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PART II 17 The late-glacial period (~16,000–11,500 cal yr BP) was a dynamic time of vegetation change, as climate shifted from glacial to interglacial conditions. In the eastern and central United States at the latitude of the Lange/Ferguson site, full-glacial spruce forest transitioned to Holocene deciduous forest and grasslands. This transitional period was distinguished by vegetation with no modern analogue , with still-significant spruce (Picea) along with substantial black ash (Fraxinus nigra), hornbeam (Ostrya or Carpinus), and other deciduous trees such as oak (Quercus ) and elm (Ulmus). These species are sympatric in the upper Great Lakes region today, but the dominant taxa there presently are pine (Pinus) and birch (Betula), which were either rare or absent in the late-glacial period (Gonzales and Grimm 2009; Grimm and Jacobson 2004; Jacobson et al. 1987; Williams et al. 2000, 2001). Our knowledge of the late-glacial vegetation of the Great Plains south of the Wisconsin glacial border in the Dakotas is limited , based on scant fossil data and biogeographic inferences . For this chapter, radiocarbon dates were calibrated with OxCal 4.2.4 (Bronk Ramsey and Lee 2013) with the IntCa113 calibration curve (Reimer et al. 2013); age ranges are given at the 95.4 percent probability level. For the northern Great Plains, some of the most intriguing biogeographic evidence is from the Black Hills, where a significant number of trees, shrubs, and herbs with boreal and eastern affinities are disjunct (Buttrick 1914; Hayward 1928; McIntosh 1930, 1931; Rydberg 1896). Boreal disjuncts include white spruce (Picea glauca), paper birch (Betula papyrifera), and Canadian buffaloberry (Shepherdia canadensis). Eastern disjuncts include, among others , ironwood or hop hornbeam (Ostrya virginiana), hazel (Corylus americana, C. rostrata), and forest herbs such as bloodroot (Sanguinaria canadensis). These species suggest a previous forested connection across the Great Plains to the Black Hills, but when? The James Lobe of the Laurentide Ice Sheet was near its last glacial maximum (LGM) extent in eastern South Dakota from at least 18,000 14 C yr BP (~21,000 cal yr BP) to 13,500 14 C yr BP (~16,300 cal yr BP). By 13,000 14 C yr BP (~15,600 cal yr BP), the ice sheet had retreated into North Dakota, but it then made another short-lived advance well into eastern South Dakota at about 12,500 14 C yr BP (~14,700 cal yr BP), finally retreating north of the Canadian border by 12,000 14 C yr BP (~13,800 cal yr BP) (Dyke et al. 2003). A forested connection could not have existed until after the final retreat of the James Lobe. Deciduous elements, of course, could have previously extended across the plains, only to be isolated by the advancing James Lobe. However, the recent discovery of the subarctic collared lemming (Dicrostonyx richardsoni ) at a relatively low-altitude site (1,427 m amsl) in the southeastern Black Hills (Fulton et al. 2013) bears upon this hypothesis. Today, D. richardsoni occurs in a relatively small area west of Hudson Bay in tundra at the northernmost limits of the boreal trees Picea glauca and Betula papyrifera. AMS radiocarbon dates on purified collagen extracted from D. richardsoni from Don’s Gooseberry Pit range from 15,007±120 14 C yr BP (18,541–17,941 cal yr BP) to 14,055±100 14 C yr BP (17,434–16,725 cal yr BP), coeval with the James Lobe LGM. Thus, boreal species such as Picea glauca and Betula papyrifera, which are disjunct today in the Black Hills, may indeed have persisted in South Dakota west of the ice sheet. However, it seems highly unlikely that temperate plant species with eastern affinities would have persisted in a climate suitable for Dicrostonyx, implying that these species spread across the northern Great Plains during the late-glacial period following the final retreat of the ice. The first fossil evidence of this connection was from the Rosebud site (Watts and Wright 1966) in Todd County, about 130 km southeast of Lange/Ferguson (fig. 17). This Late-Glacial Vegetation from Stratum C, Lange/Ferguson Mammoth Site Pollen and Opal Phytolith Evidence chapter two Eric C. Grimm and Glen G. Fredlund 18 Chapter Two site, an interdunal swale in the northernmost Nebraska Sandhills, yielded abundant Picea pollen and macrofossils . The record consists of two core sections, with radiocarbon dates from only the deeper section of 12,630±160 14 C yr BP (15,476–14,205 cal yr BP) and...

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