Desert vegetation often appears different than plants that grow in other types of environment or biomes. Desert plants grow in one of the harshest environments on Earth, and therefore benefit from special adaptations that help them to survive. These adaptation enable desert plants not only survive, but to thrive in hot and dry desert conditions. Such adaptations of desert plants are described below.
Drought Avoidance Through a Short Life Cycle
Some plants avoid dry conditions by completing their life cycle before desert conditions intensify. These plants usually mature in a single season and then die, but produce seeds that later blossom into new plants. For example, in the Sonoran Desert of North America, 90% of plant species are annuals, and many germinate during the short fall season, when a small amount of rainfall is required for germination. In some cases, not all seeds germinate at the same time, but remain dormant and germinate the following year or even years later. Plants that germinate in the fall grow slowly through the winter and flower in the spring, after which they die before the scathing summer begins. The plant life cycle continues through the seeds produced.
Adaptations to Avoid Animals
Since desert plants are usually rare and have sparse populations, it is important for them to protect themselves against animals or other predators. As such, these plants have several adaptations that prevent animals from approaching them. Hunger and thirst draw animals to plants, but many desert plants have spines and thorns, such as the barrel cactus, that can harm an animal that attempts to eat it. Many of these plants are also toxic, such as the desert thorn-apple, and some are both spiny and toxic. Certain plants also use camouflage as a means to avoid being eaten by animals, such as the Arizona night-blooming cereus.
Drought Avoidance By CAM Photosynthesis
Plants usually absorb carbon dioxide during the day through stomata in their leaves to perform photosynthesis. However, openings of the stomata also lead to the loss of valuable water through evapotranspiration. Desert plants cannot afford to lose water, and therefore some plants perform Crassulacean acid metabolism (CAM) photosynthesis for carbon fixation. In CAM photosynthesis, stomata remain closed during the day but open at night to absorb carbon dioxide, which is then stored in the vacuoles as malate. During the day, malate is transported to chloroplasts, where its reconversion to carbon dioxide allows the remaining steps of photosynthesis to occur. Yuccas, xerophytic bromeliads, and epiphytic orchids are examples of plant species that perform CAM photosynthesis.
Leaf Adaptations in Desert Plants
Desert plants usually have leaves that are adapted to hot and arid conditions.
Size and Number of Leaves
Desert plants have smaller leaves, seasonal leaves or no leaves at all. Species with small leaves, such as the little leaf palo verde tree or Parkinsonia microphylla, have less surface area on leaves and therefore lose less water through evapotranspiration. Plants like acacia and ocotillo, which are summer deciduous, drop their leaves during the hot season. As soon as weather conditions improve, these plants re-foliate. Additionally, plants such as cacti have spines or thorns instead of leaves, and photosynthesis occurs in stems or bark. Succulents like agave have fewer leaves that help them survive in dry environments.
Since dark colors absorb more heat, some plants have light-colored leaves. Such leaves reflect light and therefore lose less water from transpiration. For example, the leaves of sagebrush (Artemisia tridentata) are light green in color.
Leaves with Specialized Stomata
Some plants have a limited number of stomata, while others have stomata that close during the day. Such adaptations allow plants to reduce water loss.
Leaves with Waxy Surfaces
Many desert plants have leaves covered in waxes or special oils that reduce transpiration. An example of such a plant is the creosote bush (Larrea tridentata).
Some plants, such as the desert ironwood (Olneya tesota), have leaves with small hairs. These hairs reflect sunlight and block wind movement, both of which reduce evapotranspiration from the leaves.
Narrow and Pointed Leaves
Plants like the Joshua tree have narrow, pointed, and sharp leaves whose reduced surface area protects the plant against water loss.
Cacti and other succulents tend to have thick leaves with a large number of vacuoles that store wate. Such plants can survive for long periods of dry weather by using stored moisture content in their leaves.
The leaves of certain desert plants, like jojoba (Simmondsia chinensis), move throughout the day so that the Sun’s rays fall only on the edges of the leaves, reducing the heat transferred to the surface, which reduces evapotranspiration.
Stem Adaptations in Desert Plants
The stems of desert plants also exhibit various specializations that allow them to thrive in harsh desert climates.
Thick and Fleshy Stems
Stems of most cacti and other succulents are thick and fleshy. Such stems hold moisture that helps the plant survive drought.
Stems Perform the Function of Leaves
The stems of plants that lack leaves or have leaves that are reduced to thorns or spines take up the function of leaves and perform photosynthesis. For example, the stems of most cacti perform the vital photosynthesis function.
Stems Have Waterproof Coatings or Hairy Growths
Stems of desert plants also often have waxy coatings or hairy growths that help limit water loss and provide wind protection.
Plants with Expandable Stems
Plants like the saguaro cactus have expandable stems that have a pleated structure that expands and contracts, similar to an accordion. This adaptation allows the stems to hold more water during a rainstorm and contract during dry conditions to prevent water loss.
Root Adaptations in Desert Plants
Some Desert Plants Have Deep Roots
Desert plants like the mesquite have deep taproots that reach down to the water table to reach water. This root adaptation allows the plant to escape drought.
The roots of plants that grow in arid conditions are often fleshy and thick, as the roots store moisture and nutrition, allowing the plant to survive dry conditions. These roots are called tubers.
Shallow and Horizontal Roots
Many succulents, such as saguaro, have extensive shallow roots systems that grow horizontally rather than vertically. These roots are usually as deep as the plants are tall, but not deeper. This root adaptation allows the plant to tap and absorb water from soil across a larger area. In order to allow the root systems to spread out well, these species usually grow further apart from each other rather than in clusters.