Plant Adaptations to Arid Environments
By Fritz Kollmann - Water Conservation Garden Crew Leader
It takes a certain degree of tenacity to survive in a climate like ours. Life is not easy in the Intermountain West with blazing sun for most of the summer, and cold, snowy winters with temperatures that dip into the single digits. As 21st century humans, we can just retreat to our climate-controlled buildings, drink a glass of water when we need one, and seek shade when it gets uncomfortably hot—but plants remain in one place and only use the resources that are available.
As plants evolved and colonized the surface of Earth they were forced to develop diverse survival strategies. The plants that developed adaptations to survive in arid environments have produced interesting, beautiful, and strange forms of life in the process. Fleshy-leaved succulents, small desert wildflowers, and shrubs and trees with waxy, leathery, or even tiny leaves all thrive in hot, dry environments with little rainfall. These plants use a variety of strategies 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 living 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, desert-adapted annuals will germinate in large numbers, producing incredible carpets of flowers, while in drier years significantly fewer plants will germinate. This strategy ensures the best chance of successfully germinating, growing, and reproducing seed for the next generation.
Succulent plants use several strategies to make the most efficient use of the limited water they receive. Agaves, cacti, ice plants (Delosperma sp.), and other fleshy-leaved plants store water in their leaves or stems. The water they store is so precious that some succulents have developed spines and glochids (small, easily detached, barbed spines), or toxins within their tissues to protect themselves from thirsty and hungry animals.
To further aid in the conservation of valuable stored water many succulents use two additional adaptations, the first being a waxy layer on the surface of the leaves that inhibits water loss. The other adaptation involves a different type of photosynthesis than what is commonly employed by most plants called CAM (Crassulacean Acid Metabolism). These plants typically only open their pores for gas exchange (letting carbon dioxide in while water evaporates out) at night, resulting in significantly less water loss when compared to plants that conduct gas exchange during the day.
Woody plants in the vast shrublands of the Great Basin demonstrate interesting adaptations to the low availability of water as well. 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, some plants such as the creosote bush (Larrea tridenta) employ a resinous coating on their leaves to reduce water loss, while others drop their leaves entirely such as the bud sage (Artemesia spinescens).
Curl-leaf mountain mahogany (Cercocarpus ledifolius), and other woody plants in the Wasatch foothills demonstrate their ability to withstand scarce water through the small size of their leaves. Smaller leaves reduce the surface area through which a plant loses water. Some species like the shrubby cinquefoil (Potentilla fruticosa) have developed wooly, hairy leaves in order to reflect some of the light from hitting their leaves, reducing leaf temperatures and water loss.
Most plants take up a significant portion of their water through their roots. Tall trees with large leaves, such as Utah’s native Cottonwood (Populus fremontii) need a significant amount of water in order to survive and reproduce. Cottonwoods tend to grow near water and send their roots deep into the ground to reach the water table, ensuring a consistent supply of water. Some perennial bunchgrasses (little bluestem, Schizachyrium scoparium) can send the roots down five feet or more to find groundwater. Large cacti (saguaro, Carnegiea gigantea) have large networks of shallow roots that enable the plant to quickly absorb precipitation before it has a chance to evaporate from the soil. This works well for cacti because they are then able to store water in their fleshy stems for use long after the surface soil has dried out. They also employ CAM photosynthesis to reduce water loss.
The new Water Conservation Garden, scheduled for completion in late 2016 or Spring 2017, will offer a large variety of plants displaying a wide variety of ways to thrive in low-water situations.