidity is very low.
Microphylly (the trait of having small leaves) is primarily an adaptation to avoid overheating; it also reduces water loss. A broader surface has a deeper boundary layer of stagnant air at its surface, which impedes convective heat exchange. A leaf up to 1/2 inch (10 mm) across can stay below the lethal tissue temperature of about 115°F (46°C) on a calm day with its stomates closed. A larger leaf requires transpiration through open stomates for evaporative cooling. Since the hottest time of year is also the driest, water is not available for transpiration. Non-succulent large-leafed plants in the desert environment would overheat and be killed. Desert gardeners know that tomatoes will burn in full desert sun even if well watered; their leaves are just too big to stay cool. Desert plants that do have large leaves produce them only during the cool or rainy season or else live in shaded microhabitats. There are a few mysterious exceptions, such as jimson weed (Datura wrightii) and desert milkweed (Asclepias erosa). Perhaps their large tuberous roots provide enough water for transpiration even when the soil is dry.
Leaf or stem color, orientation, and self-shading are still more ways to adapt to intense light and heat. Desert foliage comes in many shades, but rarely in typical leaf-green. More often leaves are gray-green, blue-green, gray, or even white. The light color is usually due to a dense covering of trichomes (hairlike scales), but is sometimes from a waxy secretion on the leaf or stem surface. Lighter colors reflect more light (= heat) and thus remain cooler than dark green leaves. Brittlebush and white bursage leaves show no green through their trichomes during the dry season, while desert agave (Agave deserti) is light gray due to its thick, waxy cuticle. Other plants have leaves or stems with vertical orientations; two common examples are jojoba and prickly pear cactus. This orientation results in the photosynthetic surface facing the sun most directly in morning and late afternoon. Photosynthesis is more efficient during these cooler times of day. Prickly pear pads will burn in summer if their flat surfaces face upward. Some cacti create their own shade with a dense armament of spines; teddy bear cholla (Opuntia bigelovii) is one of the most striking examples.
Further Reading
- Plant Ecology of the Sonoran Desert Region, Arizona-Sonora Desert Museum.
- Desert Plant Survival, Desert USA.
- Phillips, S.J. and P.W. Comus (eds.) 2000. A Natural History of the Sonoran Desert. Arizona-Sonora Desert Museum Press, Tucson, and University of California Press, Berkeley.
| Disclaimer: This article contains information that was originally published by the Arizona-Sonora Desert Museum. Topic editors and authors for the Encyclopedia of Earth have edited its content and added new information. The use of information from the Arizona-Sonora Desert Museum should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content. |
