As I'm sitting here in the office avoiding gardening on a 102°F
afternoon, I'm forever thankful I'm a human. Among the finest of human
inventions is air conditioning, but what can plants do to tough out extreme
temperatures? It's not like they can flick a switch for central air or even
move into the shade. No, they're stuck where they're rooted, but just as humans
have sweat, mobility, and cold drinks, some plants have developed a fascinating
array of adaptations that keep them from withering to a crisp under that
unforgiving sun.
The name of the game in hot, dry climates is water conservation. On a basic level, plants breathe by opening pores in their tissues called stomata. Whenever a stoma opens to take in carbon dioxide and release oxygen as waste, water vapor follows the oxygen and desiccates the plant. Many plants in arid regions have evolved to use a form of photosynthesis called Crassulacean acid metabolism (CAM) to manage this. Named after the family from which it was first described (Crassulaceae, or stonecrops), CAM is a distinct metabolic pathway that allows the plant to keep its stomata shut during the day and open them at night when it's cooler and more humid. Carbon dioxide flows into the plant, is converted to malic acid, and sequestered in the vacuoles of the plant cells, where it can then be transported to chloroplasts, converted back into carbon dioxide, and used for energy throughout the day. Think of it like plugging in and charging your phone over the course of the night, so it'll be at 100% battery and ready to go for the next day.
Beyond complicated metabolic strategies, plants have evolved an array of unique forms and structures to beat the heat. Returning to high school geometry, spheres maximize volume while minimizing surface area. The volume is filled with water that the plant needs to survive, but less surface area decreases the amount of evapotranspiration transpiring, so many desert plants have a spherical or cylindrical form, like barrel cactus. In fact, some of these plants are pleated like an accordion, too. When water is taken up, the plant swells, and the pleats expand; in times of drought, the plant shrinks, and the pleats shrivel back. You may also consider how some plants grow in rosettes, like yucca and agave. These plants use their broad leaves as a way to intercept raindrops or humid air and use water's physical property of cohesion to direct water to the base of the rosette and to the ground where the root is and can easily take it up.
Hedgehog cactus (Echinocereus viridiflorus ssp. viridiflorus) in Nunn, Colorado; photo by Andrew Scott
Speaking of roots, there are two basic strategies used in making use of what little water there is. Plants like desert marigolds and prickly pears have shallow roots that spread far and wide to collect precipitation on the surface before it can evaporate, while shrubs like mesquite, creosote, and acacia invest in bulky taproots that, as the name suggests, can tap into the water table deep underground. Roots also help the plant allocate tissue underground, where they're insulated to some degree from direct heat and sunlight. Should a plant's aboveground tissues completely desiccate and burn to a crisp, the roots might still have enough energy to put out new growth when conditions are more favorable. If you were to see a peyote cactus in the Chihuahuan Desert of northern Mexico, it would only be a tiny button peeking out of the rocks, but 90% of its biomass is buried and safe.
Peyote cactus (Lophophora williamsii); photo by Martin Terry and James D. Mauseth
Leaves are typically the primary photosynthesizing organ in plants, but in atypically hot and dry settings, they have become incredibly diverse in their form and function and sometimes even take the backseat to energy production. One strategy is to keep leaves only as long as the plant needs them or forego them entirely. During the wet season, plants may push leafy growth to collect as much solar energy as they can while there's water to keep them alive and then shed them once they have fulfilled their purpose. After that, chloroplasts near the surface of the stems do all of the photosynthesis.
The leaves of succulent plants, in particular, are full of mucilage, a mesh of proteins and sugars that hold onto water. While ordinary plants are about 75% water, succulents can be up to 95% water, stockpiling precious moisture in case they are forced to go without for a long time. A great example of mucilaginous leaves most folks are familiar with is the sticky, gel-like insides of aloes used to tend to sunburns. Similarly, some desert plants produce a thick, waxy cuticle of hydrophobic lipids on their surface as a way to keep internal water inside where it belongs and slough external water down to the ground where it can be absorbed.
Cross section of an Aloe vera leaf; photo by Deanna Talerico, HomesteadAndChill.com
Leaf size and shape are also important adaptations to aridity. Evolutionary pressures might select for highly reduced leaves that absorb and hold less damaging heat than a larger leaf would, and some leaves might evolve to be so reduced that they no longer even photosynthesize. In cacti, needles are actually modified leaves, while the pads/cladodes are modified stems that do the bulk of photosynthesis. Similarly, some plants like desert globemallow have a distinctive pale coloration thanks to the dense hairs covering them, the purpose of which is threefold. Like an animal's fur coat, hairs shield living tissue from harsh ultraviolet radiation and reflect incoming light and heat away from the plant, but also create a boundary layer of still, humid air close to the stomata, deterring desiccation by preventing the wicking action of high-speed drying winds.
Stellate (star-shaped) leaf hairs on desert globemallow, Sphaeralcea ambigua; photo by AmericanSouthwest.net
Desert plants also adapt different life history strategies to succeed in harsh conditions, being either extremely short or long-lived, both of which have trade-offs. Annual plants put all of their energy into reproducing, often within the span of a couple months or even weeks when conditions are favorable before dying. Afterward, their seeds lie dormant until conditions become just right to germinate and go through the cycle again. This strategy is what leads to desert superbloom events during unusually wet years. On the other hand, perennials that are resilient enough to survive and establish themselves may slowly grow for decades, centuries, or even millennia. Saguaro cacti can easily push 200 years, but King Clone, a creosote bush clonal colony in the Mojave Desert of California, is one of the oldest living organisms on Earth, estimated to be 11,700 years old!
A superbloom in Anza-Borrego Desert State Park, California; photo by AaronP/Bauer-Griffin via Getty Images
Sitting here in a sweat-mottled shirt with a glass of iced tea at hand, I'm forever humbled by plants that can take the heat in stride. From finding ways to collect and pack away as much water as possible to creating microclimates conducive to survival with their unique physiology, these plants have evolved across millions of years to scoff at temperatures that make us humans slog to the shade.