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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



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Effects of Past Global Change on Life 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

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Effects of Past Global Change on Life group of major actors. Selection must therefore occur on a stage with continuously changing combinations of environmental variables and competitors, and the local setting (specific soils, relief, etc.), local players, and recent arrivals generally have only a short-term influence on the evolutionary play. Within this chapter, I begin by describing pollen data and their ability to record patterns in present and past vegetation. I then summarize what recent mapping studies of late Quaternary pollen data have illustrated about past changes in vegetation and their link to climate change. Simulations of past climates by climate models have aided these studies (Barnosky et al., 1987; Webb et al., 1987). These models are being used as tools for generating hypotheses that the data can test (COHMAP, 1988; Wright et al., 1993). In the final sections, I discuss (1) how an understanding of the space and time scales for taxonomic ecological units leads to a temporal separation of ecological units from evolutionary units for many plant taxa (McDowell et al., 1990) and (2) the advantages of a 4-dimensional space-time perspective of the data. SENSITIVITY OF POLLEN DATA TO VEGETATION PATTERNS Pollen data are sensitive to a wide spectrum of spatial and temporal variations in the vegetation (Figure 13.1). These can vary from local succession (Janssen, 1967; Bradshaw, 1988; Edwards, 1986) up to long-term continental-scale changes in plant formations (Jacobson et al., 1987; Huntley, 1990). The data can also record how the vegetation was influenced by humans (Behre, 1988; Birks et al., 1988; McAndrews, 1988), disease (Davis, 1981; Webb, 1982; Allison et al., 1986), fire (Patterson and Backman, 1988; Clark, 1990), soils (Webb, 1974; Brubaker, 1975; Jacobson, 1979; Bernabo, 1981; Tzedakis, 1992), topography (Janssen, 1981; Gaudreau et al., 1989; Lutgerink et al., 1989; Jackson and Whitehead, 1991), and climate (Bartlein et al., 1984; Webb et al., 1987; Huntley and Prentice, 1988; Prentice et al., 1991). With such a variety of possible processes and influences, palynologists must choose appropriate methods of data collection, analysis, and display to obtain results indicative of the vegetational variations of particular interest (Webb et al., 1978, 1993; Jacobson and Bradshaw, 1981; Grimm, 1988). Figure 13.1 Relative abundance of oak trees and oak pollen  at different spatial scales from that of a subcontinent, a state, and  a county (from Solomon and Webb, 1985). A key metaphor for understanding the interpretation of pollen data is to think of them as remotely sensed vegetation data (Webb, 1981; Webb et al., 1993). Just as the current vegetation emits or reflects radiation that remote sensors on satellites intercept, so too does (and has) the current (and past) vegetation shed pollen that accumulates "remotely" (i.e., well away from the source) in lakes and bogs. Both types of "remote" sensors record data with certain sampling characteristics (e.g., spatial and temporal resolution), and their data need calibration in terms of climate or vegetation variables. One major thrust in palynology, therefore, has been the analysis of modern pollen data to see what features of the modern vegetation are recorded (Figures 13.1 to 13.3). Palynologists have attempted to learn what the modern vegetation looks like in pollen terms (Webb, 1974), so that they can better visualize the past vegetation, which is only represented in pollen (or other fossil) terms. Palynologists have also studied modern data in order to learn how varying the sampling characteristics of the data can alter what vegetational features or processes are represented (Janssen, 1966; Andersen, 1970; Webb et al., 1978; Heide and Bradshaw, 1982; Bradshaw and Webb, 1985; Prentice et al., 1987; Prentice, 1988; Jackson, 1990, 1991). Recent mapping studies at the subcontinental scale (Peterson, 1984; Webb, 1988; Huntley, 1990; Anderson et al., 1991) have shown how well the modern pollen data match contemporary vegetation patterns. In eastern North

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Effects of Past Global Change on Life FIGURE 13.2 Zoom-lens view of the vegetation from local to global showing the different levels of subdivision of the vegetation at each mapping scale (from Kutzbach and Webb, 1991).

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Effects of Past Global Change on Life FIGURE 13.3 Maps (from Webb et al., 1993) with contours of equal pollen frequency that show the current distribution for 9 pollen types. For forbs (Artemisia, other Compositae excluding Ambrosia, and Chenopodiaceae/Amaranthaceae), sedge (Cyperaceae), birch (Betula), spruce (Picea), and alder (Alnus), black areas are >20%, dark gray 5-20%, and light gray 1-5%. For oak (Quercus) and pine (Pinus), the values are >40%, 20- 40%, and 5-20%. For fir (Abies), hemlock (Tsuga), beech (Fagus), and hickory (Carya), the values are >5%, dark gray 1-5%, and light gray 0.5-1%. America, for example, maps for 12 major pollen categories record the patterns of the major formations from the western prairie into the eastern forests and from the northern tundra into the southeastern pine-oak forest (Figures 13.2 and 13.3). At this scale, the major vegetation gradients are aligned with temperature and moisture gradients. The pollen maps show not only the pattern of the major plant assemblages (i.e., formations) but also the steepness and position of the ecotones between them. The maps also show the changes in species composition not only among but within the major plant assemblages. Within the mixed forest, for example, a gradient exists between the dominance of birch, beech, and hemlock in the east and the dominance of pine in the west. Other mapping studies show similar ability of pollen to record vegetational patterns at state and county levels in support of the general picture shown in Figure 13.1 (Webb, 1974; Webb et al., 1983; Bradshaw and Webb, 1985; Jackson, 1991). At these spatial scales, the vegetation patterns reflect differences in soils, topography, and disturbance history as well as climate gradients. Pollen data can therefore give a zoom lens view of the vegetation when spatial arrays of the samples are properly organized (Figure 13.1). By being sensitive to vegetation at various spatial scales, the pollen data are also sensitive to a variety of factors affecting the vegetation from climate at the continental scale down to disturbance history and patch dynamics at local sites (Figure 13.2; Delcourt et al., 1983; McDowell et al., 1990). MAPS OF CHANGING TAXON DISTRIBUTION THROUGH TIME Time sequence maps for the past 18,000 yr show where and when the modern patterns and vegetational regions developed. Webb (1988) and Jacobson et al. (1987) used time series of maps such as those in Figure 13.4 to infer that the modern boreal forest, for example, with its coincident patterns of spruce, birch, and alder, developed only after 6000 yr ago (ka). From 18 to 12 ka in the area south of the ice sheet between treeless boreal vegetation and pine-dominated mixed forests, spruce, sedge, and sagebrush pollen co-occurred, indicating widespread growth of a spruce parkland. From 11 to 7 ka, transitional forests near the ice front were dominated first by spruce and later by birch populations. Considerable compositional and structural change therefore occurred in the regions where spruce trees grew, and the modern boreal forest was one outcome of these changes. Similar types of sequences of compositional change occurred for other modern plant assemblages (Davis, 1983). Webb (1988) used time series of maps like those in Figure 13.4 to infer that the modern deciduous forests developed by 12 ka; prairie and tundra by about 10 ka; mixed forest with beech, birch, and hemlock after 8 ka; and the southeastern pine-oak forest after 8 ka. Maps of European vegetation for the past 12,000 yr illustrate similar types of broad-scale changes in the abundance, location, and association of taxa, with consequent changes in vegetational assemblages (Huntley, 1990). In the western United States, where low regional site density and topographic complexity have thus far precluded mapping studies, the data also show individualistic changes among taxa (Thompson, 1988). In each of these regions,

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Effects of Past Global Change on Life FIGURE 13.4 Maps of pollen frequency for sedge (Cyperaceae), spruce (Picea), birch (Betula), alder (Alnus), fir (Abies), and pine (Pinus) from 18 ka to present (0 ka). For all taxa but fir, black areas are >20%, dark gray 5-20%, and light gray 1-5%. For fir, black are >5%, dark gray 1-5%, and light gray 0.5-1%.

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Effects of Past Global Change on Life the climate changes were large, and simulations of past climates by climate models have reproduced many of the patterns of change in the data (COHMAP, 1988; Wright et al., 1993). Mapping studies of the fossil pollen data at regional and local scales also show variations in composition, location, and extent of vegetation. At these scales, variations in elevation and soil type become important along with climate in shaping the development of the vegetation (Davis et al., 1980; Webb et al., 1983; Ritchie, 1987; Gaudreau et al., 1989; Woods and Davis, 1989; Jackson and Whitehead, 1991). Studies at each spatial scale show the independent behavior of individual taxa and the resultant changes in vegetational composition. Many of the past pollen assemblages have no analogues among modern pollen samples (Baker et al., 1989; Overpeck et al., 1992). Each of these types of vegetational change has occurred during the switch from full glacial to interglacial climates, and is likely to have occurred each time that such shifts in climate occurred in Earth history (see Figure 7.2 in Stanley and Ruddiman, Chapter 7, this volume). During the past 700,000 yr, the major changes have occurred seven times, and the estimated total change in the global mean temperature is 5° ± 1°C (Webb, 1991). During times with lesser degrees of global climate change, the changes in vegetation were also less dramatic but probably still involved significant changes in community composition that produced assemblages without modern analogues. As Figure 7.2 in Stanley and Ruddiman (Chapter 7, this volume) shows, the climate has been changing continuously for millions of years; therefore, the vegetation is a continuously changing set of variables chasing a continuously changing set of other variables, namely, climate (Webb and Bartlein, 1992). Webb (1986) and Prentice et al. (1991) have interpreted these compositional changes and consequent no-analogue assemblages as resulting primarily from the different climatic response of each taxon to the changing mixture of climatic variables as climates have varied temporally. They argue for the taxa being in dynamic equilibrium with climate (Prentice, 1986; Webb, 1986). Studies matching observed pollen maps with those simulated from climate model output provide support for this interpretation (Webb et al., 1987; COHMAP, 1988). Other researchers argue for disequilibrium conditions between plant taxa and climate. They have given major emphasis to the role of biotic factors, such as differing dispersal rates and time lags for populations growth, when interpreting the development of no-analogue assemblages and patterns of species migration (Bennett, 1985; Birks, 1986). Recognition is now developing that a hierarchy of factors is operating, and that the importance of different factors varies with time and space scale (Davis, 1991). Biotic factors are most evident over short time and small spatial scales, and climatic impact is most evident over long time and large spatial scales. IMPLICATIONS FOR SPECIES AND EVOLUTION No matter which interpretation is favored (equilibrium or disequilibrium), the pollen record shows major changes in plant assemblages at all spatial scales with major plant assemblages (i.e., formations) having an average life time of ca. 10,000 yr in response to orbitally driven climate change. Consideration of the record of climate forcing for the past 2.8 million years (m.y.) (Figure 7.2 in Stanley and Ruddiman, Chapter 7, this volume) reveals that this forcing has been long-term, large, and continuous (Webb and Bartlein, 1992). The net result has been a continuously changing ecological theater for the evolutionary play (Hutchinson, 1965), and individualistic behavior has produced a continuously changing role, setting, and cast of associated characters for each taxon. Despite all this environmental and ecological change, most species have survived. Evidence from the fossil record suggests that the average longevity of species is 1 to 10 m.y. (Stanley, 1985). One reason for the longevity may be the relatively high frequency of mixing (induced by changes in species abundance, distribution, and association) that prevents long-term isolation of genetically distinct populations (Coope, 1978; Webb, 1987; Bartlein and Prentice, 1989). Gould (1985) and Bennett (1990) discuss how "progress in life's history" may be thwarted, and selection over 10,000 years or less ("ecological time") is erased or lost by longer-term processes. In a well-argued paper, Bennett (1990) identifies orbitally forced climate change, which occurs at time scales of 20,000 to 100,000 yr, as the key longer-term process. As stated by Bartlein and Prentice (1989), "The paleoecological record of the past 20,000 years demonstrates that orbitally induced climatic changes produce changes in the distribution of organisms, leading to the quasi-cyclical alternation between allopatry and sympatry, commonness and rarity, continuous distribution and fragmentation." Furthermore, if recognition is given to the orbital control of long-term variations in monsoonal climates that depends on land-sea contrast (Kutzbach, 1981; COHMAP, 1988; Kutzbach and Webb, 1991), then the orbital pulse to climate can be seen to be possible in the absence of large ice sheets and to have a long history throughout the geological record. Crowley et al. (1986) and Ruddiman and Kutzbach (1989) have explored the potential implications of known tectonic changes on this mechanism for climate change, and Barnosky (1984), Olsen (1986), and others (see Berger et al., 1984) have documented orbitally driven climate changes during several intervals of the Phanerozoic. The long-term occurrence of

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Effects of Past Global Change on Life 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.

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Effects of Past Global Change on Life 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. 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. 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,

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Effects of Past Global Change on Life and then shows the grouping of trees into communities whose upper size is determined by the area over which organisms interact directly with each other (Figure 13.6). Modern vegetation maps illustrate the different areas for vegetation regions (Figure 13.2), and Figure 13.4 and estimates from Webb (1988) for the time scale of recent formations fix the time scale for formations (see earlier discussion). The assumption that units with smaller areas exist for shorter times leads to the alignment of ecological units. This alignment illustrates the assumed role of orbital forcing in setting the upper age limit for the phenomena. Were the time scale of orbital forcing to change, then the age limit for formations should be changed. An alignment of evolutionary phenomena was constructed by plotting the temporal and spatial dimensions of genetically related units from individuals up through all levels of taxonomic units. The estimates from the fossil record for the average longevity of each species is 1 to 10 m.y. This fact leads to the evolutionary units being plotted along a different slope from the ecological units. The average pollen dispersal distance sets the size of demes for wind-pollinated trees (Levin and Kerster, 1975; Bradshaw and Webb, 1985). Incipient species represent genetically distinct populations that are evident today or at any time, but disappear within 1000 to 10,000 yr and therefore never fully qualify as species. Stanley (1979) defined them as aborted species. In general, higher taxonomic units such as families will be longer lived and more widely distributed than lower units such as genera, species, populations, or demes. A major result from constructing this figure is its illustration of how the ecological and the evolutionary axes diverge. The observations and theory underlying this divergence have been discussed in the previous section. Such figures as this should be useful in designing studies of how ecological processes influence evolution. These studies should lead to revisions in this figure as better understanding is obtained of the time constants and spatial coverage for each unit plotted. SPACE-TIME PERSPECTIVE When the temporal sequence of maps for spruce pollen is stacked to form a box, the contours among maps can be connected to form a three-dimensional surface in space and time (Webb, 1988; Banchoff, 1990). If contours for several abundance levels are connected, then we have a four-dimensional plot with abundance (a), varying in space (x, y), and time (t). Various cross sections can be removed from this box, such as maps and latitude-time or longitude-time plots. In terms of the space-time box, the maps representing plant distributional data for today are just an arbitrary cross section of a continuously changing four-dimensional distribution. 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