Larrea tridentata
| Larrea tridentata | ||||||||||||||
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 Close-up of Larrea tridentata | ||||||||||||||
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| Larrea tridentata (Sessé & Moc. ex DC.) Coville | ||||||||||||||
Larrea tridentata (creosote bush) is a flowering plant in the family Zygophyllaceae. It is a prominent species in the Mojave, Sonoran, and Chihuahuan Deserts of western North America, including portions of California, Arizona, Nevada, Utah, New Mexico and western Texas in the United States, and northern Chihuahua in Mexico. It is closely related to the South American Larrea divaricata, and was formerly treated as the same species.
Overview
It is an evergreen shrub growing to 1-3 m tall, rarely 4 m. The stems of the plant bear resinous, dark green leaves with two leaflets joined at the base, each leaflet 7-18 mm long and 4-8.5 mm broad. The flowers are up to 25 mm diameter, with five yellow petals. The whole plant exhibits a characteristic odor of creosote, from which the common name derives.
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 Ambrosia dumosa (burrow bush or bur-sage). Despite this common habitat, 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-soluble inhibitor that prevented the growth of other bushes near mature, healthy bushes. Now, however, it has been shown that the root systems of mature creosote plants are simply so efficient at absorbing water that fallen seeds nearby cannot 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 the creosote bush is a very long-living plant. As the Creosote Bush grows older, its oldest branches eventually die and its crown splits into separate crowns. This normally happens when the plant is 30 to 90 years old. Eventually the old crown dies and the new one becomes a clone of the previous plant, composed of many separate stem crowns all from the same seed. One creosote plant, named "King Clone", near Lucerne Valley has been carbon dated to 11,700 years old.
Contributing to the harshness of the germination environment above mature root systems, young creosote bushes are much more susceptible to drought stress than established plants. Germination is actually quite active during wet periods, but most of the young plants die very quickly unless there are optimal water conditions. Ground heat compounds the young plants' susceptibility 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. From this, it can be inferred that all the plants inside a stand are of equal age.
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.
The small leaves of the creosote bush 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 detritus caught from the passing wind, creates an ecological community specific to the creosote bush canopy, including beetles, millipedes, pocket mice, and kangaroo rats.
Creosote bush commonly forms clonal colonies, which may be very long-lived; a ring of creosote bushes in the Mojave Desert is believed to be at least 12,000 years old.