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Show Peter E. Wigand and David Rhode Quaternary Science Center, Desert Research Institute, 7010 Dandini Blvd., Reno, NV 89512 LATE QUATERNARY CLIMATES, GREAT BASIN VEGETATION, AND AQUATIC SYSTEM HISTORY Our understanding of late Quaternary vegetation history in the Great Basin has advanced significantly in the half century since the publication of Hubbs and Miller ( 1948) and Antevs ( 1948). At that time the number of studied late Quaternary fossil plant localities in the Great Basin was negligible ( Deevey 1949). Laudermilk and Munz ( 1934, 1938) had analyzed Late Pleistocene pollen and plant fragments from Shasta ground sloth dung balls found in Gypsum Cave and Rampart Cave, along the lower Colorado River just outside the southern margins of the Great Basin. Plant remains had been recovered from other cave sites ( e. g., Smith Creek Cave) but these had not yet been studied. Only two late Quaternary pollen sequences had been analyzed in the Great Basin, at Chewaucan and Warner marshes in south- central Oregon. Another locality lay outside its southeastern edge in the American Southwest ( e. g., Sears 1937). Use of tree- ring analysis to provide clues of the Holocene climates driving changing vegetation was still in its descriptive stage and chronologies were still relatively limited in length. The revolution in reconstruction of vegetation and climate history brought about by the recognition of the potential of woodrat middens as a fossil record was still a decade and a half in the future. By 1948 evidence derived from the interpretation of modern biogeographic indicators such as disjunct taxa and now- isolated hybrid populations had led to the recognition that Pleistocene glaciopluvial episodes and mid- Holocene warming had wrought substantial shifts in vegetation zones, but even the barest outlines of these shifts remained uncertain. In fact, a half century ago we had more direct information about Tertiary floras in the Great Basin than we did about late Quaternary vegetation history. Today our knowledge of Late Quaternary Great Basin vegetation history rivals that of any other area in the world. We are able to examine changes in late Quaternary climates through long and short term vegetation responses and lake basin histories. Paleobotanical proxy data reveals Great Basin vegetation dynamics in great detail for the last 4 ka, more coarsely for the last 35 ka, and only poorly beyond 35 ka. Paleobotanical reconstructions have been constrained by the natural decrease of palynological records southward and the decrease of studied woodrat midden localities northward. Recent studies are alleviating these problems. Vegetation changes observed in the Great Basin include large- scale latitudinal shifts of taxa that typically characterize the south and east, to primarily altitudinal shifts that characterize the northwest, as well as shifts in abundance of major taxa within plant associations. These differences are due primarily to the effects of topographic relief on vegetation distributions and proximity of various areas in the Basin to major vegetation boundaries. They are reflected in the vegetation histories below that characterize three broad regions of the Great Basin: northwestern ( Lahontan Basin), southern, and northeastern ( Bonneville Basin). Northern/ northwestern Great Basin. Abundant fir and spruce pollen dated to ca. 200 ka in the pollen sequence from Summer Lake, Oregon during Isotope Stage 6 indicates that climates had been more mesic than during Isotope Stages 2 and 4. Isotope Stage 3 climates were cool, dry, and relatively stable based upon the dominance of sagebrush and saltbush shrub communities. This pattern occurs in the Owens Lake pollen record as well. Pollen evidence from Summer Lake and from woodrat midden strata west of Pyramid Lake indicate that cool, wet conditions prevailed from 34 to 30 ka, leading to a regional expansion of spruce and fir unparalleled since Isotope Stage 6. A |