| Plant macrofossils from ancient packrat (Neotoma spp.) middens preserve a detailed record of past vegetation. They provide a far more informative record on details of upland community composition than has been available to paleoecologists previously. In the Sheep Range of the northern Mojave Desert 30 midden sites that span 900 m of relative relief, from desertscrub vegetation to fir-pine forest, yield 52 radiocarbon dated samples. This data base, encompassing more than 50,000 radiocarbon years, was used to reconstruct the vegetation of the middle Wisconsin through Holocene and the dynamics of vegetation change, to test corollaries of the individualistic concept of the plant association, and to verify vegetation reconstructions based on pollen analysis.; During the late Pleistocene Juniperus osteosperma woodland ranged from below 1500 m to elevations exceeding 2000 m in the Sheep Range. Above the woodland a subalpine forest characterized by Pinus flexilis and Pinus longaeva occurred as low as 1800 m elevation between 20,000 and 15,000 B.P. Great Basin Desert shrubs (e.g., Artemisia sec. Tridentatae, Tetradymia sp.,Chrysothamnus spp.) were common in both the juniper- and pine-dominated vegetation. Subalpine taxa (e.g., Pinus flexilis, Jamesia americana) extended down to at least 1580 m elevation in the juniper woodland.; The displacement of plant taxa relative to their current ranges was individualistic. Some plants expanded to slopes of different aspect, some occurred far below their present lower limits, while others show little elevational displacement. At least one species, Atriplex confertifolia, grew well above its present upper limits. During the Wisconsin most sites appear to have supported fewer woody plants than today and there may have been fewer shrubs and trees in the Sheep Range as a whole.; Reconstructions of late Pleistocene vegetation based on fossil pollen assemblages from the nearby Las Vegas Valley were not verified. Instead of a mesophytic pinyon-juniper woodland and ponderosa pine forest, interpreted from the Wisconsin pollen record, xerophytic juniper woodland and subalpine forest seem to have occupied elevations from 1400 to 2400 m. The climate of the Sheep Range prior to 15,000 B.P. appears to have been relatively cold and dry, rather than cold and moist. |
