Plant Adaptations to Arid Environments
By Guy Banner, Emily Alworth, and Fritz Kollmann
It takes a certain degree of tenacity to survive in a climate like ours. Life is not easy in the Intermountain West, with blazing hot summers and frigid, snowy winters that dip into single digit temperatures. As 21st century humans, we can retreat to climate-controlled buildings that provide us with shade, heat, and fresh water—but plants are limited to the environments and resources available where they grow.
Fortunately, many plants are able to flourish in arid environments due to diverse survival strategies. Fleshy-leaved succulents, small desert wildflowers, and shrubs and trees with waxy, leathery, fuzzy, or very small leaves all thrive in hot, dry environments. These plants use a variety of methods to conserve water, reduce transpiration, and get the most out of the scant, and often seasonal precipitation.
Most dry regions receive the bulk of their moisture during one or two seasons. Some species of annuals have adapted to this by growing only during these short, relatively moist seasons. Small, desert annual flowers such as the desert five-spot (Eremalche rotundifolia), have adapted to germinate, grow, flower, and produce seed in a relatively short period of time. The seeds of many plant species can remain dormant in the soil for several years waiting for the right conditions to germinate. Following wet seasons, these desert-adapted annuals will germinate in large numbers, producing incredible carpets of flowers, often referred to as super blooms. During more arid years, significantly fewer plants will germinate. This strategy ensures the best chance of successfully germinating, growing, and reproducing seed for the next generation.
Woody plants in the vast shrublands of the Great Basin, Colorado Plateau and Mojave Desert demonstrate many adaptations to the low availability of water. When the winter snows have melted off, spring rain showers are a distant memory, and the soil begins to bake under the punishing rays of the sun, plants such as the creosote bush (Larrea tridentata) employ a resinous coating on their leaves to reduce water loss. Manzanita or bearberry (Arctostaphylos spp.) and Fremont’s desert holly (Mahonia fremontii) inhibit water loss through a waxy layer on the surface of their leaves. Some deciduous plants, such as the bud sage (Artemisia spinescens), drop their leaves entirely, becoming dormant during the hot summer season.
Many woody plants in the Wasatch foothills, such as the curl-leaf mountain mahogany (Cercocarpus ledifolius), have adapted to scarce rainfall by boasting small leaves that minimize water loss due to reduced surface area. The pubescent, wooly leaves of species such as bitterbrush (Purshia tridentata) make use of the Boundary Layer Effect. “The boundary layer is a thin zone of calm air that surrounds each leaf... (and) influences how quickly gasses and energy are exchanged between the leaf and the surrounding air. A thick boundary layer can reduce the transfer of heat CO2 and water vapor from the leaf to the environment.”(1) Other species, like Dorr’s purple sage (Salvia dorrii), have a powdery leaf surface that reflects sunlight. Both adaptations reduce leaf temperatures and water loss.
Plants take up a significant portion of their water through their roots, posing a unique challenge to species living arid environments. Many plants have developed root systems that are adapted to make effective use of limited water resources.
Research at the University of Utah using stable isotopes to identify water sources within plant tissues shows that some plants rely on “deep, winter recharge water throughout the summer” rather than summer rains. Both mature Gambel’s oak (Quercus gambelii) and bigtooth maple (Acer grandidentatum) showed this effect, suggesting that both tree species have deep root systems that access water for summer growth from soils still saturated from winter recharge.(2)
Perennial bunchgrasses, such as the popular Utah native little bluestem (Schizachyrium scoparium), have the ability to send their roots five feet or more until they find groundwater. Joshua trees (Yuccabrevifolia) and coyote gourds (Cucurbita foetidissima) have adapted large tuberous roots that can expand to store rainwater for later use during drier times.
Various cacti, such as the saguaro (Carnegiea gigantea), have large networks of shallow roots that allow the plant to quickly absorb any rainfall before it evaporates. Plants in areas of infrequent precipitation can use this evolutionary solution to store water for use long after the surface soil has dried out.
Succulent plants are known for their many strategies to efficiently use limited water resources. Agaves, cacti, ice plants (plants in the Aizoaceae family, such as Delosperma), and other fleshy-leaved plants store water in their leaves or stems.
In addition to a waxy layer on their leaves, many succulents have an additional photosynthetic adaptation called Crassulacean Acid Metabolism, or CAM. CAM plants only open their pores for gas exchange (required for photosynthesis) at night, resulting in significantly less water loss when compared to plants that conduct gas exchange during the day. CAM photosynthesis can be found in many species found in dry, desert environments, such as all members of the Crassulaceae family (Sedum, Crassula, Sempervivum, Telephium, etc.), many Aizoaceae family members (Delosperma, Mesembryanthemum, Sesuvium, Lithops, etc.), most Cactaceae (Cactus) family members, some spurge (Euphorbia spp.), and some members of the Asphodelaceae family (Aloe, Gasteria and Haworthia).
The water that desert plants store is so precious that many species, such as the Utah agave (Agave utahensis) and the Spinystar (Escobaria vivipara), have evolved spines and glochids (small and easily detached spines) to protect themselves from browsing critters. The spines offer additional protection against water loss to sun and wind by interrupting airflow across the plant and providing reflective sunblock in species such as Whipple’s cholla (Cylindropuntia whipplei).
Many species, such as Antelope horns milkweed (Asclepias asperula), create toxins as well as sticky latex (milky sap) within their tissues in response to herbivory (plant-eating). Chemical defenses evolve over many generations of predation by insects and other herbivores. Though creating toxins requires extra resources and energy from plants, it can be critical to surviving the threat of being eaten especially under harsh climactic conditions (3). Defensive compounds can be desirable flavors for non-target species, such as the tangy menthol taste of mountain beebalm (Monardella odoratissima), which makes a pleasant tea.
Red Butte Garden’s three-acre Water Conservation Garden offers a large variety of plants that displaymany ways to thrive in low-water situations. Be sure to spend time in the Adaptive Beauty Garden to see plants with appearances and life cycles related to their water-conserving physiological adaptations. Take a moment to explore the many hydro zones of the Water Saver Terrace in order to see a range of adapted species grouped by their watering requirements. Journey to the top of the Water Conservation Garden and visit the Gravel Garden to see an abundance of extremely resilient specimens with unique forms. It's well worth the uphill walk just to gain a sense of the wide array of low-water plants that can thrive in our semi-arid environment.
