“I have seen the world go by in the natural world in my garden with a continuity and intensity that I have not been able to give it before…” — Sir David Attenborough

Charles Darwin solved what he called the “mystery or mysteries” when he (and Alfred Russel Wallace) developed their theory of the origin of species through natural selection. Yet, there are countless mysteries still to unravel, opportunities for naturalists, as Darwin and Wallace were, to whittle away at those mysteries, filling in pieces of an endless and infinitely fascinating jigsaw puzzle.

One of those jigsaw pieces is the improbability that life exists in a desert. Midday-midsummer surface temperatures on the sand dunes regularly reach 60-70°+ C (140-160°F). These temperatures are lethal to life (except for what are termed “extremophiles," typically bacteria or marine invertebrates that exist in hot springs or at thermal vents at the bottom of the ocean). At around 40-41°C (104-106°F) (for humans its 37°C), desert iguanas have the warmest preferred body temperature of any desert animal, but at 45°C they die, so they dig deep burrows to escape extreme midday summer temperatures. Hot fringe-toed lizards bury in the sand in the shade of a creosote or indigo bush, or if that is not cool enough, they head down a rodent burrow to depths that provide temperature-buffering insulation. As temperatures increase the lizards shift their foraging and mating activities to earlier and earlier in the morning. On a particularly warm summer morning a few years ago I walked out on the dunes not long after sunrise, but based on the tracks in the sand, the fringe-toed lizards had been active hours before I arrived. Desert lizards, snakes, mammals, and arthropods are adept at being extreme temperature “avoiders."

But what about plants? Annual plants complete their entire life cycle before the onset of those lethal summer temperatures. They too are extreme temperature avoiders. Perennial plants do not have that option. One perennial, Tiquilia plicata (plicate coldenia) is small, spreading, and prostrate (never more than a centimeter above the sand surface) and lives on sand dunes. It has small leaves that are pleated like a Scotsman’s kilt, and delicate lavender blossoms that fringe-toed lizards love to eat. Small, pleated leaves dissipate heat rapidly while retaining a large surface area for photosynthesizing. Still, somehow, they tolerate temperatures of 60°+ C. I have “met” close relatives of this plant in the Galapagos Islands, including Tiquilia darwnii and Tiquilia galapagoa. The Galapagos Islands straddle the equator, so it gets hot, but the cool Humboldt current, bringing cool ocean waters north from Antarctica, moderate those temperatures somewhat. Darwin’s tiquilia occurs only on the hottest, driest, open volcanic sands within the Galapagos Islands, however, rather than prostrate and with pleated leaves, it is upright. Tiquilia galapagoa is prostrate. How their common ancestor reached the North American deserts is another one of those mysteries, but it demonstrates again how tropical plants evolved tolerances for aridity and then were able to shift northward as the North American deserts began to develop.

How they do it, whether an extreme temperature avoider or tolerator, desert life puts up with hot temperatures. How they deal with extreme aridity is yet another mystery. Life on earth needs water. Period. Heat increases aridity by increasing evaporation. Water that evaporates is of no value to plants or animals. Water that reaches the root zone of plants, that through plants becomes available to animals, is the key. Plants can avoid water loss by not having leaves (smoke tree, ephedra, cacti), jettisoning their leaves when under water stress (ocotillo, brittlebush), having a waxy leaf or stem surface that holds water in (creosote, cacti), storing water in their stems or roots (cacti) and/or developing a different metabolic pathway that allows them to go through photosynthesis at night when temperatures are lower and so evaporation issues are reduced (cacti). These adaptations are incredible, but what keeps me up at night wondering “how they do it” is the plants’ first years of life. Tiny, fragile, high surface areas (so high evaporation rates), relatively low root volume, and little if any capacity to store water. How do they survive that first decade of summers? And, as climate change makes it hotter and so increases aridity, can those seedlings survive at all? That is the improbability that life exists in a desert, yet it does exist. It exists in a myriad of forms and species. 

I have long naively assumed that desert plants should be easy to propagate from seeds. Just give them the one resource that for them is always in short supply — water — and they should burst from their seed coats with exuberant life. It's not that easy. Some seeds need to be abused first, scratched or abraded (scarified) or cooled, or aged. Sprouting at a seed’s first soaking is not a wise strategy. Desert rains are fickle at best, and a single storm is no guarantee that it will be a wet spring. If a seed sprouts with that initial storm, the seedling will wither and die if that storm is not followed by an additional series of soaking storms. Their potential to grow and populate their habitat wasted. What environmental cues the seeds respond to is species-specific and a secret they do not give up easily. A wet winter and spring will result in annual wildflowers. However, if the very next year receives what seems like the identical rainfall pattern, the wildflowers will be a different mix of species. When a wet year follows a long drought, the wildflowers are spectacular, but if the next year is just as wet, the flowers will be much less spectacular. This variability, beyond the broad patterns I have described, has defied analyses that would allow accurate, precise, predictions. Yet when the right flowers are present, their pollinators are there in force, so they (the pollinators) seem to be responding to those same cues, whatever they are.

Then there are the perennials. I have a couple California barrel cacti in my yard (Ferocactus cylindraceus) that flower and go to fruit every year. The fruits collectively produce many 1000s of seeds each year. For the past 20+ years of wet years and dry years, cool winters and warm, dry summers and wet, I have spread those seeds throughout my yard so I can have more barrels. So how many baby barrels have resulted? – zero. I should not be too surprised; when I hike the trails surrounding our valley, adult barrel cacti are common, sometimes abundant, but tiny baby barrels are exceedingly rare. The same is true for almost all desert perennials. Obviously, the barrels do reproduce sometimes, but what is the recipe of weather conditions that allows those tiny black seeds to grow into impressive spiny columns? I did find on-line descriptions of how to grow cacti from seeds and tried their recipe. Basically, soak the seeds, plant them in a flowerpot and cover the pot with plastic wrap to create a mini greenhouse, and leave them for 2 months. It worked. When I removed the plastic wrap there were dozens of tiny jade-green spheres. However, a year later only two of those spheres had grown into something resembling a baby barrel. Obviously, nature does not provide plastic wrapped greenhouses to produce baby cacti, nor does she coddle her seedlings, so the mystery continues. I continue to use my native desert garden unravel nature’s secrets. As with any experiment, failures teach us much more than do successes. 

With increasing desert aridity, catalyzed by climate change, the fate of biodiversity will depend on if, how, and where native plants will be able to reproduce and replace adults as those adults age out of the populations. In a nutshell, that is the question the community scientists that work with me and I are answering. As long as the plants can sustain populations, the animals will too. If the summer and fall are dry, as was the case in 2020, bighorn sheep butt the barrel cacti until they break open and provide the sheep with critical water resources. On our community science surveys we see the aftermath with broken open barrels littering the hillsides. I wonder, if aridity increases as it has, will there be baby barrels to replace their fallen parents? If not, what does that portend for the thirsty sheep?

Go outside, tip your hat to a lizard (and a cactus), and be safe.