ABSTRACT
Climates at the Last Glacial Maximum (LGM) have been inferred from fossil pollen assemblages, but these inferred climates are colder for eastern North America than those produced by climate simulations. It has been suggested that low CO2 levels could account for this discrepancy. In this study biogeographic evidence is used to test the CO2 effect model. The recolonization of glaciated zones in eastern North America following the last ice age produced distinct biogeographic patterns. It has been assumed that a wide zone south of the ice was tundra or boreal parkland (Boreal-Parkland Zone or BPZ), which would have been recolonized from southern refugia as the ice melted, but the patterns in this zone differ from those in the glaciated zone, which creates a major biogeographic anomaly. In the glacial zone, there are few endemics but in the BPZ there are many across multiple taxa. In the glacial zone, there are the expected gradients of genetic diversity with distance from the ice-free zone, but no evidence of this is found in the BPZ. Many races and related species exist in the BPZ which would have merged or hybridized if confined to the same refugia. Evidence for distinct southern refugia for most temperate species is lacking. Extinctions of temperate flora were rare. The interpretation of spruce as a boreal climate indicator may be mistaken over much of the region if the spruce was actually an extinct temperate species. All of these anomalies call into question the concept that climates in the zone south of the ice were extremely cold or that temperate species had to migrate far to the south. An alternate hypothesis is that low CO2 levels gave an advantage to pine and spruce, which are the dominant trees in the BPZ, and to herbaceous species over trees, which also fits the observed pattern. Thus climate reconstruction from pollen data is probably biased and needs to incorporate CO2 effects. Most temperate species could have survived across their current ranges at lower abundance by retreating to moist microsites. These would be microrefugia not easily detected by pollen records, especially if most species became rare. These results mean that climate reconstructions based on terrestrial plant indicators will not be valid for periods with markedly different CO2 levels. The Pleistocene 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; Over-peck 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, 1980b; 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 chapter 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.
