Creosote bush
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| Missing image CreosoteForegroundKilbourneHoleBackground.jpg Larrea tridentata bushes near Kilbourne hole, New Mexico | ||||||||||||||
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The Creosote bush (Larrea tridentata, formerly Larrea divaricata) is an evergreen shrub of the Zygophyllaceae family. It is a prominent species in the Mojave, Sonoran, and Chihuahua Deserts of western North America, including portions of California, Arizona, Nevada, Utah, and western Texas in the United States, and northern Chihuahua in Mexico. The wand-like stems of the plant bear resinous, dark green leaves and small yellow flowers. It also exhibits a characteristic odor of its creosote namesake. Some specimens have been found with heights of three meters, but typical plants do not exceed one meter.
Creosote bush is most common on the well-drained soils of bajadas and flats. In parts of its range, it may cover large areas in practically pure stands, though it usually occurs in association with Burrow Bush (Ambrosia dumosa, also known as Bur-sage). Despite this common kinship, Creosote bush roots have been found to produce chemicals that inhibit the growth of Burrow Bush roots, and much of their relationship is currently unexplained.
Such chemicals, however, have failed to explain the peculiar regularity in the spacing of individual plants within a stand. Creosote Bush stands tend to resemble man-made orchards in the even placement of plants. Originally, it was assumed that the plant produced some sort of water-soluable inhibitor that prevented the growth of other bushes near mature, healthy bushes. Now, however, it has been shown that the root systems of mature plants are simply so efficient at absorbing water that fallen seeds nearby can not accumulate enough water to germinate, effectively creating dead zones around every plant. It also seems that all plants within a stand grow at approximately the same rate, and that Creosote Bush is a very long-living plant. From this, it can be concluded that all the plants inside a stand are of equal age.
Contributing to the harshness of the germination environment above mature root systems, young Creosote Bushes are much more succeptable to drought stress than entrenched plants. Germination is actually quite abundanant during wet periods, but most of the young plants die very quickly unless there are optimal water conditions. Ground heat compounds the young plants' aversity to water stress, and ground temperatures can reach upwards of 70° C (160° F). To become established, it seems the young plant must experience a pattern of three to five years of abnormally cool and moist weather during and after germination.
Mature plants, however, can tolerate extreme drought stress. In terms of negative water potential, Creosote Bushes can operate fully at -50 bars of water potential and have been found living down to -120 bars, although the practical average floor is around -70 bars, where the plant's need for cellular respiration generally exceeds the level that the water-requiring process of photosynthesis can provide. Cell division can occur during these times of water stress, and it is common for new cells to quickly absorb water after rainfall. This rapid uptake causes branches to 'grow' several centimeters at the end of a dry season.
After rainfall of at least 25 millimeters (one inch), the plants will bloom numerous small, bright yellow flowers. Various insects are responsible for pollination, after which the flowers' petals will make a quarter turn, making them much less conspicuous. This discourages accidental secondary attempts at pollination, with insects instead preferring the more conspicuous unpollinated flowers.
Creosote Bush's small leaves have a high surface-volume ratio, optimizing the rate at which heat escapes and water content is retained. Water loss is further decreased by the resinous, waxy coating of the leaves. Plants do drop some leaves heading into summer, but if all leaves are lost, the plant will not recover. Accumulation of fallen leaves, as well as other detrius caught from the passing wind, creates an ecological community specific to the Creosote Bush canopy, including beetles, millipedes, pocket mice, and kangaroo rats.
Creosote Bushes tend to grow new stems from an underground crown taproot, with newer stems occurring on the outside circumference and older stems remaining towards the center. As the inner stems wilt and break off, mature plants begin to appear to be several individual plants, but arranged in a conspicuous ring. These plants remain attached to the central crown and all share the same genetic material. Eventually, however, the ring gets too large and the crown breaks apart into several smaller bunches of plants. Despite being separate organisms, these bunches are all still genetic clones of the original plant.
By correlating the results of carbon-14 dating with growth rings, it has been shown that Creosote Bush clone rings expand very slowly. Most rings of approximately six meters (20 feet) in the Mojave Desert are estimated to be about 3,000 years old. The Mojave's oldest discovered clone, near Lucerne Valley, California, has been named King Clone and is 22 meters (67 feet) in diameter and is thought to be 11,700 years old. Another ring in the Sonoran Desert near Yuma, Arizona is thought to be 18,000 years old. If the clones are considered part of the original plant, these are Earth's oldest living things. They would have germinated at the end of the last ice age, and would be as old as the deserts they grow in.
External links
- FEIS entry (http://www.fs.fed.us/database/feis/plants/shrub/lartri/all.html)
- About.com entry (http://phoenix.about.com/cs/desert/a/creosote.htm)
- Drug Digest entry (http://www.drugdigest.org/DD/DVH/HerbsReference/0,3928,4098%7CCreosote+bush,00.html)
- The World's Oldest Living Thing (http://waynesword.palomar.edu/ww0601.htm)
- photo links
- close photo of flower (http://extension.usu.edu/rangeplants/Woody/creosotebush.htm)
- landscape with creosote (http://www.epa.gov/nerlesd1/land-sci/photo05.htm)
- various photos (http://www.suu.edu/faculty/martin/creosotebush/creosotebush.htm)