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POLLEN RECORDS OF LATE QUATERNARY VEGETATION CHANGE: PLANT COMMUNITY REARRANGEMENTS AND 221 EVOLUTIONARY IMPLICATIONS 13 Pollen Records of Late Quaternary Vegetation Change: Plant Community Rearrangements and Evolutionary Implications Thompson Webb III Brown University INTRODUCTION Radiocarbon-dated records of late-Quaternary pollen data provide a unique window on biospheric dynamics. They illustrate the vegetational response to large-scale climatic forcing; provide a space-time view of community and plant population variations; and fill a gap between short-term observations of ecological patterns and dynamics and the long-term fossil records of the Phanerozoic. With temporal sequences of maps in 1000-yr intervals, the paleoecological records of the past 18,000 yr add both a temporal dimension to ecological observations and a temporally and spatially precise mapping view to the standard fossil record. Succession, which has long dominated ecological thinking, no longer appears as the dominant mode of vegetational dynamics when viewed within the context of continental-scale maps of the late Quaternary pollen data. Rather these maps depict populations of sessile plants as mobile entities that move in response to orbitally paced glacial-interglacial climate changes (Huntley and Webb, 1989). Such movement allows the plant populations to track the climate conditions favorable for their growth and indicates that evolutionary responses among these populations are secondary to migration. Were evolution the primary response to Quaternary climate change, then plant taxa would maintain fixed populations south of the advancing or retreating ice sheets; oaks, for example, would evolve new climate tolerances rather than being replaced by pines or other trees more suited to the new climate conditions. The rates of migration are sufficiently fast that the plant taxa can match (with relatively small lags) the rates of orbitally forced climatic change; otherwise, the plant taxa would have gone extinct long ago (Webb, 1986). Studies of modern pollen and vegetation data show that the taxonomic resolution in pollen data is good enough to allow resolution of vegetational patterns across continents, states, and counties (Figure 13.1). Maps of temporal changes in the data illustrate the independent movement of individual taxa, and this individualistic behavior leads to community breakup and rearrangement. Within the context of the periodic large changes in Quaternary climates, communities are ephemeral within cycles, though often recurrent between cycles, and represent epiphenomena that arise out of the changing co-occurrence of plant taxa (Davis, 1983; Jacobson et al., 1987; Webb, 1987; Jackson and Whitehead, 1991). Despite the large changes in late Quaternary climates and communities, most mid- to high latitude plant genera and species changed quickly enough in abundance and location to survive. Such an observation raises interesting questions about the nature of the ecological theater within which evolution occurs (Hutchinson, 1965). Climate forces the theater to be a traveling road show with a changing
