3 Taxonomic anomalies
A key species in the reconstruction of LGM climates in eastern North America is spruce (Picea). The pollen of various species of spruce are very difficult to distinguish. Since all existing spruce species in eastern North America require a cold climate, they have generally been considered together (see, e.g., Davis and Shaw, 2001). In this context, pollen profiles dominated by spruce and herbaceous species have been classified as boreal parkland.
It is now apparent, however, that a major portion of the southern range of spruce may have been occupied by a nowextinct species (Jackson and Weng, 1999) which was common in the lower Mississippi valley and east at least to western Georgia. Given that fossil stumps of this species can be found growing with oaks and with strictly southern species, this was likely a temperate spruce. This spruce pollen is thus not necessarily indicative of cold climates. This may have biased climate reconstructions at the LGM to a considerable degree.
A similar problem occurs with sedges (Carex). This taxonomic group has been interpreted as diagnostic of boreal or tundra habitats. However, sedges are also common in grasslands and forests and would have been favored under a low CO2 atmosphere (Beerling, 1996; Beerling and Woodward, 1993; Jolly and Haxeltine, 1997; Robinson, 1994; Saxe et al., 1998), as will be discussed below.
In both cases, the species have been considered key indicators or diagnostic species of boreal forest but lumping species is known to produce misleading indicators of climate (Finkelstein et al., 2006). The fact that their pollen is found mixed with that of temperate species such as oaks across the “boreal parkland” (e.g., Webb et al., 1988, 1993, 1997), further calls into question their indicator status. When models are created for climate, the climatic tolerance of individual species is used to create a regression model that predicts climate, or a biome is defined from the dominant species, and that biome is used as an indicator of climate. Inclusion of ambiguous genera such as spruce in the model will create a cold bias. This analysis also helps explain part of the no-analog vegetation situation.