The dog days of summer are upon us ...

The latter half of July through the first half of August are typically the hottest days of summer in the northern hemisphere. They correspond with the days when the star Sirius, one of the brightest stars in the night sky, sets below the Earth’s horizon with the sun. Sirius is part of the constellation Canis Major, the “big dog,” and that is the origin of the idiom the “dog days of summer,” a time of especially hot temperatures. Some ancients believed that as the bright star went below the horizon with the sun it was transferring heat to our sun, giving us those sweltering summer days.

For me, one of the near miracles of the desert is how any plant or animal survives those dog days without irrigation or air conditioning. Part of my sense of awe is that there is not just one solution for surviving summer.

The easiest way out is to escape the heat of summer. Annual plants, those spring wildflowers we all long for every February and March, simply complete their entire life span before the heat arrives; they spend the dog days as seeds buried in the desert sand. The real challenge is for perennial plants. Leaves are the problem; leaves lose water, water vapor draining from their leaf pores (stomata) like a sieve. For many desert plants that answer is to shed those leaves when it gets hot. Ocotillo, brittlebush, palo verde, and chuparosa all have adopted that strategy. Ocotillos are especially adept at assessing their water stress, dropping their leaves whenever that stress threshold is reached, and then growing new leaves within days of enough rain falling to be captured by its roots, eliminating that stress. Ocotillo can drop and then regrow leaves two to three times or more a year depending on whether there is significant summer or fall rainfall.

Another solution is to not grow leaves at all. Smoke trees, rush and giant milkweeds, and cacti are examples. These plants still have stomata, they have to for conducting photosynthesis – pulling in carbon dioxide to then be used to construct sugars and protein molecules necessary for plant growth and reproduction, and expelling oxygen (and water vapor). Their stomata are located along their pale waxy stems where temperatures are reduced (pale white stems reflect heat) and water conservation is easier to manage. One of those quintessential groups of desert plants, cacti, first evolved in the tropical forests of what is now Central and South America (there are no true cacti living in Africa, Australia, Asia, or Europe unless a human moved them and planted them there – although I did see a native epiphytic cactus in the moist forests of Madagascar).

At first blush it seems incongruous that a plant that evolved in the wet tropics would then be among the first to invade, exploit, and diversify in the relatively new North American deserts. It makes sense if you travel to the American tropics in search of cacti and find that cacti first evolved as tree-top epiphytes. Even in the wet tropics, treetops are dry because there is no soil to hold onto the otherwise abundant rain falling from the sky. Dense needles keep their stems cooler. To successfully exploit those dry treetops cacti also evolved spongy tissue to capture and store water and evolved a different way of doing photosynthesis. Rather than keeping their stomata open during the heat of the day, cacti (and a few other plants) store the sun’s energy in their tissues during the day, but only open their stomata at night when it is cooler, and the humidity is higher. Scientists refer to this alternative photosynthesis mechanism as crassulacean acid metabolism (CAM). For cacti, moving into the hyper arid, hot deserts of North and South America was a relatively simple task.

Another quintessential desert plant is the creosote bush, which is the only plant species to exist and become numerically dominant in the Chihuahuan, Sonoran and Colorado, and Mojave Deserts. Yet creosote does not shed its small waxy leaves, nor has it evolved anything like CAM metabolism. Their small waxy leaves do “move,” turning just the edge of their leaf blades toward the moving sun, reducing leaf temperatures and retarding water loss. Still, by the dog days creosote leaves turn an olive-brown and appear to be dead. My first year experiencing a Colorado Desert summer I was convinced that all the creosote bushes were dying, that I was experiencing an ecological disaster. Yet come fall, with cooler temperatures and the onset of rain, each creosote greened up, looking as healthy as ever. The same miraculous transformation has occurred every fall since.

Then there are desert fan palms. Everything about palms screams that that they are thumbing their leafy noses at desert heat and aridity. Giant, bright green leaves with no special metabolic pathways to conserve water. Their secret? They have none, other than they can only exist where their roots are bathed in water – oases tucked into narrow canyons where bedrock or fault lines bring ground water to the surface, along the San Andreas fault, or in people’s yards where abundant water is provided. Take away the water, lower the aquifer, turn off the irrigation, and they perish.

Desert animals have their own ways of surviving the dog days. For the most part it is by avoidance. The Cahuilla migrated to higher, cooler elevations during the summer; those that couldn’t make the trek hunkered down in the cooler, wetter palm oases. We do the same – make trips to the coast or mountains for respites from the heat. Desert bighorn sheep shift their summer activity to deep canyons, north-facing slopes, and those cooler-wetter oases. Where surface water is unavailable the sheep will break open barrel cacti with their horns, and then consume the water-filled pulpy center the cacti worked so hard to store and live on. Desert rodents, kangaroo rats and pocket mice, are strictly nocturnal most of the year, but especially so during the summer. During the day they stay deep within their burrows where the humidity is near 100%. At night they collect the seeds that annual plants produced the previous spring. One of the metabolic by-products of eating carbohydrate-rich seeds is water, so kangaroo rats and pocket mice can live without ever drinking water.

Desert beetles, ants, lizards, and snakes are extremely good at conserving water. Non-porous exoskeletons or scaly skin holds what precious moisture they consume inside their bodies, with little or no evaporation. The beetles and harvester ants consume many of the same seeds the rodents depend on, and like the afore-mentioned rodents and reptiles, their urine and feces are non-liquid or at worst “paste-like,” so little water is wasted. Most snakes are nocturnal all year-round, but in the days, weeks, and months approaching and following the dog days, desert lizards increasingly limit their foraging and breeding activity to the early mornings when humidity is higher, and temperatures are cooler. Take an early morning walk on sand dunes and you can easily see the comings and goings of the dune lizards by their tracks in the sand. For the most part they will have already completed any social engagements and consumed some of those beetles and harvester ants long before you arrived. During mid-day the lizards are ensconced in a burrow or deep in a rock cleft where they are out of the sun’s reach and where humidity is much higher.

Desert plants and animals have adapted to and survived the dog days for many millennia. However, modern climate change is making those summers both hotter and drier. We, along with those plants and animals, are entering uncharted conditions with outcomes that are difficult to predict.