Late-Quaternary vegetation dynamics in North America:  scaling from taxa to biomes

J. W. Williams, B. N. Shuman, T. Webb III, P. J. Bartlein, P. L. Leduc

2004 Ecological Monographs in press

Abstract — This paper integrates recent efforts to map the distribution of biomes for the late Quaternary with the detailed evidence that plant species have responded individualistically to climate change at millennial timescales.  Using a fossil pollen dataset of over 700 sites, we review late-Quaternary vegetation history in northern and eastern North America across levels of ecological organization from individual taxa to biomes, and apply the insights gained from this review to critically examine the biome maps generated from the pollen data.  Higher-order features of the vegetation (e.g. plant associations, physiognomy) emerge from individualistic responses of plant taxa to climate change, and different representations of vegetation history reveal different aspects of vegetation dynamics.  Vegetation distribution and composition were relatively stable during full-glacial times (21-17 ka) and the mid- to late Holocene (7-0.5 ka), but changed rapidly during the late-glacial period and early Holocene (16-8 ka) and after 0.5 ka.  Shifts in plant taxon distributions were characterized by individualistic changes in population abundances and ranges, and included large meridional shifts in distribution in addition to the northward redistribution of most taxa.  Modern associations such as Fagus-Tsuga and Picea-Alnus-Betula date to the early Holocene, whereas other associations common to the late-glacial period (e.g. Picea-Cyperaceae-Fraxinus-Ostrya/Carpinus) no longer exist.  Biomes are dynamic entities that have changed in distribution, composition, and structure over time.  The late-Pleistocene suite of biomes is distinct from those that grew during the Holocene.  The pollen-based biome reconstructions are able to capture the major features of late-Quaternary vegetation but downplay the magnitude and variety of vegetational responses to climate change by 1) limiting apparent land-cover change to ecotones, 2) masking internal variations in biome composition, and 3) obscuring the range shifts and changes in abundance among individual taxa.  The compositional and structural differences between full-glacial and recent biomes of the same type are similar to or greater than the spatial heterogeneity in the composition and structure of present-day biomes.  This spatial and temporal heterogeneity allows biome maps to accommodate individualistic behavior among species, but masks climatically important variations in taxonomic composition as well as structural differences between modern biomes and their ancient counterparts.

Figures

Figure 1:  The number of pollen sites available in boreal and eastern North America per time interval (solid line) and the total unglaciated land area in North America (dashed line).   PDF (4 KB)
Figure 2:  Single-taxon isopoll maps, group isopoll maps, and inferred biome distributions in boreal and eastern North America since the last glacial maximum.  In the single-taxon maps, the pollen abundances of a single taxon are displayed as various shades of green, with high color saturations corresponding to high abundances.  Regions with insufficient data for mapping are left blank.  In the multi-taxon isopoll maps, each of three pollen taxa is mapped as ‘present’ or ‘absent’, and the eight possible combinations of presence and absence are each mapped as a distinct color (Jacobson et al. 1987). Primary colors (red, blue, cyan) indicate regions where only one taxon is present in abundance.  Secondary colors (orange, purple, and green) indicate associations between pairs of taxa; areas where all three taxa are associated are beige.  The abundance threshold chosen for each plant taxon is set relatively high to indicate only those regions where the plant was an important constituent of the regional vegetation (Jacobson et al. 1987).  By reading the maps horizontally, one can track the history of a single plant taxon, plant association, or biome.  Vertical comparisons across maps provide information about the interplay among ecological resolutions.  Animated versions of these maps and others not shown here may be viewed at www.ngdc.noaa.gov/paleo/pubs/williams2003/.  For all maps, map projection is Albers equal area with standard parallels 33.33°N and 66.66°N, center point=70°N,100°W.  Biome abbreviations:  CDEC=Cold Deciduous Forest, TAIG=Taiga, CCON=Cool Conifer Forest, CLMX=Cool Mixed Forest, TDEC=Temperate Deciduous Forest, WMMX=Warm Mixed Forest, XERO=Xerophytic Scrub, MXPA=Mixed Parkland, SPPA=Spruce Parkland, CWOD=Conifer Woodland, STEP=Steppe, DESE=Desert, TUND=Tundra.  PDFa (4.1 MB), PDFb (4.8 MB)
Figure 3:  As Figure 2, for Tsuga, Tsuga/Fagus/Pinus, Quercus, Carya/Quercus/Liquidambar, prairie forbs, prairie forbs/Cyperaceae/Poaceae, and biomes.  The prairie forb category comprises Asteraceae and Chenopodiaceae/Amaranthaceae.  The biome maps are repeated from Figure 2 for comparison.  PDFa (4.1 MB), PDFb (4.9 MB)
Figure 4:  Maps of the squared chord dissimilarity (SCD) between adjacent time intervals.  To measure vegetation change, dissimilarities were calculated only within-core (Overpeck et al. 1991, Grimm and Jacobson 1992).  Because all intervals are equally spaced in time (except for 0.5 ka), the mapped dissimilarity values represent both the magnitude and rate of vegetation change.  For comparison, modern pollen samples drawn from different vegetation formations typically have SCD’s>0.15 (Overpeck et al. 1985). PDF (2.1 MB) 
Figure 5:  Anomaly maps for biomes, squared-chord distances, Quercus, Picea, Pinus, and prairie forbs, for the last glacial maximum vs. pre-settlement vegetation (21 ka vs. 0.5 ka), and mid-Holocene vs. pre-settlement (6 ka vs. 0.5 ka).  The biome anomaly maps show the past biome assignments for the gridpoints which differ in biome type between past and pre-settlement vegetation.  Areas with no data or with unchanged biome assignments are left blank.  The dissimilarity maps show the aggregate palynological differences between the past and present; darker reds indicate higher dissimilarities.  In the anomaly maps for individual plant taxa, darker browns indicate that the taxon was locally less abundant in the past; green indicates that the taxon was locally more abundant in the past.  PDF (1.2 MB)
Figure 6:  For the cool mixed forest and temperate deciduous forest, plots comparing temporal variations in composition to their present-day spatial heterogeneity.  The time-series plots show long-term variations of mean pollen percentages for individual taxa and plant life forms, averaged across all pollen sites assigned to those biomes.  Vertical axes represent the percent pollen abundance for the indicated taxa.  The histograms in the top plots show the number of pollen samples (before gridding) assigned to each biome per time interval.  Fossil pollen records are distributed non-randomly in space and time, so number of samples is only partially related to biome area.  At least two pollen records had to be assigned to a biome for an average to be calculated.  The present-day spatial variability within each biome is shown in the box plots to the right of each time series.  The line bisecting each box is the median, the boxes are bounded by the 1st and 3rd quartiles, and the whiskers denote the 5% and 95% limits.  Note that the right and left Y-axes are often scaled differently, but each axis is identically scaled within pairs of time-series and box plots.  PDF (1.4 MB) 
Figure 7:  As Figure 6, for the eastern and western varieties of the tundra (dividing line set at 105°W).  PDF (1.3 MB) 
Figure 8:  As Figure 6, for the eastern and western varieties of the taiga.  PDF (1.2 MB)
Figure 9:  As Figure 6, for the eastern and western cool conifer forests.  PDF (1.4 MB)
Figure 10:  As Figure 6, for the warm mixed forest and steppe.  PDF (1.2 MB)
Figure 11:  As Figure 6, for the mixed parkland.  Because the mixed parkland is nearly extinct today, the box plots show the spatial variability for 14 ka (marked by vertical bar in time series).  PDF (1.0 MB)
Figure 12:  Schematic of the reciprocal interactions between the vegetation and atmosphere.  Species distributions at regional to continental scales are primarily determined by climate; variations in climate are the primary driver of vegetation dynamics at millennial timescales.  Individualistic species responses result in continuously changing associations among plants and long-term variations in vegetation physiognomy, described as the distribution of plant functional types and biomes.  Gross changes in vegetation structure alter the physical properties of the land surface, modulating the exchanges of energy, moisture, and carbon between the atmosphere and vegetation.  PDF (9 KB)