The Joshua Tree: Myth, Mutualism and Survival | KCET
The Joshua Tree: Myth, Mutualism and Survival
The Mojave Project is an experimental transmedia documentary and curatorial project led by Kim Stringfellow exploring the physical, geological and cultural landscape of the Mojave Desert. The Mojave Project reconsiders and establishes multiple ways in which to interpret this unique and complex landscape, through association and connection of seemingly unrelated sites, themes, and subjects thus creating a speculative and immersive experience for our audience.
On April 14, 1844, riding eastward out of the Tehachapi Pass near Oak Creek into a landscape that would later come to be called the Mojave Desert, Brevet Captain John C. Frémont of the U.S Army Corps of Topographical Engineers became the first white settler to record the existence of one of the Mojave’s more unusual plant species. He was not precisely charmed by the oddly formed tree, or at least that’s how he recounted it months later, writing[i]:
Frémont was writing of Joshua trees, though they would not come to be called that in southern California for about 80 more years. He was the first writer to vilify the Joshua tree; he would not be the last.
Los Angeles writer Francis M. Fultz, better known for his advocacy of chaparral native shrubs, had this to say about Joshua trees in Scientific American in 1919: “Whenever I see the Joshua-Trees I think how considerate they have been in choosing to make their home where few men have a desire to live.”[ii] In that same year, in his best-seller “California Desert Trails,” writer Joseph Smeaton Chase made Francis Fultz look positively enamored of the Joshua tree by comparison, writing[iii]:
That’s some prose there, as purple as the evening sky above the Mojave Desert. But despite the worthy competition from Fultz, Chase and others, it’s Frémont’s slander of Joshua trees that has lodged itself in the skin of desert plant fans. It may be a combination of the hyperbole — restrained by comparison to Chase — and the perverse pride desert folk themselves take in insulting the landscape they love most. It may be the fact that the insult was part of the first quasi-scientific mention of the tree, or simply that Frémont got there decades before his imitators. Whatever the reason, Frémont insult has proven durable, immortalized in field guides and textbooks and park interpretive kiosks.
The “repulsive tree” line is also the most-quoted bit of the book in which it appears, with the ponderous 19th century title “Report of the Exploring Expedition to the Rocky Mountains in the Year 1842, and to Oregon and North California in the years 1843-44.” That in itself is fairly remarkable. In the same work Frémont — or his wife Jessie Benton, generally acknowledged these days as co-author — named both Pyramid Lake and the Great Basin, and established once and for all that no navigable river flowed through the deserts to the Pacific, dashing American imperial hopes of luxurious passage from Utah to San Francisco. The book is a foundational primary source in the history of westward American expansion, for good or ill. Yet it’s a nearly tossed-off line about an eccentric tree that people seem to quote most often.
Aesthetics are necessarily subjective, and even the most ardent fan of Joshua trees would be hard pressed to argue his assessment of the trees’ charms is necessarily wrong. But Frémont's previous sentence, about the trees being “suited well with the dry and desert region we were approaching,” is easier to argue with. As it turns out, Joshua trees may not well suited to the Mojave. The trees struggled to survive in the Mojave Desert even in the 19th century, before our monkeywrenching of the climate became apparent. And things are only going to get worse.
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The Mojave used to be different. Nowadays, the floors of Mojave valleys bear sparse covers of creosote bush, of globemallow and senna. Twenty-five thousand years ago, the same valleys were forested. Junipers and single-needle piñon pines grew nearly to sea level, along with oaks and manzanitas. On the slopes of mountains now cloaked in low blackbrush and iodine bush and Mojave yucca, there were white firs, limber pines, cottonwoods and aspens. The Mojave was dotted with freshwater lakes, some of them hundreds of feet deep, with broad rivers filling them. Elsewhere, this period is referred to as the Ice Ages. In the Mojave, there was precious little ice but a lot of rain, hence the scientists’ localized name for this period: the Pluvial.
An improbable bestiary roamed the landscape. There were familiar animals we don’t usually think of as desert dwellers: black bears, grizzlies, gray wolves and elk. There were also mastodons and mammoths. There were mountain lions, along with the now-extinct American lion and saber-toothed cat. There were camels and horses. There were short-faced bears, some of the most fearsome mammalian predators that ever existed, standing six feet tall at the shoulder and weighing about a ton, capable of running at more than 30 miles per hour for sustained periods — but which likely relied just as much on roots and berries as its surviving cousins. There were Shasta ground sloths, lumbering cow-sized animals with prodigious claws, which were probably used both for defense and to gather food.[iv]
Among this unfamiliar biological diversity, sprinkled into the vast forests of juniper and piñon were Joshua trees, known now to botanists as Yucca brevifolia, a.k.a the short-leaved yucca. (Some botanists have proposed assigning the eastern population of Joshua trees, which is smaller of stature and more freely branching, to the new species Yucca jaegeriana. Their arguments are compelling and the split is probably inevitable.[v]) Both the eastern and western Joshua tree types thrived in those wetter, cooler times as a minor component of that ancient Mojave woodland.[vi]
Today’s Joshua trees have a fairly restricted range, growing in an archipelago of isolated patches across the Mojave. During the lush Pleistocene, though, the trees grew across far more of the desert. At one point or another over that million-year period, Joshua trees grew near the Colorado River’s delta in present day Mexico, 300 miles from the southernmost Joshua tree today. They grew in Death Valley at around 200 feet below sea level, perhaps lower. (Imagine sitting near a Joshua tree on the forested shore of Lake Manly, which filled Death Valley hundreds of feet deep.) And at one point or another during the Pleistocene, they likely grew most everywhere between those two points.
About twelve thousand years ago, everything changed. North America’s continental glaciers started receding, allowing the polar jet stream to move northward. Less rain and snow fell in the Southwest. The Mojave started getting warmer. The desert’s rivers went dry. The lakes they had fed became salt flats. Those Pleistocene animals that could not adapt to the new Mojave moved away or died out or both.
Mojave botany changed as well. Those broad open forests died back, hanging on now in just a few favored places. The first creosotes appeared from warmer deserts to the south, and colonized the valley floors. The junipers and piñons that had carpeted the plains died out, their only surviving descendants springing from seeds that birds had fortuitously planted at higher, cooler altitudes.
And the Joshua trees that had grown among the piñons and junipers? They were left behind like a child trying to follow his older, faster siblings to the mall. They retreated upslope as well, but more slowly.
Now, the trees grow mainly on the slopes of desert hills above 3,000 feet in elevation, with a maximum range topping out at around 5,500 feet. There are exceptions in each direction. In the slightly less arid West Mojave near Palmdale, Joshua trees grow on the floor of the Antelope Valley at 2,300 feet, among junipers that are themselves incongruously close to sea level, a remnant of what Pluvial Mojave valley floors might well have looked like. Along the crest of the San Bernardinos at the southern edge of Joshua trees’ range, they grow as high as 7,000 feet.
Though they’re taken as emblematic of the Mojave Desert, they do grow outside what are generally considered to be the Mojave’s boundaries. The northernmost wild population grows within shouting distance of Goldfield, NV, just inside the Great Basin Desert according to most definitions. In the Virgin River Gorge, Joshuas grow out of Chinle formation red rock with junipers and piñons at the westernmost edge of the Colorado Plateau. They intersperse with saguaros and soaptree, Yucca elata, along the Joshua Forest Parkway outside of Wickenberg, AZ, as sure a Sonoran Desert landscape as exists. Just south of Gorman, California, a grove of Joshuas peeks out of a valley in the Tehachapis among coastal California vegetation. West of Ridgecrest, one can stand on the crest of the Sierra Nevada near Walker Pass, turn one’s back to a coastal flannelbush in full bloom, look westward toward the Pacific, and see clumps of Joshua trees seemingly heading down toward the Kern River Canyon and Bakersfield.
If Joshua trees do leak out of the strictly defined Mojave here and there, it is mainly just by a few miles. The heart of the trees’ contracted, post-Pluvial range is the core of the Mojave desert. And yet there is nothing preordained in this congruence of territory. The neat overlap of the range of Yucca brevifolia with the generally accepted boundaries of the Mojave Desert is an accident of evolutionary history. The trees live in the Mojave because of all the places their seeds were able to get to, that’s where those seeds survived and grew new populations of Joshua trees. And this accidental relationship is in danger of coming undone.
Every square mile of the American West has been well known for millennia by the landscape’s indigenous inhabitants. But the settlers who began arriving in increasing numbers in the 19th century were not often the beneficiaries of that native intelligence. Until late in the 1800s, when the US government began systematically mapping the interior west of the 100th Meridian, maps of the west often owed more to wishful thinking than to the surveyor’s craft.
The most notorious manifestations of this phenomenon were the West’s legendary navigable rivers, which boosters of Western Settlement hoped would, once found, make travel from the Rocky Mountains to the Pacific Coast a relatively sedate undertaking. An emigrant need only cross the Rockies, load his or her possessions onto a Mississippi-style steamboat, and then watch the scenery of the West roll by along the route to California or Oregon. Before long, the newly American west would consist of a network of increasingly prosperous river towns, each of them fed by a constellation of nearby farms along the lines of Jefferson’s agrarian dream.
Explorers’ observations of short stretches of river, misunderstood Native descriptions of the landscape, and sheer mapmakers’ fantasy combined to create a pantheon of illusory western rivers. The Multnomah was thought to drain a large part of the west, from the headwaters of the Rio Grande almost to the Pacific. (We know its more prosaic lower reaches as the Willamette River.) The Rio Timpanogos rose in Utah, then flowed more or less westward until it reached the Pacific in the vicinity of San Francisco. The Rio Buenaventura, generally assumed to have been inspired by a section of the Green River, was likewise said to flow westward from more southerly parts of Utah to the California coast, its mouth sometimes emptying into San Francisco Bay, and sometimes farther south.[vii]
By that day in April 1844 on which Frémont first saw the West Mojave’s Joshua trees, only the Rio Buenaventura still lingered in the hopes of partisans of Manifest Destiny. The others had succumbed to an overdose of contrary information.
Though reports by Jedediah Smith and other explorers had lessened hopes for the Buenaventura, it fell to Frémont to deal the mythical river its final death blow. In the April 14, 1844 entry in the Report of the Exploring Expedition, he wrote[viii]:
Frémont then offered a short description of a descent through a pleasant valley full of oaks, bunchgrasses and flowing springs, to be rudely interrupted by the sudden appearance of those most repulsive trees in the vegetable kingdom.
With that day’s entry, Frémont ended the settler nation’s collective fantasy of a languid, effortless migration of settlers from the interior to the Pacific. The journey west would be hard. Until the advent of the railroads a quarter century later, migrants would cross what seemed a deadly waste, the pathways soon marked by the cast-off possessions of previous, desperate travelers; the bones of their livestock; the increasingly frequent grave markers.
And along some of the southern routes, those migrants would struggle past stands of bizarre, contorted Joshua trees. Frémont had killed the landscape of hopeful American imagination, and in the next breath introduced a glimpse of the actual landscape that would greet the hapless settlers.
It would likely have been scant comfort to those settlers that the same heat and dryness that bedeviled them also pose a dire threat to the tree yuccas leering as they passed.
Migration is dangerous, and migrants enjoy no guarantee they’ll find a suitable new home at the end of their journey. It’s hard enough for animals to seek new habitat when the climate changes around them. But plants don’t even have the option of migrating. Instead, those plant species lucky enough that their ancestors have survived massive climate changes — namely, all of them — generally did so because those ancestors’ seeds found themselves in places where they could survive.
If you’re a plant, in other words, seed dispersal is key to survival of your species in the face of climate change. Some plants, like dandelions and tumbleweeds, rely on the wind to move their seeds from place to place. Others make their fruit attractive to birds, which eat and then fly away, scattering pre-fertilized seeds widely across the landscape. Those Pleistocene piñons and junipers enjoyed help from seed-dispersing birds in escaping the warming desert floors, their populations migrating not only up the mountainsides but to mountain ranges far to the north.
Joshua trees aren’t so lucky. Their fruit, the production of which requires increasingly rare cool wet springs, does not fall easily from the tree, occasionally hanging on for more than a year. That’s not particularly conducive to seed dispersal. Making things worse, the fruit’s juicy flesh surrounding their seed chambers turns, less than a month after ripening, into a tough, spongy fiber that resists opening to release the seeds. It’s almost as if Joshua trees do everything they can to keep their seeds from getting away.
Some scientists conclude that Joshua trees’ ancestors must have had a very good reason to clasp their seeds tightly to themselves, namely because the trees relied on animals that would find large clumps of juicy, ripe Joshua tree fruit, swallow those fruit with minimal crunching, walk off, and deposit nicely processed, mostly unchewed seeds in a pile of moist compost some distance away. Speculation as to that animal’s identity eventually settled on one primary suspect: the Shasta ground sloth, an eight-foot-long, quarter-ton relative of today’s rainforest-dwelling Internet memes.
The Shasta ground sloth, a committed herbivore, seems deliberately engineered to live on Joshua tree fruit. Its stature would allow the sloth to reach well up into the canopy of many Joshua trees, its massive tail stabilizing it tripod-style. The sloth’s formidable eight-inch claws, likely evolved as a defensive measure against saber-toothed cats and other predators, would have made handy grappling hooks to help bend the trees’ spiny branches toward the sloths’ mouth. Sloths were large enough that they could conceivably have swallowed the fruits whole, reducing chewing damage to the seeds within. And as the Shasta ground sloth ranged over much of southwestern North America, it was in the right place to be responsible for moving Joshua tree seeds around during the Pleistocene.
The story’s melancholy ending attracted a remarkable amount of attention in the late 2000s, at least as compared to the amount of press coverage paleoecological hypotheses usually attract. By about 8,000 years ago, the Shasta ground sloth was no more, the victim of climate change or hunting by humans or something unknown or a combination of all of the above. The Joshua tree was thus a sort of ecological widow, faithfully holding on to its seeds for millennia, waiting for a symbiotic partner that would never arrive. Deprived of sloths to move its seeds to more suitable locations, the Joshua tree would inevitably decline as climate change outstripped its ability to move its offspring north and uphill.
The story of Joshua trees pining away for their missing sloth partners is a tidy one, with a clear and compelling moral: extinction of a species can have long-lasting effects within the ecosystem the species once inhabited, the damage echoing down the halls of time for millennia. It’s a story that tells environmentally concerned people about the world as we think it must be. And so, because the myth is just too perfect, it might be vanquished by new research as surely as was the mythical Rio Buenaventura. Deposits of mummified ground sloth dung are more common in the desert than one might think: they often contain bits of Joshua tree leaves and stems, but remains of fruit are rarer. Nine feet in reach seems impressive, but Joshua trees regularly exceed 20 feet in height, and their fruit develop at the ends of the trees’ stout stems. In some Joshua tree forests, Shasta ground sloths would have been unable to reach the majority of ripe fruits.
It’s possible, in fact, that the primary disperser of Joshua tree seeds during the Pluvial is still busily dispersing those seeds today. The white-tailed antelope ground squirrel, a rodent ubiquitous in the Mojave that’s often mistaken for a chipmunk, is a voracious collector of Joshua tree seeds[ix]. The squirrels have no trouble reaching fruit on even the highest branches. They can easily chew through the overripe fruits’ spongy mesocarp, releasing the seeds. They then distribute many of those seeds into caches secreted around the landscape, and when they inevitably forget where some of those cached seeds are, those seeds can germinate and create a new generation of Joshua trees. Antelope squirrels are efficient enough dispersers of Joshua tree seeds that some models suggest they could account for the species’ present distribution all on their own.
That’s not a slam-dunk disproof of the sloth hypothesis. It’s possible that both animals played a role in moving Joshua tree seeds from place to place 15,000 years ago; sloths may have distributed seeds in ripe fruit, which could then have germinated within a day or so, while the remaining fruit then became spongy, protective covers that guarded the seeds until antelope ground squirrels could get to them.
Joshua trees might be suffering from the extinction of a major seed disperser. Or they might not. Either way, the Joshua tree seeds that do get dispersed are faced with an environment that is becoming increasingly hostile to their survival.
In order to sprout successfully, Joshua tree seeds need relatively warm soil temperatures, and enough water to keep their emerging roots moist for a couple of months. While winter rains are reasonably regular in Joshua tree country, Mojave winters are cold enough that few Joshua tree seeds will germinate. Soil temperatures in summer are, if anything, too warm, and any monsoonal rain that falls generally evaporates too quickly to sustain growing seedlings. That leaves spring and fall, and while the soil temperatures an inch or so down are generally just fine for Joshua tree germination, those transitional seasons aren’t generally when the Mojave gets precipitation.
It’s only during years with exceptional rainfall, generally during El Niño years, that the desert soil stays damp enough to afford Joshua tree seedlings the moisture they need to sink roots into the soil. That’s more likely to happen if the seeds germinate at the beginning of an El Niño season, in September or thereabouts, than if they sprout when soils warm up again in spring[x].
For the next two years, the baby Joshua trees look like nothing so much as delicate tufts of grass. Their leaves lack both the spines and the sharp edges that deter most herbivores from eating the foliage of mature trees, and most of them quickly fall prey to hungry animals.
Trees that have the good fortune to germinate beneath another shrub can be afforded some protection from marauding rodents and rabbits, as well as from the elements. Blackbrush in particular is a very common nurse plant for Joshua trees; it maintains an environment beneath its scratchy canopy that is moister and more nutrient-rich than that found beneath creosotes or other shrubs. Joshua trees do use other plants as shelter in their early years, creosote and big galleta grass and even chollas, but blackbrush seems to give them the best chance of survival. The plant forms a thick cover on desert slopes at the slightly cooler altitudes Joshua trees prefer[xi].
In a decade or so, if a seedling is lucky enough to survive the desert’s drought and scorching summers long enough to sink its roots well into the surrounding soil, it will emerge from its blackbrush shelter. Joshua trees of about this age have been known to flower[xii], but the vast majority will take another 50 to 60 years to reach maturity.
When they do, yet another important partnership between Joshua trees and other living things springs into action. The trees grow flower spikes from the ends of their branches; when the cream-colored flowers eventually unfurl, dozens on each flower spike, small, dusky yucca moths emerge from the desert soil and take flight. First the moths mate, and then the males become food for lizards. The females fly to a nearby Joshua tree flower and collect a small ball of sticky pollen. They then fly to another flower, inject a fertilized egg into the flower’s ovary, and then take a bit of the pollen and pack it into the stigma, where it can then fertilize the ovules to produce viable seeds.
The females work for a few days, collecting and distributing pollen and laying eggs, and then succumb. Within the developing Joshua tree fruit, five distinct seed chambers begin to develop. In one of those chambers, the yucca moth larva hatches from its egg and begins to eat. The seeds, little flat black flakes about the size of a lentil, grow to fill their chambers like stacked coins in a banker’s paper roll. The moth larva eats its way through one of the five stacks, chews its way out of the fruit and drops to the ground, then burrows into the soil to pupate.
In the other four chambers, the seeds develop in the ripening fruit, which then dries and turns spongy and we are back where we started, waiting for a sloth or a ground squirrel to move them around. The partnership between Joshua trees and their moths is one of the most-cited examples of symbiosis in the natural world[xiii]. The trees cannot make seeds if there are no moths. The moths cannot make caterpillars if there are no trees.
The Joshua trees’ other partnerships don’t get quite as much press. There is the extinct possible partnership with the sloths, to be sure, which has attracted significant attention. There have however been no front-page stories about the similar, actual partnership with antelope ground squirrels, or about the trees’ reliance on blackbrush nurseries. The continued existence of Joshua trees is totally reliant on yucca moths, almost as reliant on antelope squirrels, and substantially dependent on the continued existence of blackbrush stands.
Phrased another way, Joshua trees depend for their continued existence on a web of ecological relationships with other species. That web of relationships allows Joshua trees to just survive in the present-day Mojave. If you make the Mojave less hospitable, or threaten the relationships that allow the trees to survive, the trees may suffer greatly. If you do both, it’s even worse.
The near-continuous cover of blackbrush beginning at 4,000 feet above sea level in the Mojave that offers shelter to young Joshua trees is itself a relic of cooler, wetter times. Blackbrush requires a series of five or more cool, wet springs to germinate; conditions that are increasingly rare in the Mojave. Blackbrush is especially susceptible to wildfire. It burns to the ground and does not re-sprout. One study has suggested that a solid stand of blackbrush may take tens of thousands of years to recover after a fire, if it ever does[xiv]. With fires becoming more frequent in a desert landscape that once burned only rarely, Joshua trees may well lose their main nurse plant habitat for good.
Fire is a direct threat to Joshua trees as well. In much of the tree’s range, fires were historically uncommon. Tree mortality after a fire is startling: as many as 80 percent of trees with burn damage can die within the following five years, and subsequent fires thin the ranks even further.
On the western edge of the Mojave, where fires were more common, Joshua trees seem to reproduce by underground shoots more often, often forming thick clumps. These clumps do re-sprout readily after a fire. (Reproducing readily by cloning themselves, these trees are also less reliant on yucca moths for long-term survival.)
But throughout the remainder of the range, fire poses an existential threat to Joshua trees.
Antelope ground squirrels are one of the most common rodent species in the Mojave, and they almost certainly aren’t going anywhere. As the desert’s droughts become more intense, however, they may begin to harm Joshua trees more than they help by dispersing seeds. During pronounced drought, desert herbivores often seek sources of moisture within the stems of Joshua trees, often stripping the outer “bark” (more properly called periderm) to reach fluid-filled tissues beneath. One study in Joshua Tree National Park found that pocket gophers did considerable harm to individual trees, often hollowing out trunks and completely stripping the periderm from the lower parts of the trees. Antelope ground squirrels don’t do as much damage as gophers, but they have been known to climb trees and strip the periderm from upper branches, an act that can easily contribute to the dieback of those branches. As Joshua trees flower at the terminal bud on the ends of their branches, killing off branches impedes the trees’ ability to flower and set seed for antelope squirrels to disperse.
Damage to a tree’s surviving periderm is especially harmful when the tree has been damaged in a fire, and fire may well increase the chances of herbivores targeting those trees if other plants sources of moisture have been destroyed[xv].
Most mysterious of all are the likely effects of climate change on yucca moths, about which we know relatively little. The moths emerge from their underground cocoons, sometimes several years or even decades after the larvae burrow into the ground, just as local Joshua trees’ flowers are ready to be pollinated. But biologists are at a loss to explain how the moths know when to emerge. Do the trees’ roots release a pheromone that triggers moth emergence? Is the timing a simple matter of soil temperature and moisture over the months previous? If the latter, a changing climate may throw moth and tree out of sync, with disastrous results for both. Higher temperatures may also hurt moth larvae as they develop within the trees’ fruits. One recent study of trees and moths in Joshua Tree National Park found that the partnership between moth and tree essentially falls apart at warmer, lower elevations, with very few moths found in Joshua tree groves at the lower limits of the tree’s altitudinal range. As the desert warms, that zone of dysfunctional relationship may expand uphill[xvi].
And of course, increasing temperatures will likely reduce the number of cold, wet Mojave winters that trigger Joshua tree flowering in the first place.
The range of possible existential threats climate change poses to every stage of the Joshua tree’s life cycle has raised alarm among scientists. In 2005, biologists Ken Cole and Kirsten Ironside co-authored a poster projecting that a warming climate would drastically shrink the extent of suitable habitat for the Joshua tree by 2100, essentially extirpating the tree from not only Joshua Tree National Park, but all of its range in the San Bernardino and San Gabriel Mountains, the southern Sierra Nevada, and Utah and Nevada.
Cole and Ironside’s projections got some pushback from biologists who suggested that they were based on incomplete data, and that Joshua trees might persist in higher-elevation pockets of the trees’ namesake National Park. In 2011, Cole and Ironside published a paper[xvii] that accounted for some of the previous criticisms, but underscored the basic conclusions of the earlier study. Joshua trees are likely to be absent from up to 90 percent of their current range by the end of this century, with remaining populations much more sparse than they are today. The study identified areas to the north of the Mojave that are likely candidates for artificially established Joshua tree populations. Whether such an effort could be successful long-term depends on whether we find a way of transplanting moth populations as well.
The mounting evidence of threats to the Joshua tree were enough to prompt the environmental group WildEarth Guardians in 2015 to petition the U.S. Fish and Wildlife Service (USFWS)[xviii] to list the Joshua tree as Threatened under the U.S. Endangered Species Act. A year later, USFWS found that listing the species might be warranted, and embarked on the process of determining whether or not to do so. That decision is still pending, three years past the legal deadline. It is unlikely in the extreme that the current administration will decide in favor of listing the tree. A delay past 2020 is not necessarily a bad thing.
All in all, the Joshua tree is becoming less well-suited to the increasingly hot and dry desert landscape it currently inhabits. Its future may well lie on the margins of that desert. South of Gorman, the westernmost grove of Joshua trees sits just south of the San Andreas Fault, approaching the Pacific Ocean at about an inch per year, on average. It is a tightly packed thicket of clonal sprouts which may all have originated from the same seed, or just a handful of them, some tens of thousands of years ago. It has traveled about a quarter mile since the last Shasta ground sloth died. It is now abundantly re-sprouting from a fire a dozen years ago[xix], better-adapted to life in the fire-prone coastal mountains than its relatives farther into the Mojave.
West of Walker Pass, similar thickets of trees grow at roadside and in cow pastures between the Sierra Crest and Lake Isabella. A climate-changed southern Sierra may prove to be surprisingly hospitable to fire-adapted clumps of Joshua trees, with its projected hot, droughty summers and rainy winters.
Given this slender reed of hope, perhaps a bit of deliberate optimistic mythmaking is in order. But imagine those Sierra Nevada clumps of Joshua trees persisting, growing by cloning themselves, flowering often enough to maintain a population of yucca moths. Every so often, a fruit with viable seed offers itself up to the local antelope ground squirrels. Every so often, one of those fruits rolls downhill and to the west. In time — a long, long time, well after the last industrial farm is long forgotten and the freeways are fossilized outcrops — clonal clumps of Joshua trees appear at the mouth of the Kern River canyon on the floor of the San Joaquin Valley. They populate the southern Sierra foothills. Moth larvae delve the deeper soil of the Valley. San Joaquin antelope squirrels decide to cache Joshua tree seeds.
It’s a long shot. But while we’ve raised the stakes considerably regarding their success in migrating, the trees were already heading in that direction. Little did Frémont know, when he encountered those grotesque representatives of the vegetable kingdom, that he was merely passing them while they headed the other way.
Read about Diane Best’s landscape projects in this KCET Artbound feature by Kim Stringfellow. To view more of Catherine Ruane’s drawings and art, click here. Visit Juniper Harrower’s website to learn more about her work and research including a 2018 Ecosphere paper co-authored with Gregory S. Gilbert exploring the mutualisms of the Joshua Tree and the yucca moth within a shifting climate.
[i] John C. Frémont. “Report of the Exploring Expedition to the Rocky Mountains in the year 1842, and to Oregon and California in the years 1843-’44.” Accessed September 1, 2018.
[ii] Francis M. Fultz. “The Lilies of the Field; Beautiful and Striking Wild Lilies of California’s Fields,” Scientific American Supplement, Vol. 88 No. 2275; August 9, 1919.
[iii] J. Smeaton Chase. California Desert Trails, Houghton Mifflin Company, 1919.
[v] Lee Lenz, "Reassessment of Yucca brevifolia and Recognition of Y. jaegeriana as a Distinct Species," Aliso, Volume 24 Issue 1, 2007.
[vii] A compelling description of these mythical rivers, and the process by which they were proven not to exist, can be found in Part IV, Chapter 4 of Wallace Stegner’s Beyond the Hundredth Meridian (Penguin Books, 1954).
[viii] Frémont, “Report of the Exploring Expedition to the Rocky Mountains in the year 1842, and to Oregon and California in the years 1843-’44.” Accessed September 1, 2018.
[ix] Ben A Waitman et al. “Seed dispersal and seed fate in Joshua tree (Yucca brevifolia),” Journal of Arid Environments, Volume 81, June 2012.
[x] M Bryant et al.“Short Seed Longevity, Variable Germination Conditions, And Infrequent Establishment Events Provide A Narrow Window For Yucca brevifolia (Agavaceae) Recruitment,” American Journal of Botany, October 2012.
[xi] Steve Brittingham and Lawrence R. Walker.“Facilitation of Yucca brevifolia Recruitment by Mojave Desert Shrubs,” Western North American Naturalist, Vol. 60 No. 4, October 2001.
[xii] Todd C. Esque et al.“Direct and Indirect Effects of Environmental Variability on Growth and Survivorship of Pre-Reproductive Joshua Trees, Yucca brevifolia,” American Journal Of Botany, January 2015.
[xiii] The yucca-yucca moth relationship was first described in 1872 by pioneering entomologist Charles Valentine Riley, and has since become a literal textbook example of symbiosis.
[xiv] Robert H. Webb et al,“Dynamics of Mojave Desert Shrub Assemblages in the Panamint Mountains, California,” Ecology, June 1987.
[xv] Lesley A DeFalco, et al. “Desert Wildfire and Severe Drought Diminish Survivorship of the Long-Lived Joshua Tree” American Journal of Botany, 2010.
[xvi] Jennifer Harrower and Gregory Gilbert. “Context-dependent mutualisms in the Joshua tree-yucca moth system shift along a climate gradient,” Ecosphere, September 2018.
[xvii] Kenneth L. Cole et al. “Past and ongoing shifts in Joshua tree distribution support future modeled range contraction,” Ecological Applications, 21(1), 2011.
[xviii] Wild Earth Guardians. “Petition to List the Joshua Tree (Yucca brevifolia) Under the Endangered Species Act,” September 2015. Accessed September 1, 2018.
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