Creosotebush (Larrea tridentata)
Main sources: Dimmit, Mark, "Zygophyllaceae", and McAuliffe, Joseph, "Desert Soils", in Steven Phillips and Patricia Comus, eds., 2000, A Natural History of the Sonoran Desert, pp. 262-4, 99-103, Arizona-Sonora Desert Museum Press, Tucson; Robichaux, Robert, 1999, Ecology of Sonoran Desert Plants and Plant Communities, University of Arizona Press, Tucson; Shultz, Jack, 1999, "Desert Survivor", Natural History Magazine vol 108:1:pp.24ff; USDA Forest Service FEIS Database: Creosotebush
Creosotebush is an evergreen, many-branched, open shrub whose upper elevation limit in our area is about 4,500', and which is found most strikingly on dry plains and mesas, where it sometimes occurs in almost pure stands. It is a dominant or co-dominant member of most plant communities in the Mohave, Sonoran, and Chihuahuan Deserts, being excluded only from North America's northernmost desert (the Great Basin) by that desert's very cold winters (its northernmost limit is where Nevada temperatures drop to 5 degrees F. in winter). The most drought-tolerant perennial plant in North America, it can survive at least 2 years of no rainfall, losing leaves and shedding branches as its last resorts. For discussion of different varieties of creosotebush found in two of these deserts, see this link: Sonoran vs. Chihuahuan creosotebush.
Creosotebush will thrive where few other plants will. For example, see this location on a Sonoran Desert terrace, below, where the absence of these plants would leave little but desert pavement in their place:
The stems and evergreen leaves of this plant contain a sticky resin that smells like (but doesn't actually contain) the wood preservative creosote. This resin screens the leaves against ultraviolet radiation, reduces water loss, and poisons or repels microbes and most plant-eating animals. (Shultz, cited above)
Creosotebush is extremely longlived. Apparently, despite its prolific flowerings (which may occur after rains anytime during the summer), it becomes a stable member of desert communities mainly by cloning. After extreme droughts, old branches and roots die back, then when rains return, sprouts near the outside of the root crown appear, and the clones gradually expand to form large, open rings of plants. Some such plant-rings in the Mohave Desert are estimated to be nearly 10,000 years old. However, Creosotebush is also quite vulnerable to fire by virtue of its resinous foliage, and it appears to require long periods of surface stability to become well established.
In our area, Creosotebush seems to thrive best on gradually sloping terraces where soils are sandy-gravelly and lack clay horizons near to the surface. Its lateral roots may fan out into more than fifty square yards of surface soil, and it has a fairly deep root system that enables it to tap even meager sources of water from well down, including soils which have caliche layers, which its roots can penetrate through cracks. In such places it can remain active during the entire year, even when shallow soil moisture has disappeared. It does less well on steeper slopes with coarse, shallow soil, where for example Palo Verdes and Saguaro cacti thrive by capturing shallow water immediately during and after rains. Joseph McAuliffe (in Robichaux, cited above, pp. 79-84) suggests that Creosotebush is found mainly on old, stable slopes that have not experienced significant soil aggradation for very long periods of time, whereas different types of vegetation dominate places subject to recent change -- whether aggradation, as in floodplains, where short-lived shrubs predominate, or degradation, as on somewhat unstable gravelly slopes where Saguaros and Palo Verdes are more common. The very high frequency of Creosotebush on surfaces of the remnant Quiburis-period terraces along the San Pedro River definitely supports this view, for example below left, this apparently almost pure stand overlooking the west side of the San Pedro River floodplain photographed from near Mile 16 on the Cascabel Road; below right, a view from further north along the river shows a terrace mesa, now fully isolated from the higher slopes. If you look closely here you can see hints of Ocotillo scattered among the dominant vegetation. (Click on each image to enlarge it.) These are very old surfaces -- eroding mainly at their extreme edges.
At the right, you see an image showing a Mesquite bosque in the floodplain of lower Tres Alamos Wash (in the foreground and at middle far left, with Creosotebush occupying a terrace of some age at middle right. (Click on the image to enlarge it.) As you can see, the creosotebush is also colonizing a lower, younger terrace in middle-left of the picture. (It is also present on the much older terrace at top left, a terrace probably of comparable age to the ones shown just above.)
When winter rains are reasonably good, as in April of 2004, a creosotebush may burst into a brilliant bloom, as below left on April 9th; below right, a photograph taken at the same time but on the Ridge Road above Pool Wash, where the dryness remained very severe, note that no blooms are evident, and that some of the leaves have turned brown. (Click on each image to enlarge it.)
The image at left (and also the banner image at the top of the page) shows a creosotebush in bloom in a sidewash of Hot Springs Canyon in mid-May of 2002, despite the severest winter-long drought of quite a few years. This side-wash must have residual groundwater available -- see in contrast some images further below. The leaves -- waxy, resinous, strong-scented, 2 leaflets joined at the base -- run in color from dark green (at left) to yellowish (almost burnt) green (below), depending on drought. Flower-pods (left) open to present solitary yellow, 5-petaled flowers, which are followed by globe-shaped, fuzzy-white dry seed pods, shown below left -- click on the image for a seed close-up.
The distinctive leaf-form shows more clearly in the closeup below (also much drought-stressed): the leaves are paired, and they fold together during the heat of the day, thus lessening the area exposed to transpiration:
Below left, also taken on the Pool Wash ridge road in May 2002, a view of the typically open structure of the creosote canopy, which provides but very weak shade at midday. Note the typical grayness of the larger branches here also. Below right, a closeup view of the characteristic gray larger branches of creosotebush, often marked with dark cross-bars.
While occurring in otherwise very barren-appearing places, Creosotebush tends to create an enriched, softened soil in its immediate vicinity that creates microhabitats. Thus it serves as an important nurse plant for a number of species, producing a chemical inhibitor of germination that apparently works mainly against its own seeds. At left, it nurses a young saguaro. Click on the image to enlarge it.
Creosotebush is especially closely associated with bursage -- White Bursage, in the Mohave Desert. In our area it nurses otherwise now-uncommon grasses like Bush Muhly. A variety of small rodents, reptiles, and insects often burrow into these associated soils for shelter. Desert Tortoise burrows are often associated with creosotebush. Many small mammals browse the bush or eat its seeds.
At left, this Creosotebush stands on a ridge of Soza Mesa at an elevation of several hundred feet above the San Pedro River. This is an often wind-swept and sun-baked slope, as can be seen by the almost barren desert pavement exposed in the foreground, but immediately beneath and around the shrub rises a low cone composed mostly of soft fine soil that bears witness to the shrub's protective powers.Various forms of otherwise vulnerable vegetation will also be found growing in these low mounds.
At right, a very small desert hole in one edge of this same mound, signalling the den of perhaps one of our small pocket mice.
Below left, a creosote bush on the slopes of Soza Mesa photographed in February 2004 provides soft soils which harbor what appears to be a Desert Tortoise burrow. Below right, another Soza Mesa bush both protects small desert grasses (visible near its stems, though dark and shriveled in this February photo) and some rodent occupants whose holes are scattered close by. (click on each image for a seed close-up)
Creosotebush arrived in our area only during the last 14,000 - 10,000 years, coming toward the end of the Pleistocene out of South America, (where it has 5 species relatives, but none in North America) in a manner not yet understood (there are now no suitable habitats in the intervening thousands of miles), but possibly in the tail feathers of migrating plovers. However, even in this evolutionarily brief period of time in our area it has evolved associations with "more than 60 species of insects, including 22 species of bees that feed only on its flowers." (Dimmit p. 263) Many insects have become specific to it. Describing creosotebush from study sites in New Mexico, Jack Shultz (cited above) reports that "The creosote bush walkingstick, for example, is virtually identical to the shrub's stems: the smaller males mimic young, green shoots, and the much larger females mimic older, gray-brown stems. The creosote bush grasshopper mimics leaves in minute detail, down to silvery reflective patches that match the shine of the plant's resinous coating. Its olive wings, with spots that resemble the plant's rows of resin glands, help the creosote bush katydid blend in with leaves and stems." A scale insect, Tachardiella larreae, creates sticky secretions on it that the O'odham people highly valued and used for sealing cracks in leaky pots. Tiny creosote gall midges (~15 closely releated cecidomyiid flies in the genus Asphondylia) produce galls which were smoked like tobacco by some Native American groups (Dimmit, ibid.). We plan to investigate the resident insect populations to be found on creosotebush in our part of southeastern Arizona.
Jack Shultz also states, again, for New Mexico, that "Side-blotched lizards, desert iguanas, snakes, toads, and termites burrow beneath the shrub, and the chuckwalla feeds on its flowers and fruits. Ten or twelve species of small mammals depend on it for food, nesting sites, and refuge from the elements. Kangaroo rats consume its seeds, and jackrabbits prune its branches. Local birds, such as the verdin, black-throated sparrow, and black gnatcatcher, as well as scores of migratory visitors like the yellow warbler, frequent stands of creosote bush for seeds and insects, while roadrunners prowl the vicinity in search of snakes and lizards." Native Americans valued creosotebush as a medicine for treating a wide variety of illnesses, and The O'odham so revered it that they called it the first plant ever created by Earthmaker. See The O'odham for more details.