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 and there have been only four documented drop-everything-and-go-see-it displays in southern Arizona: , , , and During that period only the displays of and were widespread throughout both the Sonoran and Mohave deserts.
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. 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. 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. 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. 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.
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. A couple of exceptions are brittlebush when it occurs in pure stands, and exten sive 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. 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-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.
Summer and winter annuals almost never overlap. The dramatic wildflower shows are only a small part of the ecological story of desert annuals. 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. The buckwheat family Polygonaceae is well- represented.
There are more than a score of skeleton weeds Eriogonum spp. Fiddlenecks Amsinckia spp. 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.
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. 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.
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. 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. 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 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.
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. Flowers are very useful for identifying plants and providing aesthetic pleasure for humans, but they have a more vital function—they are the sexual reproductive organs of plants.
Many plants also have methods of asexual vegetative reproduction, which produces offspring that are genetically identical to the parent: root-sprouting limberbush, palo verde, aspen , stolons and rhizomes agaves, strawberries, many grasses , and aerial plantlets some agaves, mother-of-millions, kalanchoe.
All of the progeny of asexual reproduction are clones of their parent plants. A clone is a group of organisms that are genetically identical; in the case of flowering plants each clone originates from a single seed. Horticulturists have developed additional methods of plant cloning that are valuable in perpetuating superior varieties of plants: cutting, grafting, and tissue culture.
In contrast, sexual reproduction combines half the genes from each of two parents, so sexually produced offspring are different from either of their parents and from one another. This variation is the raw material of natural selection which in turn results in evolution. A species that cannot reproduce sexually—there are quite a few among both plants and animals—is at greater risk of extinction if its environment changes, because it cannot adapt to new conditions.
Pollination is the transfer of pollen from an anther onto the stigma of a flower. The pollen then grows a tube that penetrates the style down to the ovary; sperm cells swim down the tube and fertilize the ova. Fertilized ova develop into seeds, which are the sexual propagules of flowering plants. Outcrossing pollination by pollen from another plant is evolutionarily advantageous because the offspring are more variable than those from self- pollination.
But self- pollination is still sexual reproduction which results in different combinations of genes and therefore allows evolutionary change, as vegetative cloning does not. Plants have many adaptations that increase the likelihood of outcrossing.
From this need widespread and complex kinds of mutualism mutually beneficial interactions have evolved between plants and animals.
The pollen-transporting agent is frequently an insect or other flying animal. Flying animals are more mobile than grounded species, and thus more likely to visit widely-separated plants.
In order to get pollinated, a flower must both make its presence known advertise , and provide an incentive a reward for an animal to make repeated visits to flowers of the same species. Two kinds of food are the usual reward. Nectar is a sugar solution that provides energy for flight. Flying requires much more energy than terrestrial locomotion.
Pollen, besides being the male gene-bearer of a flower, is also rich in proteins essential for maintaining animal tissues and for raising young.
In place of nectar some flowers offer oil fat , another energy food. Others provide fragrances that the pollinator gathers to use for its own reproductive advertisement, and a few fascinating species employ deceit and provide no reward see the species account on pipevine for an example. The sugar in nectar and the protein in pollen are expensive to produce, so there is selective pressure to use these resources efficiently.
It is important that animals other than the pollinators do not eat steal the nectar and pollen, and that the pollinators transport pollen to other flowers of the same species and deposit it in the right place. Natural selection has produced specialization: most plants with animal-pollinated flowers attract only a few species of animals which have the right size and behavior to reach the reward and pick up pollen.
The more than million years of coevolution between flowering plants and their pollinators has greatly contributed to the huge number of species in both kingdoms , flowering plants, hummingbirds, and 15, known bees in the world. It also explains why there are so many different shapes and colors of flowers. Flowers can be classified into several pollination syndromes according to their pollinators. Even though there is a wide variety of plant life in the deserts , the plants tend not to grow close together as they would, for instance, in a rain forest.
Desert plants also typically grow close to the ground. Some plants are able to live in the desert because they have evolved special structures to help them store water. Cacti—such as the prickly-pear, saguaro, and barrel cacti—are among the best-known desert plants. Cacti are a type of succulent, or water-storing plant. Instead of leaves, cacti have a thick, waxy cuticle, or outer layer, that protects against water loss.
Cacti are also able to store water in their stems. The spines thorns on a cactus can help collect water, too—while also providing a bit of shade for the growing plant and a degree of protection from thirsty animals. In addition, the stomata pores on the underside of leaves that allow the plant to take in air on a cactus are usually sunken and can close during the day to prevent water loss.
Cacti have shallow roots that are far-reaching enough to quickly take up any available water when it rains. Plants such as the agave and the euphorbia use many of the same or similar mechanisms for capturing and storing water. Other types of plants, such as perennials—plants that grow for several years—also have special adaptations for surviving in the desert.
For instance, sagebrush Artemisia have tiny leaves to cut down on the amount of water the plant loses through transpiration the evaporation of water into the air. Other plants have evolved waxy leaves—the chaparral bush Larrea tridentata is a notable example. This bush has other features that help it flourish in the desert, including an unpleasant smell and taste that discourage hungry or thirsty animals from visiting it.
The stomata on its leaves open only at night, minimizing the moisture lost to the daytime heat. Like cacti, the chaparral bush has widespread shallow roots for catching rainwater; it also has roots that grow deep into the ground, tapping water from the water table.
These features help the chaparral bush survive long periods without precipitation. Another desert plant that relies on deep roots for survival is the mesquite tree Prosopis. Its roots can extend 30 to feet 9 to 30 meters in their search for underground water. Some desert plants survive the harsh environment by entering dormancy —a cessation of growth during dry spells. Such plants come to life suddenly when it rains, often producing flowers and seeds in very short order.
The ocotillo plant Fouquieria splendens sheds its leaves and stops growing during periods of dry weather, but grows leaves, sprouts flowers, and bears seeds in the weeks after a precipitation event. After the seeds fall off, the plant becomes dormant again until the next time it rains. Depending on the weather, the ocotillo can go through this process several times a year.
Desert plants that belong to the lily family enter dormancy during dry times, too, losing their leaves so that only the bulb of the plant remains in the ground, unseen. Some desert plants remain green the entire year. Digging Deeper. Digging Deeper: Depression and the Past. Digging Deeper: Germs and Disease.
Digging Deeper: Milk and Immunity. How Do We See? How Do We Sense Smell? How Do We Sense Taste? How Do We Sense Touch? What is Evolutionary Medicine? What's a Biologist? What's a GMO? What's a Genome? Perennial: living or being present for many years.
Tap root: a long and mostly straight root, like a carrot. Xerophyte: a plant that is able to survive in areas with very little water.
They grow quickly, flower and produce seeds before dying and scattering their progeny to the desert floor. These seeds are extremely hardy. They remain dormant, resisting drought and heat, until the following spring -- sometimes 2 or 3 springs -- when they repeat the cycle, germinating after winter rains to bloom again in the spring. There are hundreds of species of ephemerals that thrive in the deserts of the American Southwest.
If you examine desert soils closely, you will dispel forever any notion you might have of the desert as a barren environment, for you will likely find dozens of both annual and perennial seeds in every handful of desert soil. In the Sonoran Desert, seed densities average between 5, and 10, per square meter. The world record is over , seeds per square meter. This "seed bank" attests to the remarkable reproductive success of desert flora, made possible by their symbiotic relationship with desert fauna -- birds, insects, reptiles and even mammals.
Animals aid in both fertilization and dispersion of seeds, assuring the continued profusion and diversity of plant life throughout the deserts of the Southwest. How to Keep Ice Cold in the Desert. Desert Survival Skills. Get the Best Hotel and Motel Rates. DesertUSA Newsletter -- We send articles on hiking, camping and places to explore, as well as animals, wildflower reports, plant information and much more.
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