Thibaud Aronson describes the botany, ecology, and degradation of southern California’s unique desert woodlands.
There are four deserts in North America – the Great Basin, Chihuahuan, Sonoran, and the Mojave. Both the Great Basin and the Chihuahuan deserts can have bitterly cold winters, and as a result their vegetation is quite stunted compared to the other two, with precious few trees. A few years ago, my father and I spent a fair amount of time in the Sonoran desert, both in Arizona and in Baja California, documenting its remarkable trees.
To fill an important gap in our survey of desert trees of the world, I recently visited the Mojave desert, home to one of the most iconic desert trees on the continent. Indeed, just about every time one mentions desert trees, at least in the US, the most common response is: “Oh, like Joshua trees?” Like Joshua trees, indeed.
The iconic Joshua tree (Yucca brevifolia) in the national park that bears its name in southern California’s Mojave desert.
As is typical in California, very different landscapes succeed each other in a relatively small area. So, over a week and surprisingly small distances, I traveled the region to get a sense of its rich tapestry of habitats (see my itinerary).
Heading east from LAX Airport, I drove up a winding road into the San Gabriel Mountains. By the time I reached 2000 meters (6500 feet), I was completely surrounded by tall, dense forests comprising six or seven intermingled conifer species, and a number of ski resorts, deserted in the summer season. At my campsite that night, the temperature dropped to just a hair above freezing, quite a contrast from the stifling August heat I’d experienced that very morning!
The next day, I followed the spine of the mountains through the San Gabriels and on to the San Bernardinos. Driving along the glittering blue waters of Big Bear Lake, I went past “Starvation Flats Road”, a warning of what lay ahead. And indeed, I soon reached the eastern slopes, which gave me a striking view of the Mojave desert below – stark yellow plains that disappeared in the haze. Making my way down, the conifers began to thin out, and soon the first Joshua trees began to appear, some of them nearly as tall as the oaks they grew with.
On the eastern foothills of the San Bernardino mountains, Joshua trees grow together with Valley oaks (Quercus lobata) and California Black oaks (Q. kelloggii).
Turning back to look at where I had come from, I could see Old Greyback, the tallest summit in southern California. Then I headed southeast, hugging the base of the mountains, to the famous town of Palm Springs. Originally the town was named for the freshwater springs that flow down from the mountains and the California Palms (Washingtonia filifera) that grow along the canyons. Today the palms line every street in town, towering above the low buildings.
The skinny, carefully trimmed palms that define the look of Palm Springs are in fact unnatural, as wild fan palms develop thick skirts from their dead leaves that can extend almost all the way down to the ground. See below.
The landscape to the east of town is desolate, highlighting how precious and unusual the springs are. Creosote shrubs, their leaves a characteristic greyish yellowish, grow on the dusty flats as the incessant wind spins the thousands of turbines of the San Gorgonio Pass wind farm (the third largest in the state). Suddenly, a green ribbon appeared in the distance, and as I got closer, it resolved itself into a copse of large cottonwood trees, towering over an incredibly thick mesquite thicket. This is Big Morongo Canyon, the largest freshwater spring in the region.
The thick canopy of cottonwoods (Populus fremontii) at Big Morongo canyon.
As soon as I stepped under the trees, it was clear that I had entered an oasis of life: orioles, goldfinches and hummingbirds flitted in the undergrowth, while jays pestered a great horned owl that they had found perched in a cottonwood. Cottontail rabbits hopped on the path ahead of me, and a coachwhip (Masticophis flagellum) rattled its tail at me, this tiny, harmless snake attempting (as many other local snakes do) to look like a rattlesnake to scare off potential predators.
A female Anna’s hummingbird (Calypte anna) and some bees drinking at Big Morongo.
Before it got too hot, I continued on my way farther east, until I reached Yucca Valley, and from there Joshua Tree National Park. It is a surreal experience, as they appear almost out of nowhere, the phantasmagorical silhouettes of these giant tree Yuccas stretching to the horizon, the unique bluish grey tinge of their leaves giving a peculiar appearance to the air itself. It is a landscape quite unlike any I had ever seen. Though it is hard to understand how the first pilgrims decided that one of these trees was the prophet Joshua, pointing them in the direction of the promised land, as each tree seems to point a different way!
Right on the edge of the national park, a Greater Roadrunner (Geococcyx californianus) looks for its next prey perched in a Joshua Tree.
While the northern parts of the Mojave are low-lying, making up the famously inhospitable Death Valley, Joshua Tree National Park sits at the southern edge of the desert, on a plateau about 1200 meters (4000 ft) above sea level. The cooler temperatures at that altitude are what allow the Yuccas to thrive, in truly remarkable numbers.
A typical scene from the northern section of the park. I couldn’t find any data on this, but there must be tens, if not hundreds of thousands of Joshua Trees in the park.
While Joshua trees are the most distinctive – and seemingly the hardiest, growing even in the most exposed flats – they are not the only trees of the park. Many sandy desert washes cross the plateau, and along each of them grow various dicot trees. Most noticeable among these is Chilopsis linearis, in the Bignonia family. In the vernacular, it is known as ‘Desert willow’ because of its unusually long, narrow leaves and the fact that it grows in riparian habitats. Another tree found in the Mojave is the Papilionoid legume Psorothamnus spinosus, known as ‘Smoke tree’, as its pale grey leaves look like a cloudy puff of ashes, brightened in summer when the trees are covered with gorgeous indigo-tinted, pea-like, flowers.
Smoke trees in the aptly-named Smoke Tree Wash, inside the Joshua Tree National Park.
Finally, there are two species of the legume tree Palo Verde (Parkinsonia), one of which I found providing shelter for a desert bighorn one evening near my campsite.
A young desert bighorn sheep (Ovis canadensis nelsoni). Bighorn populations have recovered quite well in recent decades, though they are still facing various threats in the California deserts. There are fewer than 300 found in Joshua Tree National Park (ca. 800,000 acres, or 3,200 km² in size).
Furthermore, the park is known for its elaborate formations of basaltic rock, which add to the surreal beauty of the landscape, and attract rock-climbers from all over the world. These rock piles, with the shelter and extra moisture that they provide, also allow oaks, pinyon pines, and junipers (all trees that typically grow on the higher mountain slopes) to survive down on the plateau as well.
At Hidden Valley, in the central section of the park, large pinyon pines (Pinus monophylla) grow among the basalt rock piles.
There are also several fan palm oases in the park, from Fortynine Palms, standing on its own amid the bare rocky slopes, reminding me of the mountains of northern Oman, to the glorious Cottonwood Springs, where massive cottonwoods barely top over the enormous palm trees, who formed a dense cluster sheltering a family of barn owls.
The saddest fan palm oasis is undoubtedly Mara, also known as Twentynine Palms. It is said that the Serrano Indians who used to live in the area planted one palm tree for each son that was born to the tribe after they first settled there. Last year, a criminal fire swept through the area, killing several of the palm trees. Some blackened trunks still stand, while several others had to be completely cut down. All in all, it is a sadly apt metaphor for what has befallen these first nations of indigenous people who called the area home for centuries prior to the arrival of Europeans.
Compare the glorious Cottonwood spring, protected as it is in a remote part of the national park, with the sad oasis of Mara, still showing the scars from last year’s fire that was set by an arsonist.
From there, I drove south through the park, and the landscape began to change markedly, as this is the where the Mojave gives way to the Sonoran desert. It got warmer and drier as the elevation decreased. The Joshua trees disappeared, replaced by ocotillos and in some areas, fields of teddybear cholla cacti (Cylindropuntia bigelovii). Passing through spectacular layers of exposed rock, I reached the Bajada, a grassy savanna with large ironwood trees (Olneya tesota, also a tree legume) a scene more akin to an African savanna than the landscapes I had left behind that same morning! This was quite striking as we had found ironwood trees to be much scarcer in Baja California!
From there, it was only a short way to the final stop on my trip: the Salton Sea. Lying about 70 meters below sea level, this is actually a depression, which was periodically filled by exceptional flooding on the Colorado River. The last time this happened was in 1905. Since it has no outlet, the lake progressively becomes more saline and eventually evaporates, until the Colorado floods its banks and fills it again.
Furthermore, because of repeated Colorado River floods, the surrounding soils are very fertile, despite the extremely dry climate. As a result, in the early 20th century, at the height of the hubris that characterized the development of the American Southwest, massive irrigation projects were put in place in the Coachella and Imperial valleys, at the northern and southern ends of the Salton Sea, respectively. The latter – formerly called Valley of Death, was rebranded as Imperial Valley – in a remarkable feat of marketing well described by Fred Pierce in When the Rivers Run Dry (2006). Today, the landscape as seen from the sky is surreal: the deep blue waters contrasting with the yellow of the surrounding desert, while the valleys at both ends are an incongruous green. The perfectly rectangular fields stretch all the way to the Mexican border, for a combined irrigated area equal to nearly three times that of the Salton Sea itself! These fields produce a large portion of the state’s lettuce, broccoli, carrots, and especially alfalfa, to feed California’s behemoth dairy industry. The sight of the hundreds of sprinklers going full tilt in the midday heat, to water alfalfa that could be grown in the East for a fraction of the cost, was rather off-putting, to say the least. (For a comprehensive and depressing history of water usage in the American West, read Cadillac Desert (1986), by Mark Reisner).
Irrigated vineyards in the southern end of the Coachella Valley. Note the extremely arid ranges in the distance, which stretch between Joshua Tree National Park and the Salton Sea.
As is by now well known, the Colorado has been well and truly “tamed”, its once wild run dammed in fifteen places, and every ounce of its water used to irrigate the thirsty cities and fields of the West, so that hardly a drop reaches its once mighty delta. In other words, the Salton Sea may soon be gone for good. And what else can we look forward to?
In a nutshell, the whole area is a mess. For a century, agricultural runoff has ended up in the Salton Sea itself and now, as the Sea is shrinking, more and more of its bed is being exposed and sediments heavily contaminated with salt and pesticides are being picked up by the winds. This is a massive public health issue. Not to mention the ecological disaster as the waters become too saline to support the fish that dwell in it, depriving the millions of birds that pass through the area of some of the only food available on that portion of the migration flyway. Furthermore, the well-publicized water shortages and catastrophic wildfires of California get worse with every passing year.
Obviously, agriculture in southern California will continue, but in what fashion? According to the California Department of Agriculture, more than half of the irrigated cropland in the state is badly affected by salinization. Surely this should be a wake-up call to explore alternative futures. For one thing, as Richard Felger, doyen of US-based Sonoran desert botanists and explorers, puts it, we need to learn to “Fit the crop to the land, not the land to the crop.”
Even though desert organisms are tremendously well-adapted to the harsh conditions they face on a daily basis, even they can only take so much. According to a recent study, rising temperatures are rapidly making the national park unsuitable for Joshua Trees themselves. In the best-case scenario, major efforts to reduce greenhouse gas emissions could save around 20% of suitable habitat for Joshua trees within the park after the year 2070. In the worst case, with no reduction in carbon emissions, 50 years from now, the Joshua Tree National park will retain a mere 0.02 percent of its Joshua tree habitat.
The first white explorers who saw the Western deserts of North America thought them so hostile that they could never support settlement, disregarding the Native Americans and the wealth of fauna and flora who had long been living in the desert, in a very delicate balance. But in the frenzy of the twentieth century, through truly prodigious amounts of effort, descendants of the European colonists radically reshaped these landscapes to suit their needs with very little understanding of the long-term consequences of their actions. The entire enterprise is clearly and dangerously unsustainable, as water reserves that accumulated over hundreds of thousands of years were drained in just a few decades.
We often hear about the fight against desertification, which is conflated with some sort of fight against advancing deserts. But it is important to remember that deserts, while harsh, can still be beautiful and full of vibrant and unique life. Joshua Tree National Park, the Mojave National Preserve, and a few other protected areas give us a glimpse into what once was. But they are mere handkerchiefs. If the region is to have a chance at a sustainable future, we need new paradigms, new laws based on a much better understanding of how life can balance itself in arid lands. Based on that understanding, it is imperative that we move away from the pattern of careless exploitation and transformation, stretching farther and farther away from what these deserts once were. Instead, it is past time to commit to ecological restoration and allied activities for the Mojave and indeed all degraded and mis-used deserts and semi-deserts, especially as climate chaos unfolds.
Natural History of Ecological Restoration 