Plant Species of the Borrego Desert: Burseraceae: Bursera microphylla, elephant tree
Introduction, Origin and Meaning of Name
The Elephant Tree, Bursera microphylla, is one of the signature plants of the Sonoran Desert. Shreve (1964), in his Vegetation and Flora of the Sonoran Desert, says:
In its best development this is one of the handsomest and most striking trees of the Sonoran Desert.The distribution of Bursera microphylla is almost coincident with the extent of the Sonoran Desert (see map below). The tree is absent only from the most northern and eastern parts of the desert, and extends beyond it only in the Cape Region of Baja California. It is uncommon north of Rosario and the Rio Magdalena, and shows its best development as to size and numbers in central Baja California and central Sonora. No pure stands have been seen, and there are rarely more than 8 or 10 trees to a hectare [3-4 per acre].
Almost all plants in the Bursera genus are tropical or subtropical; only B. microphylla and a few sister species adapted to desert environments. The distribution of B. microphylla is primarily controlled by frost. Janice Emily Bowers, in Shrubs and trees of the Southwest Deserts, writes:
Frost frequently prunes the top branches, ensuring that elephant tree seldom grows more than five or six feet tall. When winter temperatures fall to 21° F or lower, the shrubs are killed to the ground.
Thus in the Borrego Desert, at the northernmost part of its range, it only grows in the warmest microclimates, on well-drained slopes at fairly low elevations with the warmest night-time temperatures.
The Elephant Tree stores water in its very thick trunk and lower limbs, which resemble the legs of an elephant, giving rise to the common name. This water accumulation allows it to survive over a year without any rainfall (Turner, Bowers and Burgess 2005, p. 128). However, the twigs are not thickened and its leaves are only about an inch long, both completely out of proportion to the rest of the plant. The species was named for its tiny leaves; microphylla = small leaves. Its genus name, Bursera, honors botanist Joachim Burser (1583-1649).
Frankincense, Myrrh and Kelewat
The Elephant Tree is a member of the Torchwood Family, whose scientific name of Burseraceae comes from the genus name. Frankincense and myrrh are in the same family. Humans used the Elephant Tree in America in the same way as frankincense and myrrh were used in the Middle East, in sacred and medicinal roles. Lindsay (2001, p. 154) writes:
Cahuilla Indians called the Elephant Tree kelawat eneneka and believed that the sap, which bleeds red like blood, had great power and was dangerous to be kept in the open. It was always hidden and used by tribal shamans in curing skin disorders and other diseases.
The tree has a wonderful fragrance that persists on your hand for some time after you touch it. It is difficult for me to describe what it smells like, since it has an odor unlike anything I have previously smelled. Lindsay says it resembles cedar or a cross between pine needles and orange peels.
The resin from the dried sap is burned as incense, or used as cement and as a base for varnish (Lindsay 2001, p. 154).
History of Discovery in California
Bursera microphylla was first named by Asa Gray in 1861 from a Mexican specimen.
Edward H. Davis first vouchered it in California in 1911 from between Fish Creek and Carrizo Creek. One of his specimens consists of just seeds; another of (probably) just seeds along with two photographs!
Another voucher was taken in 1930 by H.T. Rogers with the same locality. Vouchers were also taken in 1932 from canyon due s Indian Gorge; twice in 1934 from 7 mi NW. of Gypsum Mine. Imperial Co. and region near Gypsum Mine Fish Cr. Mts., and in 1936 from near gashills. tanks..
Despite these six vouchers from five different years, several sources (e.g. Lindsay 2001), a number of websites (e.g., DesertUSA, and the sign at the turnoff to the trailhead), say that the Elephant Tree was "lost to science" and that there were people who "doubted its existence". This, of course, could certainly have happened, since herbarium specimens in the pre-digital era were hard to discover, especially ones at far-away UC Berkeley.
However, you would think that a tree that is typically 5-10 feet tall and 8-30 feet wide, with massive trunks and branches that resemble an elephant's legs, would have a hard time getting lost to humans. But the locations in California are pretty far off the beaten path. Even now, Schad (1998) says that the Elephant Tree Natural Area is in the heart of one of the least-visited areas of Anza-Borrego Desert State Park.
Lindsay writes that a voucher in January 1937 led to an expedition to hunt for elephant trees, which located 75 specimens in the Fish Creek area. Clearly there was a lot of interest in those trees at that time; the Consortium of California Herbaria lists eight specimens from November and December 1937; two more in 1938; and five more in 1939.
Interestingly, two of the 1939 specimens were from Martinez Canyon, Santa Rosa Mts. in Riverside County, and apparently that location got forgotten, too, for many people even now (e.g., Cunningham and Cunningham 2006).
Distinguishing Characteristics and Similar Species
All you have to do is look at the plant to reliably identify it in the Borrego Desert. No other species looks remotely like it, with its intricate branching from a massively wide trunk.
Pictures of Young Plants, Mature Plants, and Dead Plants
The pictures in the set below were taken on 1 December 2009 in the Elephant Tree Area southwest of Ocotillo Wells, 87 days after a summer thunderstorm on 5 September 2009 stimulated mature plants to leaf out, flower and fruit. The trees are growing on an alluvial fan.
Pictures of a mature specimen in fruit:
Picture of a dead specimen:
Pictures of two young specimens:
The two pictures below show another habitat for these trees, on steep south-facing rocky slopes in Alma Canyon just below Starfish Cove. The pictures were taken on 27 December 2009. This area also benefited from the monsoon rain on 5 September 2009.
Also see the pictures in The Elephant Trees of Clark Valley, showing trees that had yet to leaf out as of 19 December 2009; Martinez Canyon; and flowers.
Habitat, Distribution and Abundance
In the Borrego Desert area, Elephant Trees grow in three quite different habitats whose common denominator is probably simply that the yearly minimum temperature is high. The main populations are listed below from north to south.
The Martinez Canyon plants, on the north side of the Santa Rosa Mountains, apparently grow in an old alluvial fan just above the canyon bottom, probably at around 650 feet elevation, on the south-facing slopes above the east-flowing drainage. The alluvium consists of large rocks plus finer material. One hike description says this is a small grove.
The Elephant Trees of Clark Valley are at much higher elevation, on very steep southwest-facing cliffs made primarily of large boulders, at 1600 to 2000 feet elevation. The population is estimated to be over 160 (Lindsay 2001, p. 154.)
The Elephant Tree Area in Alma Canyon and Alma Wash, about ten miles southwest of Ocotillo Wells, has the largest population of almost 2,000 trees (Lindsay 2001, p. 154.) These trees grow in the alluvial fan down to just 400 feet elevation, but increase in density at higher elevations on the fan. They are found just above the steep canyon walls of Alma Canyon to at least 1400 feet elevation in older alluvium, on both north and south-facing slopes. The Alma Canyon and Wash drainage is primarily to the east.
The only other significant population is at Torote Bowl and Torote Canyon, south of the Borrego Desert in the Indian Gorge / Bow Willow area.
The distribution of vouchers in California from the Consortium of California Herbaria are shown in the following map, augmented by the Martinez Canyon and Clark Valley locations plotted by me (pink disks):
The approximate coordinates for the Martinez Canyon and Clark Valley herds are given here.
The plotted location in Ocotillo Wells is incorrect; that voucher should plot with the vouchers to its immediate southwest.
The distribution of vouchers in the Sonoran Desert, from SEINet, are shown in the following map:
As advertised, the vouchers pretty much fill in the entire Sonoran Desert.
Relationship with Gray Vireo
Philip Unitt writes that in 1992, John Bates discovered that the Gray Vireo, in its wintering range:
feeds mainly on the tree's fruit, while the tree relies on the vireo to disperse its seeds, through regurgitation after the bird has digested the pulp.
Unitt reasoned that the Elephant Trees in Alma Canyon and Wash might host a wintering population of the Gray Vireo, and by golly, it did.
Unfortunately, if the Elephant Trees rely on the Gray Vireo for reproductive success, they may be in trouble, since Unitt also writes that the Gray Vireo numbers have plummeted due to brood parasitism by the Brown-headed Cowbird.
Print References
Cunningham, B. and Cunningham, P. 2006, Hiking California's Desert Parks: A Guide to the Greatest Hiking Adventures, Falcon Press.
Lindsay, D. 2001, Anza-Borrego A to Z: People, Places and Things, Sunbelt Publications.
Schad, J. 1998, Afoot and Afield in San Diego County, Wilderness Press.
Shreve, F. 1964, Vegetation and Flora of the Sonoran Desert, Stanford University Press.
Turner, R.M., Bowers, J.E. and Burgess, T.L. 2005, Sonoran Desert Plants: An Ecological Atlas, University of Arizona Press.
Online references are linked in the text.