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C. Michael Hogan PhD
October 11, 2009
Bursera microphylla has a rich legendary role for prehistoric Mexicans and Native Americans in ceremonies, industry and healing. The physique of this tree is likened to multiple elephant trunks, waving and contorted, with smooth but flaking whitish bark in mature individuals. In many parts of its disjointed desert range, this species is rare. Pollen records indicate that B. microphylla appeared in some of its current range as early as about 8000 years before present in the early Holocene post-glacial.
B. microphylla occurs on rocky slopes and desert washes from the extreme southwest USA to northwestern Mexico. In California it is found at the western edge of the Sonoran Desert in San Diego County and western Imperial County; the northern limit of the California range is defined by the southern slopes of the Salton Sea basin. (Hogan) Specific California occurrences include desert washes within the Anza-Borrego Desert State Park. The disjunctive species distribution extends further eastward into southwestern Arizona. Specific Arizona occurences include the Gila, Tule and Tinajas Altas Mountains of Yuma County. The tree is also found in the Salt River Mountains and South Mountain Park of Maricopa County. In Pinal County it is found in the Waterman Mountains of Ironwood Forest National Monument. It also occurs in Pima County. The species range extends southward to the Mexican states of Baja California, Baja California Sur, Sonora (Pinacate Mountains), Sinaloa and Zacatecas. Within Baja California B. microphylla is locally common on loose basaltic soil in parts of the Sierra de San Francisco somewhat to the north of San Ignacio. The Elephant Tree is found chiefly below 700 meters in elevation.
This mysterious tree of the desert can readily attain a height of six meters in the southwestern USA, with its thick tortuous smooth trunks arrayed in a multifurcate branching habit; moreover, a mature specimen may reach six to seven meters in lateral spread with a characteristic open crown. In some parts of Mexico the height may reach as much as 15 meters with correspondingly greater spread. Young trees have a light reddish trunk, but mature trees have a characteristic white peeling bark. Glabrous leaves manifest an aroma of camphor, and are pinnately compound in a planar geometry with length of two to eight centimeters; the paired leaflets may number anywhere from seven to 33, with the odd terminal leaflet being lone
The drupe type leathery fruits are oval in shaped and tri-valved, with the stone being yellow. Stamens number from six to ten (Jepson) Flower buds are yellow, while the opened cream to white star shaped flower is five petalled with minute green five millimeter sepals and petals measuring four millimeters; blooming time is typically in June and July. Diagnostic features differentiating this plant from its genus member B. fagaroides include the notably longer (up to four cm) and more acute leaflets of the latter.
NATURAL HISTORY AND ECOLOGY
The Waterman Mountains of the Ironwood National Monument population has offered an opportunity to study the plant association progression since the Late Wisconsin glacial period. The late Wisconsin glacial flora in the Waterman Mountains has been reconstructed from macrofossils and pollen in packrat middens to have been woodland/chaparral dominated by Singleleaf pinyon (Pinus monophylla), Redberry Juniper (Juniperus coahuilensis) and Utah Juniper (Juniperus osteosperma) Other Ice Age associates no longer extant here include Celtis reticulata, Coryphantha vivipara, Krascheninnikovia lanata, Salvia pinguifolia, and Yucca brevifolia. Late Wisconsin plants included Canotia holacantha, Monardella arizonica, Opuntia chlorotica and Vauquelinia californica; however, these four species are not presently found near the midden sites studied, but rather occur relict populations at higher altitudes within the Watermans.
B. microphylla is not documented to have occurred in the Waterman Mountains until approximately 8310 years before present. However, the B. microphylla population still extant has an unusual morphology, resembling aspects of B. fagaroides. Since the nearest true B. fagaroides populations are in Fresnal Canyon within the Baboquivari Mountains not far from the border with Sonora. The only explanation for this observation is that the two Bursera species have hybridized or there has been evolution of a new species through natural selection or genetic drift since the 8310 BP population.
Some extant associates of B. microphylla in the Watermans are Cheilanthes villosa, Mimulus rubellus, Euphorbia revoluta, Hybanthus verticillatus, Panicum obtusum, and Talinum aurantiacum, all of which are also found in mesic woodlands to the north and east of the Waterman Mountains. (Turner) Other contemporary associates in the Watermans include dominant flora that appeared with arrival here of the Sonoran desert scrub in the early Holocene: Carnegiea gigantea, Fouquieria splendens, Olneya tesota, Parkinsonia florida and P. microphylla.
B. microphylla is typically dioecious, with pollination typically carried out by insects; however, some isolated stands have been found containing perfect flowers, which have been shown to be able to set fruit. The species is classified as drought deciduous. (Mielke)
The Cahuilla Indians extracted the sap to be used as a generalized cure for a gamut of illnesses. (Bean) In present day the resin is dried and prepared as a substance similar to myrrh, mirroring the use of its Asian family member tree. In Sonora tannin has been historically extracted from the bark for export; (Kearney) in the same Mexican state the gum has been used to treat venereal disease. The copal form resin of intermediate polymerization has been harvested from several Mexican regions historically for manufacture of cement and varnish, and has also been used in medicinal treatment for scorpion stings; there is data to suggest B. fagaroides may have been more common for the latter uses, as well as incense burned in Aztec and Mayan temples in prehistorical times.
B. microphylla is under protection in Arizona. In California. B. microphylla is on List 2.3 of the California Native Plant Society, indicating the taxon is rare, threatened, or endangered in that state. B. microphylla occurs in certain tropical dry forests in Mexico, which locales are noted for their high endemism and biodiversity. The Sierra de Laguna dry forest in Baja California Sur is a good example of rugged terrain, which has self-protected much of the forest; nevertheless, this forest is subject to ongoing conversion to grazing land, even though the location was nominally entered into protection in 1994. Habitat fragmentation is therefore a risk to this ecoregion. Chief flora associates of B. microphylla in the Sierra de Laguna are The main tree species in the subtropical forest are Lysiloma divaricata, L. candida, and Albizia occidentalis.
* C. Michael Hogan. 2009. Washington Fan Palm: Washingtonia filifera, GlobalTwitcher, ed. N. Stromberg
* Willis Linn Jepson. 1993. Jepson Manual: Bursera microphylla. On-line republication
* Raymond M. Turner, J.E. Bowers and T.L. Burgess. 1995. Sonoran desert plants: an ecological atlas, University of Arizona Press, Tucson, Arizona
* Lowell John Bean. 1972. Native Californians: a theoretical retrospective, 452 pages
* Thomas Henry Kearney and Robert Hibbs Peebles. 1960. Arizona flora, 1085 pages
* Judy Mielke. 1993. Native plants for Southwestern landscapes, University of Texas Press, 310 pages