Plant Adaptations

Xerophytes are plants that have developed special means of storing and conserving water. To reduce transpiration, direct cooling air flow, shade themselves, and/or reflect hot sunlight, they often have few leaves, or have spines, thorns, or hairs instead; in some cases they use only their green “skin” to perform photosynthesis. Some xerophytes have leaves that are tough and waxy or coated with shiny oils, which also cuts down on transpiration, or they have small, fluttery leaves that help to cool the plants. Some species have spongy, shallow roots to take advantage of even the smallest amounts of rain, plant tissues that can store large amounts of water for later use, or ribs that direct moisture to the plant’s roots. The Joshua tree (Yucca brevifolia), various types of cacti, and Mojave sage (Salvia mojavensis) are all examples of Mojave xerophytes.

Phreatophytes are plants that grow extremely long roots, called tap roots, that allow them to obtain water from deep in the ground. Mesquites (Prosopis sp.) have the longest tap root of any desert species; it may reach down 25 m. The creosote bush (Larrea tridentata) is one of the most successful of all desert species because it uses a combination of several adaptations, including a deep tap root, a shallow root system, and wax-coated leaves that close their pores during the day to avoid loss of water. The creosote bush has the added advantage of being both bitter-smelling and -tasting; most animals won’t eat it, and other plants often won’t even grow near it, reducing competition for water.

Desert perennials survive by remaining dormant during dry periods of the year, then “coming back to life” when water becomes available. For example, the ocotillo (Fouquieria splendens) grows leaves quickly after a rain to produce food through photosynthesis. In a few weeks, ocotillo flowers bloom, seeds ripen and fall, leaves drop, and the plant becomes dormant once more. Ocotillo stems also have a waxy coating to help conserve water during dormancy. Other perennials include brittlebush (Encelia farinosa), some varieties of California poppy (Eschscholzia californica), and desert paintbrush (Castilleja angustifolia), a parasitic plant that appropriates water from its host. By contrast, annuals, or ephemerals, germinate only after a heavy rain, and then complete their reproductive cycle very quickly to capitalize on optimal heat, light, and moisture conditions. They bloom profusely for a few days or weeks in the spring, and then die, leaving their drought-resistant seeds in the desert soils until the next spring rains. Annuals are responsible for the colorful desert wildflower displays that often appear in spring. Examples of such wildflowers are sand verbena (Abronia villosa), Mojave aster (Xylorhiza tortifolia), and dune evening primrose (Oenothera deltoides). Generally, thousands of annual and perennial seeds can be found in every handful of desert soil; the world record is over 200,000 seeds per square meter, though the Mojave’s soils contain far fewer.