Desert Habitat

March 12, 2002

Tricks of the Trade....How Desert Plants Survive

How do desert plants save water?
Desert plants work hard to make use of what's available. They use the sun's energy to convert carbon dioxide to (CO2) and water (H2O) into sugar, a process called photosynthesis. During this process, small pores (stomata) on a plant's leaves and stems open to absorb CO2 from the air and in return release oxygen (O2). Each time a plant opens its pores, some H2O is lost. This is called transpiration. Replacing this lost H2O is not easy with so little annual moisture; and if the H2O can not be replaced, the desert plants will die. A unique fact of desert plants is that they have acquired special adaptations that help them in reducing H2O loss.

• Smaller, fewer, and deeper pores - Many desert plants have very small, fewer, and deeper pores. With such pores, hot and dry winds are inhibited from blowing directly across the pores and reducing H2O loss.
• Waxy cover - Plants do not only lose H2O through their pores, they also lose it through the cell walls on their leaves. The leaves and stems of many desert plants have a thick covering that is coated with a waxy substance, allowing them to still open and absorb CO2.
• Nocturnal - Unlike most plants that carry out photosynthesis, plants lose a large amount of H2O through transpiration, and if transpiration occurs during daytime hours, high temperatures can cause water to evaporate quickly. If the process can occur at night, less H2O is lost. Often times, desert plants do not open their pores until the sun goes down and temperatures fall.
• Little leaves - Most desert plants have small leaves or no leaves at all. The smaller or fewer leaves a plant has, the less H2O is lost during transpiration since it has less surface area exposed to the sun and wind. For desert plants with small leaves or none at all, the twigs and stems help to carry out photosynthesis.
• Hide and rest - During the hottest part of the day many desert grasses and other plants "roll Up" their leaves (hide and rest) to reduce the amount of surface area exposed to sun and wind. Some plants simply position themselves so they have less exposure to the climatic elements on a hot, sunny day.
• Drop'em in drought - Some desert plants grow leaves during the high moisture period of the year and then shed them when it becomes dry and hot again. Such plants are called drought deciduous. These kinds of plants will carry out photosynthesis only during the moist period.

How do plants get water?

One way desert plants, trees, and shrubs suck up as much water as possible is by growing very deep taproots. Sometimes these roots can get to be more than 100 feet long. The above ground plant parts may remain small for years simply because the plant puts most of its energy into developing its taproot system. Desert plants may have a huge, tangled network of shallow roots that spread out from the plant in all directions. The roots can be as long as the plant is tall, and can quickly absorb water from the slightest rainfall.

Why do plants shrink and swell?

Desert plants can soak up water, store it, and prepare to use it during drought. For example, cacti and many other desert plants store water in their fleshy leaves and stems. Desert plants may also have other adaptations for water storage, such as pleats or folds that will allow the plant to swell with added water when it can. The pleats or folds can almost disappear if the plant soaks up a lot of water; then the plant can shrink, and its pleats or folds can become visible again as drought sets in and the plant makes use of water it has stored. Though many desert plants die to the ground during the hottest part of each year, the water they have stored in underground roots, tubers and bulbs will sustain them until the next moist period.

Why do plants grow hairs and spines? The hairs and spines that grow on desert plants help reduce moisture loss by breaking the effects of the wind. They also help to cast minute shadows on desert plants, which can protect them from the sun. The hairs and spines can even serve to reflect the sun's rays away from plants because of their shininess. Lastly, hairs and spines can help protect plants from hungry animal predators.

Why do plants produce special chemicals?

Scientists believe that desert plants may produce and give off chemicals from their leaves or roots that keep other plants from growing nearby. It is thought that plants do this to reduce competition, especially when water is scarce.

Why do seeds of plants sleep?

Some desert plants cope with the desert's dryness by not coping at all. As a result, during drought they are present only as seeds in the soil. For months, years, or even decades these seeds "sleep" to wait out the dry spell in a dormant state. When the right amount of rain falls and soaks into the soil, they sprout and bloom. When this happens the desert's dry brown landscape can quickly change into colorful fields of wildflowers, herbs, and grasses. Most of these fast-growing desert plants do not last very long. So aside from having seeds that are adapted to drought, they have few or no special adaptations to desert conditions. This is why desert plants of this kind sprout, flower, and leave behind a generation of seeds as quickly as possible. Short-lived desert plants like this are called ephemerals. With little water available to help them grow, dormant ephemerals are covered and protected by natural chemicals called inhibitors. The primary function of inhibitors is to keep seeds from germinating until enough moisture and specific temperatures are present. One the inhibitor has been washed off, the seeds can sprout.