Desert plants have adapted to the extremes of heat and aridity by using
both physical and behavioral mechanisms, much like desert animals.
Plants that have adapted by altering their physical structure are called xerophytes.
Xerophytes, such as cacti, usually have special means of storing and
conserving water. They often have few or no leaves, which reduces
Phreatophytes are plants that have adapted to arid environments by
growing extremely long roots, allowing them to acquire moisture at or near
the water table.
Other desert plants, using behavioral adaptations, have developed a
lifestyle in conformance with the seasons of greatest moisture and/or
coolest temperatures. These type of plants are usually (and inaccurately)
referred to as perennials, plants that live for several years, and annuals,
plants that live for only a season.
Desert perennials often survive by remaining dormant during dry periods of
the year, then springing to life when water becomes available.
Most annual desert plants germinate only after heavy seasonal rain, then
complete their reproductive cycle very quickly. They bloom prodigiously
for a few weeks in the spring, accounting for most of the annual
wildflower explosions of the deserts. Their heat- and drought-resistant
seeds remain dormant in the soil until the next year's annual rains.
The physical and behavioral adaptations of
desert plants are as numerous and innovative as those of desert animals.
Xerophytes, plants that have altered their physical structure to survive
extreme heat and lack of water, are the largest group of such plants
living in the deserts of the American Southwest.
Each of the four southwestern deserts offers habitats in which most
xerophytic plants survive. But each is characterized by specific plants
that seem to thrive there. The Great Basin Desert is noted for vast
rolling stands of Sagebrush and Saltbush, while in the Mojave Desert,
Joshua Trees, Creosote Bush, and Burroweed predominate. The Sonoran Desert
is home to an incredible variety of succulents, including the giant
Saguaro Cactus, as well as shrubs and trees like mesquite, Paloverde, and
Ironwood. The Chihuahuan Desert is noted for mesquite ground cover and
shrubby undergrowth, such as Yucca and Prickly Pear Cactus.
Cactus, xerophytic adaptations of the rose family, are among the most
drought-resistant plants on the planet due to their absence of leaves,
shallow root systems, ability to store water in their stems, spines for
shade and waxy skin to seal in moisture. Cacti originated in the West
Indies and migrated to many parts of the New World, populating the deserts
of the Southwest with hundreds of varieties, such as the Beavertail Cactus
and Jumping Cholla.
Cacti depend on chlorophyll in the outer tissue of their skin and stems to
conduct photosynthesis for the manufacture of food. Spines protect the
plant from animals, shade it from the sun and also collect moisture.
Extensive shallow root systems are usually radial, allowing for the quick
acquisition of large quantities of water when it rains. Because they store
water in the core of both stems and roots, cacti are well-suited to dry
climates and can survive years of drought on the water collected from a
Many other desert trees and shrubs have also adapted by eliminating leaves
-- replacing them with thorns, not spines -- or by greatly reducing leaf
size to eliminate transpiration (loss of water to the air). Such
plants also usually have smooth, green bark on stems and trunks serving to
both produce food and seal in moisture.
Phreatophytes, like the mesquite tree, have
adapted to desert conditions by developing extremely long root systems to
draw water from deep underground near the water table. The mesquite's
roots are considered the longest of any desert plant and have been
recorded as long as 80 feet. Botanists do not agree on the exact
classification of the three mesquite trees: the Honey Mesquite, Screwbean
Mesquite and the Velvet Mesquite, but no one disputes the success of their
adaptation to the desert environment. Mesquites are abundant throughout
all the southwestern deserts.
The Creosote Bush is one of the most successful of all desert species
because it utilizes a combination of many adaptations. Instead of thorns,
it relies for protection on a smell and taste wildlife find unpleasant. It
has tiny leaves that close their stomata (pores) during the day to avoid
water loss and open them at night to absorb moisture. Creosote has an
extensive double root system -- both radial and deep -- to accumulate
water from both surface and ground water.
Some perennials, such as the Ocotillo,
survive by becoming dormant during dry periods, then springing to life
when water becomes available. After rain falls, the Ocotillo quickly grows
a new suit of leaves to photosynthesize food. Flowers bloom within a few
weeks, and when seeds become ripe and fall, the Ocotillo loses its leaves
again and re-enters dormancy. This process may occur as many as five times
a year. The Ocotillo also has a waxy coating on stems which serves to seal
in moisture during periods of dormancy.
Another example of perennials that utilize dormancy as a means of evading
drought are bulbs, members of the lily family. The tops of bulbs dry out
completely and leave no trace of their existence above ground during
dormant periods. They are able to store enough nourishment to survive for
long periods in rocky or alluvial soils. The Desert Lily, also known as
the Ajo, is found at a depth of 18 inches or more. Adequate winter rains
can rouse it to life after years of dormancy.
The term "annuals" implies
blooming yearly, but since this is not always the case, desert annuals are
more accurately referred to as "ephemerals." Many of them can
complete an entire life cycle in a matter of months, some in just weeks.
Contrary to the usual idea that deserts are uniformly hot, dry and
homogeneous in their lack of plant life, they are actually biologically
diverse and comprise a multitude of micro-climates changing from year to
year. Each season's unique precipitation pattern falls on a huge variety
of mini-environments. And each year in each of these tiny eco-niches, a
different medley of plants bloom as different species thrive.
Desert plants must act quickly when heat, moisture and light inform them
it's time to bloom. Ephemerals are the sprinters of the plant world,
sending flower stalks jetting out in a few days. The peak of this bloom
may last for just days or many weeks, depending on the weather and
difference in elevation. The higher one goes, the later blooms come.
Different varieties of plants will be in bloom from day to day, and even
hour to hour, since some open early and others later in the day.
Ephemerals such as the Desert Sand Verbena, Desert Paintbrush and Mojave
Aster usually germinate in the spring following winter rains. 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 5,000 and
10,000 per square meter. The world record is over 200,000 seeds per square
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.