How Desert Plants Survive

The Desert Food Chain Part 3

Southwestern desert basin plants are the producers in the desert food chain. They live in one of the most difficult environments in the United States.They face scorching hot air and ground temperatures during the summer season, and unpredictable and limited water even during the rainier seasons. There are only a few rare streams or ponds with dependable water, and there are high rates of evaporation. The soils of the Southwestern deserts contain relatively few nutrients and there are continual threats from predatory plants and animals.

The plants can survive the environmental challenges because they have developed both structural and behavioral strategies for enduring the desert ecosystem. Desert plants are amazing examples of the resourcefulness and adaptability of nature.

Desert plants include cacti, yuccas, agaves, shrubs, grasses and annual forbs (non-grass plants that live for only a season); all plants with a broad span of differing survival strategies.

Cacti like the prickly pear, have leaves that have evolved into spines. They help minimize water loss from evaporation, and screen the plant from the sun.

Cacti

The cacti, symbols of the desert, range from the teacup-size thimble plains cactus to the 60-foot-tall 10-ton saguaro. They are among the most drought-resistant plants on earth. Cacti have leaves that have evolved into spines, which help minimize water loss by evaporation and screen the plant from the sun. Their thorny spines also help protect them from predators.

A cactus stem has fleshy tissue that soaks up water and a waxy coating that helps seal in moisture. As the cactus uses up its store of water during prolonged drought, its stems (the pads of the prickly pear, for example) start to look drawn and shriveled. As it soaks water into its tissues following a rain, its stems regain a more swollen look. Unlike most plants, a cactus opens its pores, called stomata, at night rather than during the day to absorb the carbon dioxide gas it will need for photosynthesis (see Part 2 in this series). This strategy also helps minimize evaporation.

The cactus has a root system that radiates away from the plant, just below the surface of the ground, ready to capture any water from rain or snowfall and conduct it to the plant’s stems. Within hours after a rainfall, the cactus’ roots sprout tiny rootlets to help absorb water even more quickly.

Yucca leaves, like cacti stems, have fleshy tissues and waxy coatings to help store and preserve water.

Yuccas and Agaves

The desert’s yuccas and agaves, related plants with leaves that look like daggers or swords, also rank highly on the list of drought-resistant plants.

Their leaves, like cacti stems, have fleshy tissues and waxy coatings. The leaves of both the yuccas and the agaves are tipped with sharp points, and the leaves of the agaves are further armed with wickedly barbed margins. The leaves of the agaves serve as a storehouse for resources, not only as a hedge against drought, but also as an energy source that the plant will need for its spectacular life-ending bloom.

Like the cacti, the yuccas and agaves open their stomata at night to absorb carbon dioxide for use the following day in photosynthesis.

Some yucca species may have stems, which have specialized tissues that store and conduct water. Agaves have virtually no stem.

Like cacti, yuccas and agaves have shallow radiating root systems to intercept the water from rains and snowfalls. Some yuccas also have deep tap roots that reach for deeper water as well.

Agave (century plant) surrounded by a prickly pear cactus. Both plants are succulents that close their stomata during the day and open them at night.

Shrubs

Shrubs are typically the dominant plants in the Southwestern desert landscape. They have perhaps the widest range of strategies for survival. Many come equipped with small leaves, protective thorns, multiple branches, unpleasant smells and tastes, and extensive root systems.

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Desert shrubs typically have small leaves, protective thorns, multiple branches, unpleasant smells and tastes, and extensive root systems, all adaptations to the arid environment.

Some have small, shiny, waxy leaves that reflect heat and block 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 the broad surfaces toward the sun, minimizing exposure to the intense light. Others shed leaves and even twigs to reduce exposure and resource requirements during prolonged periods of drought.

Many smaller shrubs have a dense network of shallow roots that allow them to compete with the cacti, yuccas and agaves for water. They are sometimes called intensive exploiters. Larger shrubs usually have extensive shallow as well as deep root systems that allow them to compete for both rain water and for deeper water. They may be called extensive exploiters. A velvet mesquite, a thorny native to the Sonoran Desert, sends roots more than 100 feet or more into the earth in its reach for water. A few shrubs, like the heavily branched and pungent-smelling creosote bush for example, have roots that secrete chemicals that are toxic to nearby plants.

In addition to its other adaptations, a typical desert shrub produces a great abundance of hard-coated seeds that help ensure the future of the species.

Given enough moisture, desert grasses grow swifly and produce abundant seeds. They have a dense tangle of shallow roots that compete effectively for rainwater.

Grasses

Compared with shrubs, grasses – which dominated much of the Southwestern landscape before they were heavily overgrazed by domestic livestock – have relatively straightforward adaptations to the desert environment. Given enough water, they grow swiftly, producing seeds that are distributed by animal life and wind. Some species curl their leaves, shrinking away from sunlight and heat. Grasses produce a dense tangle of shallow roots that compete effectively for rainwater. Before the suppression of wildfires, grasses could often benefit from lightening-sparked desert blazes, which did less damage to grasses than to shrubs that would otherwise compete for resources.

Given sufficient rainfall at the right time, the annual forbs race to sprout, develop, flower and produce seeds before drought comes again.

The Annual Forbs

The annual forbs – often called ephemerals, which means short lived – don't need to deal with desert drought. They just wait for the right season and the right rainfall to arrive at the right time – an event that may not occur for years in the desert, with its erratic climate. When the moment does come, however, the annual forbs race to sprout, develop, flower and produce seeds before drought comes again.

During this time, at least one of the forbs, the indian paintbrush, dispatches roots to attack nearby plants’ roots, helping itself to its neighbors’ moisture and nutrients.

An annual forb buys its future with its seeds. 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, or asleep, awaiting just the right cues from nature for germination. Some species even deposit different sizes of seeds into this “savings account,” 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 robust sprouts. In either event, the species reinforces its opportunity for survival.

Basic Strategies

While there are diverse species of plants in the desert, they tend to follow a relatively few basic strategies in their ongoing struggle for survival.

Some, like cacti, yuccas and agaves, withstand the desert environment by economizing their expenditures of moisture. They rely on their waxy coatings, their spongy stem or leaf tissues, their root structures and their nighttime stomata openings to carefully regulate their water use.  Such plants are sometimes called drought endurers.

Other plants, such as shrubs, seek to avoid desert conditions by turning their leaves to diminish full exposure to the sunlight, shedding leaves and twigs to minimize water requirements, and putting down deep tap roots to reach underground water.  These plants may be called drought avoiders. 

Still other plants, including some grasses and the annual forbs, escape the hardship of drought altogether by growing, blooming and seeding rapidly only when conditions are ideal, with adequate moisture, even if it takes years for that moment to arrive.  They deposit their seeds in a seed bank, like a savings account, which they will spend when nature provides all the right cues.  These plants are called drought escapers. 

Over time, the plants – the producers – of the desert have developed various approaches to surviving under the stress of their environment, with the intense heat, drought, unpredictable rainfall patterns, and poor soils. 

Simultaneously, in their diversity, they have influenced the animals – the consumers – in their development of adaptations for survival. 

by Jay W. Sharp

Food Chain Introduction
How Do Green Plants Manufacture Their Own Food?
How Do Desert Plants Survive?
How Do Desert Animals Survive?
The Arithmetic of the Food Chain
The Classification of Desert Plants & Animals

 

 

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