James W. Cornett
Our Colorado Desert – which stretches from the Colorado River to Palm Springs and south into Baja California – is an arid, low-lying region of shifting sand dunes, stony flats and sparsely vegetated hillsides. In June and July, temperatures greater than 110 degrees Fahrenheit are expected and average annual rainfall barely exceeds three inches. It is one of the last places on the North American continent where one might expect to find verdant groves of palm trees, yet desert fan palms (Washingtonia filifera) do exist here.
The trees thrive near the occasional stream, spring or seep and sometimes grow to more than 75 feet with a trunk more than 40 inches wide. Their stout trunks are typically shrouded by ground-length skirts of dead fronds though sometimes these coverings have been burned away. The species is the only endemic palm in the western United States and the most massive in all of North America.
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These palm oases – “tropical islands” often tucked away in such places as Lost Palms Canyon, Hidden Spring and Surprise Canyon – had piqued my curiosity since high school when I first visited Palm Canyon in what is now the Indian Canyons Tribal Park. I wondered how the palms established in such out-of-the-way places. Were the trees ancient residents or new arrivals? And what determined the size and vitality of the groves?How did they spread from one distant spring to another, if they indeed did?
With the aid of an initial grant from the R. K. Mellon Foundation and later the Garden Club of the Desert, my co-workers and I embarked upon a project that would last three decades and continue to the present time.
The desert can be a lonely place in which to conduct ecological studies, but to our delight, we were seldom alone in an oasis. These verdant outposts attract an abundance of birds, which come to feed, drink and rest among the palms. More than 80 species use these “pit stops” on their migratory journeys. Some of our more startling sightings included snow geese, American avocets and belted kingfishers. Nearly two dozen other species are known to nest in the oases, and for most of them there are no other satisfactory nesting sites for miles around. Mourning doves, owls, roadrunners and house finches frequently rear their young in the thick skirts of the palms. Brightly colored hooded orioles use palm leaf fibers for their basketlike nests, which they attach to the underside of a palm leaf.
Oases also attract many kinds of mammals. Where there are palms, there is often drinking water nearby, and the trees’ great height – the equivalent of a six- or seven-story building – can serve as distant visual signals to birds, coyotes and bighorn sheep. Rams and ewes are occasionally seen near oases and their droppings are regularly observed. Insect-eating western yellow bats are most often found in and around palms oases where they roost under the trees’ skirts during daylight hours. The ubiquitous raccoon, though not really a desert animal, has been able to disperse into the largest of these remote sites. In several cases racoons crossed miles of inhospitable desert and then became trapped in the island-like environment of an oasis. Coyotes are the most conspicuous large mammals drawn to palm oases. Because they gorge themselves on palm fruits, they visit every grove, even those where drinking water is not available.
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Healthy, mature fan palms produce up to a half million fruits each fall. The pea-sized dates possess a sweet-tasting flesh, similar to commercially marketed dates.Unfortunately for humans, the seed occupies most of the fruit and is rock-hard. Coyotes readily consume fallen fruits but can only digest the scant flesh and pass out dozens of intact seeds in their droppings. In the late fall and winter, when the palms are in fruit, coyote droppings often contain dozens of palm seeds. We were surprised to learn that not only did these seeds germinate after their intestinal travels, but they did so in less time and with twice the success of unconsumed seeds. Coyotes are also known to travel up to 36 miles in three days, probably the maximum time palm seeds can remain in the digestive tract of any canine. Thus, coyotes may be the most important dispersal agents of the palm’s seeds and could have transported them to almost all the known fan palm sites.
There are, however, very remote palm oases far beyond the absolute maximum range of coyote transport. How did the palms reach Corn Springs 100 miles east of Palm Springs or the Kofa Mountains in western Arizona? The available evidence indicates two possibilities. The seeds were transported to the remote locations by either migrating birds that had ingested seeds or carried there byHomo sapiens, the greatest seed dispersal species the world has ever known.
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At the Living Desert in Palm Desert, biologist Vic Rohrback and I spent 14 mornings over a two-month period watching to see which, if any, migrating bird species consumed entire fruits and flew away without disgorging the seeds. Several migrants did this including hermit thrushes, northern flickers and American robins. It was the latter species, however, that performed the task with the greatest frequency. Robins are well-known long-distance migrants leading us to conclude many remote oases could have been initiated by robins voiding out palm seeds as they interrupted their migratory journey to drink from a desert waterhole. But nearly all migrating birds fly north to south, not east to west. How could we explain the presence of the remote eastern palm oases?
As mentioned previously, humans – not birds – are the best seed dispersers in the world. Any examination of odd plant distributions must consider the involvement ofHomo sapiens. With the help of archaeologist and former Palm Springs resident Jim Toenjes, we visited historical sites and examined written records, particularly those involving the Cahuilla Indians. There is abundant evidence the Cahuilla and neighboring tribes either permanently or seasonally occupied nearly all the 168 palm oases known to exist as of 2018. Today visitors can view ancient rock art at Corn Springs east of the Coachella Valley or place their hands in bedrock mortars in Palm, Murray and Andreas canyons in the Indian Canyons Tribal Park. Palms were important to the Cahuilla because they could use the dead fronds to thatch their houses, harvest and grind fruits into flour and modify the spine-covered frond stems into weapons. In conversations with early anthropologists, Cahuilla elders related how they both transported and planted palm seeds at desert springs and seeps to insure palm resources would be available at as many locations as possible.
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Since desert fan palms usually survive wildfires, a fact known to the Cahuilla, controlled burning in palm oases was practiced. Burning enhanced the survival of palms by killing some of the wood-boring insect larvae. More importantly, fire increased fruit production. We confirmed the latter information by comparing burned and unburned oasis palms and found that burned palms produced nearly twice as much fruit. We concluded the Cahuilla and related groups may very well be responsible for planting palm seeds at many, even most, of the locations where palms occur today.
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Genetic evidence supports the “most” part of the conclusion. Leroy McClenaghan, professor of biology at San Diego State University, found that palms were genetically similar regardless of whether they were in the same oasis or in distant, remote oases. Such genetic similarity is best explained by all palms having the same, relatively recent ancestor. This is the kind genetic pattern one would see if seeds had been collected in a small source area (somewhere on the Baja Peninsula where the desert fan palm first appeared as a distinct species) and then within a few thousand or even hundreds of years, distributed the seeds into a larger geographical region – the California deserts. This and other lines of evidence strongly suggest Cahuilla Indians and their ancestors played a central role in bringing palms into California and the Coachella Valley.
It may very well be that Cahuilla ancestors brought palms to Palm Canyon, the largest natural desert palm oasis in the world and the focal point of the renowned Indian Canyons Tribal Park.
James W. Cornett is a desert ecologist and author with more than 40books published as of 2018.