“If seeds waited for perfect conditions to grow, there would be no plants in the desert.” — Matshona Dhliwayo

In 1977, I was in my fourth and final year of earning my Bachelor of Science degree at the University of California Davis. I had finished all my required coursework and so was thumbing through the catalogue for some interesting elective courses to take. I came across a class called “paleobotany” that included fieldtrips. Sounded interesting so I signed up. The professor, Dr. Daniel Axelrod, had a reputation for being “crusty," and that certainly came through at that initial class meeting (more than a few students dropped the class that first day), but during each ensuing class that crustiness fell away to the point near the end of the term he was more like a teddy bear. What I did not know going in was that this would be the last class he would ever teach (he retired), and that he was the world’s leader in describing the sequence of climates and vegetation communities that gave us the amazing diversity of our current western North American flora, including our deserts. 

Professor Axelrod acknowledged during that first class meeting that his scientific reputation had been tarnished because he was too slow to accept what only a decade or so before then had been a new theory of “plate tectonics," the theory of how the earth has an outer shell of autonomous plates that “float” on a viscous mantle and have and continue to shift, moving plants and animals with them. Of course, today we all know this theory to be correct based on countless independent supporting observations, but because Axelrod had been able to describe the floristic changes of western North America without plate tectonics, he was slow to accept the theory because he didn’t need it. Finally, he reanalyzed his data within a context of tectonics and realized that his ideas of the development of the western floras worked even better. He embraced plate tectonics, but at least from his perspective, his reputation never quite recovered. Science can be a fickle institution.

One of our class field trips took us out into the Great Basin Desert, a bit east of Reno, Nevada. Professor Axelrod took us down an old road to a cut embankment that exposed layers of shale that had once been part of a shallow lake embayment sometime during the Miocene Epoch, at least 15-20 million years ago. The Sierra Nevada mountain range was young and still rising then, and apparently the current rain shadow was not yet complete. We began to sift through those shale beds identifying fossil plants, which were abundant. Standing there in a sagebrush desert community, we were finding fossils that were a close match to what can now be found on the north coast of California: maples, alders, bay trees, ferns, and coast redwood trees. Redwoods have been part of what is now North America for about 240 million years (at that time there was a single supercontinent, Pangea). Then, dinosaurs were just entering the scene, barely a glimmer in the eye of evolution’s future path. Yet there we stood in a sagebrush desert.

One of Professor Axelrod’s discoveries was that our deserts, the Great Basin, Mojave, Sonora, and Chihuahua Deserts, are relatively new additions to the earth’s biodiversity. Aridity began increasing in the late Miocene or early Pliocene (7-5 million years ago) but desert floras as we know them today probably did not began to congeal until the later Pleistocene, sometime in the last 1-2 million years. The current position of our deserts is even more recent, perhaps in just the past 15-20,000 years. Our desert plants are the “new kids on the block” with respect to the other floras of North America.

One of the most interesting parts of this story is, since our deserts are of such a recent origin, where did the plants that comprise these floras come from? For the most part they came from the tropical and semitropical regions further south. At first that seems incongruous; how could plants that are now so superbly adapted to some of the driest places on earth evolve in the wet tropics? The answer is that not every portion of the tropics is wet, or at least not wet all the time. If you have a chance to visit the tropics in Central or South America, or on the islands of the Caribbean, you can easily find cacti. Except those cacti are often growing as epiphytes near the tops of the canopies or trees. Little or no shade and no soil. Even if it is warm and wet on the forest floor, it is hot and dry (no soil to hold water) in the canopy. Epiphytic cacti evolved sponge-like stems to hold on to water, adaptations that were then ideal for shifting onto the ground within our desert landscape. Cacti are one of the iconic members of our hyper arid deserts, but they thrive (more species, higher densities) in the wetter portions of those deserts, a pattern that betrays their tropical heritage. In the driest areas our deserts, cacti are rare or absent. 

The single most iconic and enigmatic desert plant is the creosote bush, the most common species in the Mojave, Sonoran/Colorado, and Chihuahua Deserts. From its resistance (so far) to the effects of climate change, to its multiple cultural/medical uses by indigenous peoples, to the dozens of arthropod species restricted to it – and so its keystone species character, creosote has much to admire. But again, how did it get here? Its closest relatives occur only in deserts of southern the Chile and Argentina. How it got here, and when, have been the topics of much speculation. Its earliest appearance in packrat middens dates to 18,700 years ago. Using a molecular clock method using the average rate of gene mutations per year multiplied by the number of gene mutations that separate it from its closest relative, the estimate increases to 0.4 to 3.36 million years ago. Using the same approach but using a gall midge (a tiny fly that creates galls in the creosotes branches) that is restricted just to creosote and estimating mutations from its closest relative, the years since creosote arrived are even larger (3.88-4.23 million years). So, there is no clear answer as to when it got here, although it could have been hanging out in the southern Chihuahua desert for a long time before conditions further north got warm enough and dry enough for it to thrive and expand northward. 

How it got here is perhaps a tougher question to answer. One sure way to kill a creosote bush is to water it too much. There is no reasonable scenario that would have allowed it to move through the Amazon Basin and through Panama, Costa Rica and finally into the southern Chihuahua Desert in Mexico, on its own. An alternative scenario would include a seed eating bird gulping down a creosote seed and immediately flying for several days, without defecating, getting across the Amazon and landing in Mexico. Unlikely. For my scenario, the first step is identifying if there are any birds that migrate between southern Argentina and the desert regions of North America. There is one, the Swainson’s Hawk. So, the hawk grabs some Argentine rodents (there are many to choose from) that recently consumed creosote seeds, and then taking advantage of tailwinds, immediately flies north, soaring above the Amazon, above Central America and lands in what might have been the beginnings of the Chihuahua Desert in Mexico. Once on the ground the hawk coughs up a pellet of rodent hair, bones and creosote seeds. It could happen.

In all, these plant immigrants from the south with their local adaptations and speciation have resulted in a rich, high biodiversity flora, especially in the warmer Sonoran/Colorado and Chihuahua Deserts. Biodiversity drops off as you head further north into the Great Basin Desert. Still, in relation to other bioregions of California, our southern deserts are by any measure, biodiversity hotspots. Comparing a similar area in California’s redwood zone, according to the “what grows here” feature within the CalFlora web page, near Garberville in the middle of the redwood zone there are 45 species of annual herbs, 51 perennials, 35 shrubs, and 20 trees. Respectable numbers. In Joshua Tree National Park, there are 361 annual herbs, 259 perennials, 223 shrubs, and 37 trees. In the mostly desert portions of the Santa Rosa and San Jacinto National Monument there are 505 annual herbs, 358 perennials, 245 shrubs species, and 63 trees. Wow.

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