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POLLEN RECORDS OF LATE QUATERNARY VEGETATION CHANGE: PLANT COMMUNITY REARRANGEMENTS AND 227 EVOLUTIONARY IMPLICATIONS these climate oscillations indicates that the continuously changing ecological theater on orbital time scales may be a long- term feature in the geological record and hence not restricted to the Quaternary. For example, the long time series of vegetation changes from Sabana de Bogota in Colombia reveals large quasi-periodic changes dating back more than 1.5 m.y. (Hoogheimstra, 1989). Species, therefore, have evolved in the face of orbitally forced climatic changes, and orbital cycles may be as familiar to million-year-old species as the annual cycle is to 100-yr-old trees. Along with intraspecific plasticity in gene frequency, species migration and major population expansion under favorable conditions are two key mechanisms by which species respond to the orbitally induced climate changes (Dexter et al., 1987; Huntley and Webb, 1989; Bennett et al., 1991). TIME AND SPACE SCALES OF VEGETATIONAL AND TAXONOMIC UNITS One of the unifying figures for global change research is a scatter diagram of various earth-system processes plotted along log-scaled axes in units of time and area (e.g., Figure 2.3 in Earth System Sciences Committee, 1988). This type of plot shows the regions in space and time in which selected phenomena occur because of their characteristic time constants or spatial extent. It also provides a useful way to view the interaction of ecological and evolutionary phenomena. The diagram mixes many different phenomena. McDowell et al. (1990) recently separated the climatic, vegetational, and geomorphic phenomena and plotted them on different graphs. The graph for climate (Figure 13.5) shows the different spatial and temporal scales of Figure 13.5 Characteristic spatial and temporal scales for variations in weather and climate (from McDowell et al., 1990). Each bubble encloses a group of related types of variations, in which the shorter-term, smaller-area units are part of the longer-term, larger-area units. The shortest-term units (weather) are limited primarily to the atmosphere, but the longer-term variations involve progressively more earth systems.
POLLEN RECORDS OF LATE QUATERNARY VEGETATION CHANGE: PLANT COMMUNITY REARRANGEMENTS AND 228 EVOLUTIONARY IMPLICATIONS weather features up through the different spatial and temporal scales of climate phenomena on a tectonic (tens of millions of years) time scale. For each time scale, smaller-scale features have shorter time constants than large-scale features. The idea that a minimum time and area can be assigned to each feature may seem arbitrary at first because we experience large-scale features such as low pressure systems locally as well as regionally. However, we experience these large-scale features in terms of regional changes such as a frontal passage, and these regional features are included in the large-scale system. The lower limits also make sense as the smallest area or shortest time over which one is forced to recognize that a larger or longer- term feature exists. Figure 13.6 Characteristic temporal and spatial scales for vegetational (ecological) and taxonomic (evolutionary) units (from McDowell et al., 1990). The former appear in boldface, the latter in italics. The chosen scales apply most particularly to wind-pollinated plants. Using the plots of climatic and oceanographic features as an example (Figure 13.5, and Haury et al., 1978), Delcourt et al., (1983) published the original version of the scale diagram for ecological phenomena, and McDowell et al. (1990) used the evidence on vegetational rearrangements (Figure 13.4, Webb, 1988) to revise the Delcourt et al. (1983) figure (Figure 13.6). McDowell et al. (1990) also added the set of evolutionary or taxonomic categories, in part inspired by Eldridge's (1985) Unfinished Synthesis, in which he noted that individual organisms can be organized either into communities or into taxa (i.e., into either ecological or systematic units). The lineup of ecological phenomena begins with trees being 10 to 200 yr old and covering on average 10-5 km2,
