Did Native Americans introduce Fan Palms to California?
How "wilderness" and "native ranges" can be human-inflected
This post is from 2017, and is still one of my favorites. For decades, it was believed that California Fan Palms were a “relict” species, many millions of years old, left over from when the region was a moist jungle. However, one dedicated biologist (with help from his friends) discovered that this wasn’t true, and that the species was much much younger. It’s a fascinating story that reads like a mystery.
It seems apparent that the species was introduced into southern California by Native Americans. If true, this raises questions about how to conserve / restore “wild” ecosystems that are actually the result of historic indigenous land management practices.
All photos are taken by me in Anza-Borrego State Park in San Diego County, California.
I recently spent some highly enjoyable time with a friend exploring groves of Desert Fan Palms in Anza-Borrego Desert State Park in San Diego County. Tucked into canyons in the foothills of the mountains, these striking trees seemed out of place. Nothing else is as tall and lush as they and the cool microhabitat made by their shade provides a welcome escape from the sun and heat that feels almost magical. I wanted to know more about them.
So I was thrilled when I found a book called "Desert Palm Oasis" (Nature Trails Press, Palm Springs, CA: 2010). The author is James W. Cornett, an expert on Desert Fan Palms who spent over 25 years studying them and who published more than three dozen technical papers about them. The book is short, at only 75 pages, but lushly illustrated with the author's photographs, and is dense with information. Topics covered are lifecycle, ecology, animal associations, use by Native Americans, and more.
Much to my surprise, he also overturned the conventional wisdom on the history of Fan Palms and their origins, and that's what fascinated me the most.
The botanical name for the Desert Fan Palm is Washingtonia filifera, which inexplicably honors George Washington, though he had no known relationship to the plant whatsoever. There are at least 168 groves, located mostly in the state of California in the US and the state of Baja California Norte in Mexico, with a small number in Arizona and a handful in Nevada.
For the last fifty years, the conventional explanation for the origin of the Fan Palm has been that it is a "relict," which is to say an ancient species leftover from an earlier time. In this case, the tree had supposedly grown in the area for millions of years, since what is now desert was moist jungle. This theory was originally popularized by two reseachers, Richard Vogl and Lawrence McHargue, with an article in Ecology magazine published in 1966. They also stated that the species was endangered by ground water depletion from urban development.
Vogl and McHargue had in turn borrowed the relict concept from David Axelrod, a highly respected paleobotanist (one who studies ancient plants), who had collected and identified hundreds of fossils of palm trees in the western US claimed by Axelrod to be in the genus Washingtonia. This single specimen was reputedly at the University of California at Berkeley but in 1985 when Cornett went to inspect it by appointment, it could not be found, nor did its ID number match their system's format. So Cornett visited Axelrod himself, who, in Cornett's words, "was unable to provide information concerning its whereabouts and became angry when I asked why he felt the desert fan palm was a relict species when only a single fossil existed (a fossil no one could locate). His face flushed red and he demanded I leave his office at once."
Cornett knew that without fossils, there was no evidence that W. filifera had existed in prehistoric times. In fact, he writes: "The available evidence points to an opposite conclusion—the desert fan palm is a recently evolved invasive [sic] species that is rapidly expanding both in range as well as in numbers." What available evidence does he present? Read on...
In the mid-1980's, Cornett led an effort to census all known fan palm oases. Having not visited Axelrod yet, he was working under the assumption that they would find the species in decline, as would befit a relict. For historical numbers to compare with, he drew on three different sources:
location names (such as the town of Twentynine Palms and the oasis of Fortynine Palms),
old photographs
counts made by Randall Henderon.
Henderson was the publisher and editor of Desert Magazine from 1937-1958 who, for whatever reason, made it a habit to record the number of trees in every palm oasis he visited, and he visited as many as he could over those decades. Apparently he was "meticulous in his counts, even using a mechanical counter so that he would not lose track."
Going from these three sources of information, Cornett and his team found that the number of trees at each oasis had generally increased by 20, 30 or 40%. In some cases the increase was smaller (10% or single digits) and in a few much higher (84% at the Hellhole Grove and 311% at Andreas Canyon). Said Cornett about their findings:
"The trend of increasing numbers of palms continued in the overwhelming majority of oases for which we had Henderson counts. Our evidence seemed conclusive. Palm numbers were increasing significantly.... In spite of drought, groundwater pumping and the introduction of exotic weed species, palm numbers continued to increase."
This is not the behavior one would expect in a relict species that has outlived its original ecosystem. And not only should the numbers of palms be shrinking but the number of groves, too, but Cornett also found evidence for the opposite—that the range of the plant is expanding both north and west from its central concentration in the Salton trough and Baja California. This evidence included:
The lack of older trees and stumps at the farther flung groves.
The absence of the Palm Borer beetle, an insect that is dependent almost exclusively on W. filifera, in groves farther out. If the palms had once been more widespread but had since retreated, one would expect the beetle to be present everywhere.
Genetic evidence: In the mid-1980's, Cornett was contacted by a biologist who was comparing genes of palms from different oases and made a discovery that made no sense with a relict species:
"There was more genetic divergence between palms in the same oasis than between palms in different oases. This was the exact opposite of what one would expect. Oasis palms should be most closely related to the palms in their own oases, the ones from which they are most likely sired through pollination. They should be less related to palms far away in distant oases with whom cross pollination would be rare if not impossible.
"It was at this moment I had an epiphany... The palms showed a lack of genetic diversity because they had only recently broken out of isolation in the Baja Peninsula, the likely place of origin of the genus Washingtonia.
"The palms had not had time or opportunity to diverse and so, genetically speaking, were all close relatives—as if they had been separated for only a relatively short time (probably less than a few thousand years)." [my emphasis]
So how did they travel north from Baja California?
Cornett looks at this question closely, too, starting with animals. Many species of plants are dependent on animals to spread their seed, either by hooking it into their fur or skin or by enticing them to swallow it by enclosing it in fruit. In the latter case, the seed is defecated out later, some distance away. Some seeds even need to pass through the digestive tract of an animal before they will germinate. Though the fruits of the Fan Palm are slightly sticky, they are not especially so, and would not hang on to anyone reliably or for long. The seeds themselves are entirely smooth but the fruit is sweet and is part of the diet of many creatures.
Rodents? Plenty of rodents are found in the desert and at the oases, including various mice, squirrels and the idiosyncratic Kangaroo Rat, but the seeds are too large for them to swallow.
Birds? Over years of careful observation, Cornett saw at least a dozen avian species eating the fruit, but most either pecked off the sweet flesh or spat the seeds out on the spot. He witnessed four species swallow the whole fruit: American Robins, Hermit Thrushes, Northern Mockingbirds and Cedar Waxwings. All of them except the Mockingbird are migratory so they are potential candidates for long-distance dispersal, but—Cornett was careful to note—that it happens this has not been confirmed.
Coyotes? These canines eat lots of Fan Palm fruit and defecate out the seeds whole. In the laboratory, coyote-treated seeds germinated better than ones that had only fallen from the tree. According to veterinarians with whom Cornett consulted, a canine species like the Coyote can go no longer than 36 hours without defecating. Studies of Coyote travel have found the maximum daily distance to be 11 miles, meaning they could carry seeds as far as 16 or 17 miles. However, there are Palm oases that are farther away from each other than this, so Coyotes can't explain them all.
Humans? On this subject, Cornett discusses several oases, including the ones in Death Valley, that were either established by modern Americans or were seeded by animals from trees planted at modern settlements. Interestingly, he does not mention Native Americans.
The Fan Palm was of vital importance to the Native Americans of the southwest. Not only was the fruit an important food source (which they ate fresh, sun-dried and ground into flour for porridge) but they used the leaves for roofing their homes, the stalks for making tools and the fibers for weaving basketry. Contemporary visitors to Palm oases can still see smoothed indentations in granite rocks that were used for processing. Pottery shards and other signs of human habitation have been found at the majority of groves. Not surprisingly, the tree figures large in tribal mythology.
I am not surprised that Cornett, who is careful not to make statements without an abundance of incontrovertible evidence, is unwilling to make the final leap and surmise that it was Native Americans who spread the Fan Palm into California from its birthplace in the Baja. However, he does come right up to the line when he says:
"It appears that Washingtonia filifera is an excellent example of a plant species whose ecology can best be understood in light of its interaction with Native Americans. Indeed, in many instances it appears that Indians have taken this species to the geographical limits of its range. Randall Henderson speculated that Indians also planted palms at one of the highest altitude oases, Dos Palmas Spring in the Santa Rosa Mountains of Southern California. In addition, Indian-initiated fires likely maintained palms in far greater numbers than would have occurred under natural conditions. It is not inconceivable that the entire status of palms in the Sonoran Desert of southeastern California might have been vastly different were it not for the Indians' use and management of this spectacular desert plant."
Native Americans moved other plants around the New World, and over longer distances than from Baja to southern California. Maize, beans and squash were all first bred in Central America but had made their way to New England well before the arrival of the Pilgrims, with whom the Native Americans shared them. Smoking Tobacco (Nicotiana tabacum) is from South America (where, modern genetic studies have shown, it was the result of hybridization between two other species).
Pyroculture
It is well-established that the Native Americans set fires in the groves on a regular basis (i.e., at least every decade or two in some cases, and yearly in others) because post-fire trees produce more fruit (as modern study has confirmed) and burning away the skirts makes harvest easier. Fan Palms reproduce by seed better after a fire. The ground is cleared of competing undergrowth and dead Fan Palm detritus, the canopy is opened to let more sunlight down the ground, and ash is a natural fertilizer.
Fan Palms are highly-resistant to fire because, unlike most trees, they do not transport moisture from root to crown though a thin layer just under their bark, close to the surface, but through vascular tissues in the interior of the trunk. For this reason they also lack rings. Other desert plants, being far more sensitive to fire, were removed from the groves by burning, which resulted in Fan Palm dominance, an advantageous situation for the Native Americans. Doubtless, the burning event was carefully timed and monitored.
Cornett writes:
"Oasis photographs taken prior to 1936 usually reveal palms that had not been burned in more than fifty years. These same photographs, along with a few early counts of oasis palms by Randall Henderson, indicate palms may have been declining in numbers prior to World War II. If this is true, it may reflect the disappearance of Indians (as a result of the introduction of European diseases and forced migrations) from oasis environments in the late 1800s and the cessation of regular burning. After 1945, large numbers of settlers and tourists coming into southeastern California resulted in more frequent visitation and a return of regular fires to palm oases."
What now?
So, given that the existence of the Desert Fan Palm was so closely intertwined with Native Americans that a) it may well have suffered in their absence, and b) it might not have even been in California without their introduction, what are the implications for current conservation efforts? Contemporary conservationism usually focuses first on discouraging human interaction: don't pick, don't plant, don't feed, etc. Just stay out! Given the track record of the European invaders of this continent since 1492, such policies seem prudent, even imperative in many cases.
But while this approach might make sense with purely wild plants and ecosystems that did not evolve with human interaction (if such places exist), does it make sense to treat plants and ecosystems that clearly were dependent on human interaction the same way? What, in that case, are we preserving? Something akin to a ruin, I would say, or an abandoned agricultural project.
At the heart of the global ecological crisis is the lack of a healthy relationship between humans and the earth. Ultimately the only solution is for us to collectively foster a healthy relationship again. This will require hands-on interaction: picking, planting, feeding, etc. Diving in! The Desert Fan Palm oases offer an opportunity for those activities. The most logical approach, obviously, is for Native Americans to take the lead on that project and I hope that some of them would be interested in doing so. I also hope they would be willing to take on students from among the European colonizers (but I would understand if they refused). Currently, the traditional ways of tending the Fan Palm groves are illegal, so exceptions would need to be made for these experiments to take place.
Tending the wild was the way our species lived for many tens of thousands of years before agriculture. We gathered (mostly) and hunted (some) and in so doing were just one set of creatures interacting with a multitude of other creatures on a marvelous planet. Bringing this past forward is the only way we will be able to realign ourselves with life again. The Desert Fan Palms are there, waiting for us, with that invitation held forth. I pray that we accept it.
Wow, outstanding and interesting. Thanks for sharing.
I always learn so much when I read your posts, Kollibri. Thank you!