The existing warm (Larrea) deserts of the Southwest are Holocene expansions replacing late-Pleistocene, evergreen woodlands of low-statured junipers, pinyon pines, and live oaks; these woodlands have been isolated by complementary contraction to the slopes of higher mountains that rise like islands from the modern desert sea. Because pinyon-juniper woodland is now so widespread on the similar fault-block mountains of the Great Basin, even as far north as southern Idaho, it would seem reasonable to suppose that the modern "cold" (Artemisia, Atriplex) deserts were similarly wooded during the last glacial. However, conclusive Neotoma macrofossil evidence (45 14 Cdated assemblages are reported here) documents major latitudinal displacement of vegetation that precludes pinyon-juniper woodland in the northern and central Great Basin at that time. On the other hand, the entire Mohave Desert sector (south of =3rN) served as an extensive Pleistocene refugium for pinyon-juniper woodland, as documented by an additional 48 dated Neotoma deposits.During the Wisconsinan glacial in the southeastern corner of Oregon, at 42°27'N, there was a subarctic landscape of hyperboreal, prostrate shrublet-junipers (Juniperus horizontalis and J. communis) and widespread patterned ground, even at the near-basal elevation of 1460 m. The pleniglacial vegetation of the central Great Basin at 39°N in eastern Nevada and western Utah, was dominated by a regional subalpine forest of bristlecone pine (Pinus longaeva), associated with minor but consistent boreal juniper (J. communis) down to 1660 m, close to the base level imposed by pluvial Lake Bonneville. Spruce has not been recorded below 1900 m during the last glacial. At a lower range of elevation (1350-1525 m), available south of the southeastern rim of the Bonneville basin at 37°30'N, Pinus longaeva was replaced by limber pine (P. flexilis), Douglas-fir (Pseudotsuga), and montane red cedar (J. scopulorum); existing woodland juniper (J. osteosperma) was lacking, but the subalpine J. communis was present at this local base level.Theory of island biogeography, as applied to ecological islands atop the high mountains of the Great Basin, is reexamined in the light of the drastic vegetational displacements documented in the detailed Quaternary macrofossil record. Species/area plots of montane-subalpine conifers presently distributed on 54 Great Basin mountaintops show an overall insular pattern that is especially well developed on the subset of 38 ecological islands east of I l6°W; the slope of z = 0.26 is close to the theoretical value for islands in equilibrium.All I I taxa of montane-subalpine conifers that penetrate the Great Basin deeply have their main distributions in the Rocky Mountains; only three wide-ranging species occur also in the Sierra Nevada. A long sundering trough in the western Great Basin parallels and isolates the Cascade-Sierran uplift with low-elevation barriers that impede migration, but in the eastern Great Basin there are high connecting divides to the western Rockies, es...

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Seventeen ancient wood-rat middens, ranging in radiocarbon age from 7400 to 19,500 years and to older than 40,000 years, have been uncovered in the northeastern, north-central, southeastern, and southwestern sectors of the Mohave Desert. Excellent preservation of macroscopic plant materials (including stems, buds, leaves, fruits, and seeds) enables identification of many plant species growing within the limited foraging range of the sedentary wood rat. An approximately synchronous zonal differentiation of vegetation in response to a gradient of elevation on limestone in the northeastern Mohave Desert is apparent from the macrofossil evidence, preserved in wood-rat middens and ground-sloth coprolites, covering a time span bracketed by radiocarbon ages of about 9000 and 10,000 years. XerophilQus juniper woodlands descended to an elevation of 1100 meters, some 600 meters below the present lower limit of woodland (1700 meters) in the latitude of Frenchman Flat. But desert or semidesert shrubs coexisted with the woodland trees throughout much of the span of elevation corresponding to the pluvial lowering of the woodland zone, and the more mesophytic phase of pinyonjuniper woodland was evidently confined to montane habitats at elevations above 1500 meters. Joshua trees, accompanied by desert shrubs, prevailed down to about 600 meters at Gypsum Cave, Nevada, but only the shrubs of the existing warm-desert vegetation occurred at 530 meters near Rampart Cave, Arizona. Pleistocene middens from the southeastern Mohave Desert record a relatively large downward shift of the pinyon-juniper woodland zone, paralleling the remarkably low minimum elevation of the existing woodland zone in that area. The macrofossil evidence speaks for former continuity of the many disjunct stands of woodland vegetation in the Mohave Desert region, at least along the higher divides connecting most of the ranges. However, there is no macrofossil evidence of pluvial continuity of range for the more mesophytic, montane, coniferous-forest zone of ponderosa pine or white fir now occupying islands of relatively mesic environment on the highest mountains of the region. On the contrary, the uneven stocking of the lofty mountains of the Mohave Desert with mesephytic or boreal species and the trend toward endemism suggest a long history of isolation.