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Educational Bulletin #82-2 A publication of the Education Foundation of the Desert Protective Council, Inc. |
| Frank C. Vasek,
PhD Department of Botany and Plant Sciences University of California at Riverside |
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Creosote Bush - A Link To The Ice Age |
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| Distribution and Taxonomy The creosote bush (Larrea trifdentata) is a common, characteristic and often dominant shrub at moderate and low elevations in much of the desert areas of Mexico and the southwest United States. Our North American creosote bush was long considered identical with Larrea divaricata of Argentine and other parts of South America. The two are very similar in appearance; and the few differences in shape of stipules, growth pattern, salt tolerance, etc., tend to be obscured by the normal variation in populations of both. Recently, the weight of opinion seems to have been swayed by the argument that geographical separation (about 36_ of latitude) is sufficient for status as distinct species. Chromosome Races The North American species includes three chromosome races. The normal chromosome number of 26 occurs in creosote bushes of the Chihuahuan Desert. The normal number of 26 chromosomes consists of two sets: one set of 13 maternal chromosomes and one set of 13 paternal chromosomes. A plant with two sets of chromosomes has two times a basic number and is said to be diploid. Plants in the Sonoran Desert have four sets of 13 chromosomes (=52) and are said to be tetraploid. Creosote bushes in the Mojave Desert have six sets of 13 chromosomes (=78) and are, therefore, hexaploid. Plant evolutionists have discovered mechanisms whereby the chromosome number can be increased by doubling the chromosome set in a germ cell (1-ploid x 2 = 2-p1oid) or doubling the chromosome sets of a body cell (2-ploid x 2 = 4-ploid). In effect, increase in number can occur by the addition of one or more whole sets of chromosomes. However, mechanisms for the reduction of chromosomes numbers by entire sets of chromosomes are unknown. Such would lead to inviable cells and very early death of organism. Consequently, the polyploid series of diploidtetraploid-hexaploid clearly indicates that North American creosote bushes first occurred in the Chihuahuan Desert area, or nearby, and sequentially spread north and west through the Sonoran Desert to the Mojave Desert. (The Chihuahuan Desert origin of |
creosote bush by long distance dispersal from
South America seems probable because 4 species of Larrea occur there, including
L. divanicata with 26 chromosomes.) But how long have these deserts had their present vegatation? Fossil Evidence. One line of evidence to determine past distribution of vegetation is that provided by plant fossils found in in durated urine deposits in pack rat middens. These fossils can be identified by comparison with modern plants. Furthermore, they can be aged or dated by radiocarbon dating methods. Basically, this line of evidence tells us whether the vegetation at the sites of ancient pack rat nests, at given points in time, was dominated by pinyon and juniper, or by creosote bush and its desert scrub associates. On this basis, vegetation of the Chihuahuan Desert area was dominated by pinyon and juniper during the last ice age. However, an increasing number of desert shrubs were mixed with junipers (and fewer pinyon) at about 12,000 and 11,500 years BP (before the present). Fossils of creosote bush were not found in the Chihuahuan Desert pack rat middens of that time, and pack rat middens apparently were not deposited after 11,500 BP. The earliest creosote bush fossils in pack rat middens were found near Yuma, Arizona, at low elevation in the Sonoran Desert. These were dated at about 10,850 BP. At higher elevations in the Sonoran Desert, pinyon and juniper were common. Such ice age woodlands of pinyon and juniper persisted in the Mojave Desert until about 9,000 BP in the present creosote bush zone, and still occur at higher elevations. In view of the ice age vegetation pattern, the development of the creosote bush distribution pattern apparently occurred rather rapidly. The diploid spread through the Chihuahuan Desert, and gave rise to the tetraploid which spread through the Sonoran Desert in less thaa 1,000 years. Origin of the hexaploid and its spread through the Mojave Desert evidently occurred within another 1,000 years or so. |