The defining precipitation rate for desert is ten inches. Much of the Colorado Desert of southeastern California averages less than four inches of rain in a year.
Plants must find ways to cope with the same daunting conditions that animals face in trying to survive in a desert environment – searing temperatures, desiccating winds, and most important of all – scarcity of life’s elixir – water. The challenge is even greater for plants as they are literally rooted to the ground and unable to move in search of more favorable locations when times get tough. Whether it’s scorching summer days, freezing winter nights, foraging herbivores or months or even years of rainless drought, desert plants have adapted multiple strategies for existing and even thriving in an arid land.
Some desert plants are adept at efficiently absorbing water during even light rainfalls and storing the water in specialized cells in their stems, leaves or roots. This succulent, water-holding tissue provides the plants with security against extended dry spells. Cacti are by far the best known of the succulents, but many other plants have also adopted this strategy including aloes, agaves, yuccas, boojums (Fouquieria columnaris), elephant trees (Bursera spp.) and many members of the euphorbia (Euphorbiaceae) and milkweed (Asclepiadaceae) families. Remember the axiom: all cacti are succulents but not all succulents are cacti.
Maximal water uptake is achieved through shallow, wide-spreading root systems that are highly responsive to moisture. Thickened epidermal cells, waxy coatings, ephemeral leaves and no leaves are some of the methods used to reduce water loss. Many succulents also conserve water through the use of a different form of photosynthesis known as CAM (Crassulacean Acid Metabolism). Simply stated, this process allows the plant to keep its pores (stomata) closed during the heat of the day and then open them at night for the exchange of gases. This can mean a savings of up to ten times the amount of water that may have otherwise been lost due to transpiration.
Acquiring water is part of the battle, keeping it is the other. Cacti and other desert plants are notorious for their spines and thorns as a deterrent to thirsty or hungry browsers. It doesn’t take more than one encounter with a cholla to literally drive the point home as to the effectiveness of this line of defense. An additional benefit to the plants is the degree of shading afforded relative to the density of spination. Plants may rely on toxic or bitter tasting plant juices, camouflage or inaccessible growing sites as forms of protection in addition to or instead of spines and thorns.
Plants in this group may use a variety of strategies in coping with dry conditions. They too have wide ranging root systems but the roots also penetrate deeper into the soil to reach lower layers of moisture. The overall look and feel of the landscape is dictated by the competition for water. Under similar soil conditions, stature and spacing of plants are determined by precipitation rates.
Some shrubs, like jojoba (Simmondsia chinensis) and creosote (Larrea tridentata), have waxy or resinous coatings on their leaves to slow moisture loss. Others like desert lavender (Hyptis emoryi) have hairy leaf surfaces which help keep leaf temperatures down through shading and light reflection, thereby reducing water loss. Brittlebush (Encelia farinosa) will produce leaves of various sizes and hairiness depending on the level of moisture availability.
Most desert trees and shrubs have small leaves to begin with to aid in leaf cooling and to lessen surface area exposed to evaporative losses. With the onset of drying conditions, leaves are dropped to further lessen surface area. Increased dryness may induce plants to shed all of their leaves and some may begin to shed branches in an effort to conserve resources. Ocotillo (Fouquieria splendens) may shed and re-grow five sets of leaves in a given year in response to wet/dry cycles.
Trees and shrubs with green stems such as palo verde (Parkinsonia spp.), chuparosa (Justicia californica) and sweet bush (Bebbia juncea) can continue to carry out photosynthesis while leafless. Plants will eventually enter a dormant state where their metabolic rate is slowed significantly while they wait for the rain to return.
This strategy involves growing when and where moisture is available and not having to cope with the extremes of heat and drought that reign over desert biomes. The annual wildflowers or ephemerals that sporadically enliven typically muted vistas with their breathtaking displays of color fall into this category. Laying dormant in dry desert soils, their seeds can wait for years for the right combination of rainfall and temperature to break their dormancy. Once germinated, the ephemerals’ goal is to grow as big as available moisture levels allow, produce as many flowers as possible, set as much seed as their quickly amassed resources can manage and be ready to complete their life cycle by the time the weather heats up and soils dry out. In years of bountiful rainfall, plants such as desert sunflower (Geraea canescens) and dune primrose (Oenothera deltoides) may almost be shrubby in stature and an individual may produce hundreds of flowers over the span of its short life. In lean years, the same plant might be barely discernible, standing a couple of inches in height and managing to eke out only a single blossom.
No two wildflower seasons are alike as the dynamic interplay of the variables: sand deposited seed bank, amount and timing of precipitation, degree and duration of temperatures and a soil surface that is regularly carved and shaped by wind and, far less often, by water, set the stage for an infinite array of possibilities.
Another group of drought evaders are the plants that grow where streams, springs and high water tables supply water year round. Species such as California fan palm (Washingtonia filifera), Fremont cottonwood (Populus fremontii) and arrowweed (Pluchea sericea) are riparian in nature and cannot persist for long without access to water. They exist in a desert environment by offsetting the rigors of extreme heat by transpiring copious amounts of water. Other plants that thrive with sustained access to water are also able to survive in a more xeric situation. Cat claw (Acacia greggii) and mesquite (Prosopis glandulosa) will exhibit reliable seasonal patterns of leafing out, flower and seed production and leaf drop with a consistent supply of water. Away from such a source, they resort to the adaptive strategies of drought tolerant plants.