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(Left) King
Clone, an irregular elipse formed by satellite shrubs around a central bare
area. (Below) Aerial view of King Clone (23 meters long) and several smaller clones (near Lucerne Valley, California). |
| Interpretation The significance of these old clones is that the incursion of desert vegetation into the present desert areas probably occurred earlier than indicated by fossils from pack rat middens. The apparent discrepancy in timing of residency of desert vegetation may be explained by the relative occurrence of old clones and of pack rat nests. Old creosote clones tend to occur on stable, level, sandy-gravelly surfaces or long, persistent sandy slopes, where pack rat activity is nil. In contrast, pack rats tend to build nests, especially those that survive thousands of years, in the shelter of rock ledges and crevices on rocky slopes. Consequently, valley bottom populations of creosote bush would not be sampled by rocky slope/dwelling pack rats. Therefore, the absence of creosote bush from pack rat midens of the Chihuahuan Desert is not sufficient to conclude the absence of creosote bush from that region. The foregoing information leads to the interpretation that the ployploid series in creosote bush was already well established during the last ice age. On this interpretation, creosote bushes probably did exist in valley bottom refuges at low elevations in all three desert areas during the late stages of the last ice age. They probably spread very rapidly northward about 12,000 BP at a time when drastic climatic changes were shrinking the continental ice sheets. All photos by author |
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