Plant Species of the Borrego Desert: Burseraceae: Bursera microphylla, elephant tree

Table of Contents

Introduction, Origin and Meaning of Name
Frankincense, Myrrh and Kelewat
History of Discovery in California
Distinguishing Characteristics and Similar Species
Pictures of Young Plants, Mature Plants, and Dead Plants
Repeat Photographs of Young Plants, Mature Plants, and Dead Plants
Habitat, Distribution and Abundance
      The Elephant Trees of Clark Valley
Relationship with Gray Vireo
Print References

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.

Repeat Photographs of Young Plants, Mature Plants, and Dead Plants

The Elephant Trees Discovery Trail used to have a number of Elephant Trees on it. Jerry Schad, in his 1986 first edition of Afoot and Afield San Diego County hiking guide, said the Trail winds through a small group of these trees. The 1994 Trail Guide produced by the Anza-Borrego Desert Natural History Association had two stops for Elephant Trees: stop 6 had a small herd and stop 13 had one specimen that was larger and less damaged than the first ones. Diana Lindsay recalls five or more trees at that time.

However, in December 2009 there was but a single tree left, at stop #9 on the Discovery Trail.

The Nature Trail itself has changed over the years, perhaps because the number of Elephant Trees was decreasing. There is a dead Elephant Tree at a stop 15 on an old section of trail that is 0.5 miles farther than the closest point on the current trail. Ted Caragozian remembered a number of live Elephant Trees in that area on the old trail in roughly the year 1990, but in 2013 most of those trees seemed to be no longer present, with only a few live ones on the old trail and the single dead one at stop 15.

As a result, Ted and I became very curious as to how long it took for a dead Elephant Tree to rot away.

I had pictures of two dead trees, one from December 2009 and one from March 2010, so on 27 January 2015 we, along with Keir Morse, set out to relocate those trees and see if they were "still dead".

I also had a photograph from December 2009 of a very young tree that was just about two feet high that we tried to relocate to see how much it had grown in almost five years. Richard Zmasek also kindly sent me a photograph taken in April 2008 of the live tree at the current stop #9, giving us an almost seven year baseline for it. Bill Sullivan had a picture of a six foot tall live tree taken in December 2010 (from The Sand Paper, Spring 2011) that we also tried to relocate.

We were successful in relocating all of those trees! It took a real group effort to do that, since I did not have precise GPS locations for the two dead trees. The corresponding photographs are given below. The changes we see from comparing the repeat photographs are:

Comparison of the live mature tree at stop #9 on the Discovery Trail

Fig. 10. Pictures of the live mature tree at stop #9 on the Discovery Trail. Top: Picture from April 2008 by Richard Zmasek, with his son Christian (6 feet tall) on the left and wife Monica on the right. Bottom: Picture from 27 January 2015 by Tom Chester, with Ted Caragozian (5 feet 8 inches) on the left and Keir Morse on the right. The Elephant Tree appears identical for overall shape, with the major change simply being the fresher green leaves in the 2015 picture. The branch above Ted's head is a bit larger in 2015, since Ted had to bend forward a bit, whereas Christian was standing straight up in 2008. Both were standing in the same position, up against the #9 post.
The major change in the picture in plant growth is in the acacia at lower right. It partially obscures Keir in 2015, but did not approach Monica in 2008. Oddly, the cheesebush just beyond Keir's foot appears not to have grown at all in this interval.
Click on the pictures for larger versions.

Comparison of a six foot tall tree about 0.2 miles south of the Discovery Trail

Fig. 11. Pictures of a live six foot tall tree about 0.2 miles south of the Discovery Trail that is growing through a fallen dead smoke tree trunk. Top: Picture from December 2010 by Bill Sullivan. Bottom: Picture from 27 January 2015 by Tom Chester.
The pictures are not as easy to compare as the previous set, since the 2010 photograph was made with a zoom lens, which we did not realize when we were attempting to reproduce the photograph in 2015. As a result, we ended up not matching the angle of the 2010 picture exactly, since we were confused in the field about the apparent change in the boulders, especially the "missing" boulder labeled 3. Note the boulders labeled 1 and 2 are at a different orientation in the two pictures.
The tree has the same architecture in both pictures, and we estimated it was still six feet tall in 2015, the same as the estimate in 2010. The middle main stem appears to have longer branches near the top, although the different orientation may also make the branches appear longer.
Click on the pictures for larger versions.

Comparison of a two foot tall tree just below the mouth of Alma Canyon

Fig. 12. Pictures of a live two foot tall tree just below the mouth of Alma Canyon. Top: Picture taken in December 2009 by Tom Chester. Bottom: Picture taken on 27 January 2015 by Tom Chester, with a rough outline of the 2009 size of the Elephant Tree drawn in red.
The angle of the two photographs is not the same, with the 2009 picture taken from a vantage point closer to the ground. I've made the boulder immediately behind the tree the same size in the two pictures shown here, but the scale is not exactly the same for the other parts of the picture. There has been significant growth in all the branches, both in length and in the lengths of the second degree branches.
Click on the pictures for larger versions.

Comparison of a dead tree in 2009 just below the mouth of Alma Canyon

Fig. 13. Pictures of a dead tree just below the mouth of Alma Canyon. Top: Picture taken in December 2009 by Tom Chester. Bottom: Picture taken on 27 January 2015 by Tom Chester, with some changes noted. The red lines indicate branches seen in the 2009 picture that are not in that vicinity in 2015. There are many examples in the picture of more drooping branches, with decayed bark.
Overall, the dead tree has roughly the same form, but the branches have all sagged toward the ground, and the bark on the branches is mostly gone or at least significantly weathered.
Click on the pictures for larger versions.

Comparison of a dead tree in 2009 at 0.5 miles below the mouth of Alma Canyon

Fig. 14. Pictures of a dead tree at roughly 0.5 miles below the mouth of Alma Canyon. Top: Picture taken in March 2010 by Tom Chester. Bottom: Picture taken on 27 January 2015 by Tom Chester. This picture has been scaled so that the distance between the two boulders just in front of the trunk is the same as in the 2010 picture, making the size of the dead tree comparable in these pictures.
This tree has decayed significantly more than the previous example. Over half of the highest stem has broken off, and the entire tree is now closer to the ground. Some of the secondary branches have broken off, but many are still present.
This tree was not in the main nearby channels, and the small rocks near the tree have not been moved, so it is unlikely that water flow caused this change. The black on the lower trunk is a remaining portion of the bark that has changed color.
Click on the pictures for larger versions.

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.

Fig. 15 shows the locations of Elephant Trees we GPS'd, mostly from 27 January 2015 on a trip with Ted Caragozian and Keir Morse.

Fig. 15. Map showing locations of Elephant Trees GPS'd in the alluvial fan from Alma Canyon, along with the Parking area for the Elephant Tree Nature Trail (P), and the location of the tree (shown in photographs above) at Post 9 on that trail.. Although there is only a single point shown in Alma Canyon itself, there are hundreds of trees there, and probably the bulk of the almost 2,000 trees mentioned in Lindsay 2001.

The only other significant population is at Torote Bowl and Torote Canyon, south of the Borrego Desert in the Indian Gorge / Bow Willow area. There are a handful of plants in the lower Smuggler Wash / June Wash area.

Fig. 16 shows the distribution of iNat observations for which I have access to non-obscured coordinates, for the southern part of Anza Borrego Desert State Park.

Fig. 16. Map of the southern part of Anza Borrego Desert State Park showing iNat observations of Elephant Trees that have non-obscured coordinates accurate to within 100 m (330 feet).

It is quite possible there are more locations in the lower Smuggler Wash / June Wash area, since the three plants at the westernmost location in this map were just discovered on 7 January 2022 by Don Rideout, and the plants in the wash just to the east of Smuggler Wash were discovered in 2005 by Patrick McConnell, with additional plants discovered in 2018 by Carla Hoegen and Fred Melgert.

Vouchers do not contribute any additional accurate locations in the area shown in Fig. 16. There are much more widespread voucher locations, but the locations outside of the areas shown in Fig. 16 are from vouchers with vague localities, which therefore have georeferenced locations with large uncertainties. One voucher, SD170008, has a fairly accurate location consistent with one of the iNat locations.

The approximate coordinates for the Martinez Canyon and Clark Valley herds are given here.

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.

Sullivan, B. The Sand Paper, Spring 2011, pp. 1-3, published by Anza-Borrego Desert Natural History Association.

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.

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Updated 10 January 2022.