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<strong>1 Introduction</strong> <p>The Pleistocene Last Glacial Maximum (LGM) period of 18 000 years ago has been widely interpreted as a time of bitter cold in eastern North America when tundra and boreal forest extended hundreds of miles south of the ice sheets and the temperate forest of the East retreated to the southern coastal plain, to Florida, and westward into Texas and Mexico (Davis, 1983, 1984; Davis and Shaw, 2001; Deevey, 1949; Delcourt and Delcourt, 1984, 1993; Jacobson et al., 1987; Maher et al., 1998; Maxwell and Davis, 1972; Overpeck et al., 1992; Prentice et al., 1991; Ritchie, 1987; Royall et al., 1991; Schoonmaker and Foster, 1991; Tallis, 1991; Watts, 1970, 1971, 1973, 1979, 1980a, b; Watts and Stuvier, 1980; Webb et al., 1988, 1993; Whitehead, 1973; Wilkins et al., 1991). This reconstruction, which may be called the standard model, is commonly presented in textbooks (e.g., Bradley, 1999; Delcourt and Delcourt, 1993; Pielou, 1991; Ritchie, 1987; Tallis, 1991). The standard model is based largely on pollen and macrofossil records. These pollen data have been interpreted qualitatively in some cases, and in other cases transfer functions or response surface models (e.g., Farrera et al., 1999; Nakagawa et al., 2002; Peyron et al., 1998; Tarasov et al., 1999; Webb et al., 1993, 1997) have been used to infer climate from pollen composition, with similar results. By any of these three methods, the inferred LGM climate is much colder than that simulated (Ganopolski et al., 1998; Huntley et al., 2003; Kageyama et al., 2001; Pinot et al., 1999; Webb et al., 1993, 1997). Determining the correct paleotemperature is important for calibrating general circulation models (e.g., Crowley, 2000; Farrera et al., 1999; Ganopolski et al., 1998; Pinot et al., 1999). In addition, the vegetation composition at this time is consistently described as having no analogs in modern periods. In this paper it is argued that both of these anomalies arise from a combination of ambiguous pollen interpretation and the effects of low ambient CO2 at the LGM which would have altered the relative dominance of different taxa in a manner that mimics colder and drier climates. Biogeographic and phylogenetic data are cited as independent tests of the CO2 effect model. The idea that glacial levels of CO2 could affect vegetation structure and composition was proposed over a decade ago (e.g., Beerling, 1996; Beerling and Woodward, 1993; Bennet and Willis, 2000; Cole and Monger, 1994; Cowling, 1999; Cowling and Sykes, 1999; Farquhar, 1997; Harrison and Prentice, 2003; Jolly and Haxeltine, 1997; Polley et al., 1993, 1995; Street-Perrott et al., 1997). Nevertheless, LGM climate as estimated from pollen data generally still does not factor in CO2 effects (e.g., Elenga et al., 2000; Nakagawa et al., 2002; Peyron et al., 1998; Tarasov et al., 1999) and discussions of simulated LGM climate vs. vegetation (the anomaly problem) either do not mention the role of CO2 (e.g., Ganopolski et al., 1998; Pinot et al., 1999) or only touch on this effect (e.g., Crowley, 2000; Huntley et al., 2003).</p> <p>To date, the discussion has featured dueling models. Regression models based on pollen (which seem straightforward) are pitted against simulations of plant communities under low CO2. It is tempting to be wary of the simulations because they cannot be directly verified. In what follows, several types of independent evidence are presented that imply that LGM climates inferred from pollen data are colder than the likely actual climates. These various types of evidence are shown to reconcile with effects of low ambient CO2, thereby supporting the models of CO2 effects. The results of the analysis also bear on questions of glacial refugia. The focus of this study is largely eastern North America for the sake of concreteness, but no-analog vegetation found elsewhere at the LGM can be explained by similar mechanisms.</p>
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