Thompson Webb

Quantitative Interpretation of Fossil Pollen Spectra: Dissimilarity Coefficients and the Method of Modern Analogs

Abstract Dissimilarity coefficients measure the difference between multivariate samples and provide a quantitative aid to the identification of modern analogs for fossil pollen samples. How eight coefficients responded to differences among modern pollen samples from eastern North America was tested. These coefficients represent three different classes: (1) unweighted coefficients that are most strongly influenced by large-valued pollen types, (2) equal-weight coefficients that weight all pollen types equally but can be too sensitive to variations among rare types, and (3) signal0to-noise coefficients that are intermediate in their weighting of pollen types. The studies with modern pollen allowed definition of critical values for each coefficient, which, when not exceeded, indicate that two pollen samples originate from the same vegetation region. Dissimilarity coefficients were used to compare modern and fossil pollen samples, and modern samples so similar to fossil samples were found that most of three late Quaternary pollen diagrams could be “reconstructed” by substituting modern samples for fossil samples. When the coefficients indicated that the fossil spectra had no modern analogs, then the reconstructed diagrams did not match all aspects of the originals. No modern analogs existed for samples from before 9300 yr B.P. at Kirchner Marsh, Minnesota, and from before 11,000 yr B.P. at Wintergreen Lake, Michigan, but modern analogs existed for almost all Holocene samples from these two sites and Brandreth Bog. New York.

Reid's Paradox of Rapid Plant Migration Dispersal theory and interpretation of paleoecological records

James S. Clark, Jason Lynch, and Peter Wyckoff are in the Department of Botany, Duke University, Durham, NC 27708; Chris Fastie and Stephen T. Jackson are in the Department of Botany, University of Wyoming, Laramie, WY 82701; George Hurtt and Stephen Pacala are in the Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544-1003; Carter Johnson is in the Department of Horticulture and Forestry, South Dakota State University, Brookings, SD 57007; George A. King is at Dynamic Corporation, US EPA National Health and Environmental Effects Research Laboratory, Corvallis, OR 97333; Mark Lewis is in the Math Department, University of Utah, Salt Lake City, UT 84112; Colin Prentice is at the School of Ecology, Lund University, Lund, Sweden; Eugene W. Schupp is in the Department of Rangeland Resources, Utah State University, Logan, UT 84322; and Thompson Webb III is in the Department of Geological Sciences, Brown University, Providence, RI 029121846. ? 1998 American Institute of Biological Sciences. A plausible explanation

Late-quaternary vegetation dynamics in north america: Scaling from taxa to biomes

This paper integrates recent efforts to map the distribution of biomes for the late Quaternary with the detailed evidence that plant species have responded individual- istically to climate change at millennial timescales. Using a fossil-pollen data set 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, physiog- nomy) emerge from individualistic responses of plant taxa to climate change, and different representations of vegetation history reveal different aspects of vegetation dynamics. Veg- etation distribution and composition were relatively stable during full-glacial times (21 000- 17 000 yr BP) (calendar years) and during the mid- to late Holocene (7000-500 yr BP), but changed rapidly during the late-glacial period and early Holocene (16 000-8 000 yr BP) and after 500 yr BP. Shifts in plant taxon distributions were characterized by individ- ualistic changes in population abundances and ranges and included large east-west 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 in- dividualistic behavior among species but masks climatically important variations in taxo- nomic composition as well as structural differences between modern biomes and their ancient counterparts.

Vegetation and Climate Change in Eastern North America Since the Last Glacial Maximum

Response surfaces describing the empirical dependence of surface pollen percentages of 13 taxa on three standard climatic variables (mean July temperature, mean January temperature, and mean annual precipitation) in eastern North America were used to infer past climates from palynological data. Inferred climates at 3000-yr intervals from 18 000 years ago to the present, based on six taxa (spruce, birch, northern pines, oak, southern pines, and prairie forbs), were used to generate time series of simulated isopoll maps for these taxa and seven others (hickory, fir, beech, hemlock, elm, alder, and sedge). The simulations captured the essential features of the observed isopoll maps for both sets of taxa, including differences in migration patterns during the past 10 000 yr that have previously been attributed to differential migration lag. These results establish that the continental-scale vegetation patterns have responded to continuous changes in climate from the last glacial maximum to the present, with lags < 1500 yr. The inferred climatic changes include seasonality changes consistent with orbitally controlled changes in inso- lation, and shifts in temperature and moisture gradients that are consistent with modelled climatic interactions of the insolation changes with the shrinking Laurentide ice sheet. These results pose new ecological questions about the processes by which vegetated land- scapes approach dynamic equilibrium with their changing environment.

Climatic response surfaces from pollen data for some eastern North American taxa

Ecological response surfaces are nonlinear functions describing the way in which the abundances of taxa depend on the joint effects of two or more environmental variables. Continental-scale patterns in the relative abundances of plant taxa are dominated by the effects of macroclimate on the competitive balance among taxa. Pollen analyses record such regional variations for major vegetation components. Empirical ecological response surfaces were derived from high-resolution climate models to yield testable reconstructions of vegeta- eastern North America. The surfaces were obtained by second- or third-degree polynomial regression on two predictor variables, mean July temperature and annual precipitation, with various nonlinear transformations of variables to allow flexibility of shape. Response surface analysis consists of a remapping of abundance patterns from geographic space into climate space, and complements efforts to explain distri- butions in terms of biological processes. Each fitted surface is unique. The surfaces focus attention on the climatic location of range limits and optima, and on less obvious phenomena such as the spatial pattern in the relative sensitivity of different taxa to spatial variation in the climatic variables. Given certain assump- tions, response surfaces based directly on pollen data may be used collectively in a global nonlinear method for estimating past climates from postglacial pollen data. Such response surfaces may also be coupled to palaeoclimatic simulations from high-resolution climate models to yield testable reconstructions of vegeta- tional history.

Changing patterns in the Holocene pollen record of northeastern North America: A mapped summary

Abstract By mapping the data from 62 radiocarbon-dated pollen diagrams, this paper illustrates the Holocene history of four major vegetational regions in northeastern North America. Isopoll maps, difference maps, and isochrone maps are used in order to examine the changing patterns within the data set and to study broad-scale and long-term vegetational dynamics. Isopoll maps show the distributions of spruce (Picea), pine (Pinus), oak (Quercus), herb (nonarboreal pollen groups excluding Cyperaceae), and birch + maple + beech + hemlock (Betula, Acer, Fagus, Tsuga) pollen at specified times from 11,000 BP to present. Difference maps were constructed by subtracting successive isopoll maps and illustrate the changing patterns of pollen abundances from one time to the next. The isochrone maps portray the movement of ecotones and range limits by showing their positions at a sequence of times during the Holocene. After 11,000 BP, the broad region over which spruce pollen had dominated progressively shrank as the boreal forest zone was compressed between the retreating ice margin and the rapidly westward and northward expanding region where pine was the predominant pollen type. Simultaneously, the oak-pollen-dominated deciduous forest moved up from the south and the prairie expanded eastward. By 7000 BP, the prairie had attained its maximum eastward extent with the period of its most rapid expansion evident between 10,000 and 9000 BP. Many of the trends of the early Holocene were reversed after 7000 BP with the prairie retreating westward and the boreal and other zones edging southward. In the last 500 years, mans impact on the vegetation is clearly visible, especially in the greatly expanded region dominated by herb pollen. The large scale changes before 7000 BP probably reflect shifts in the macroclimatic patterns that were themselves being modified by the retreat and disintegration of the Laurentide ice sheet. Subsequent changes in the pollen and vegetation were less dramatic than those of the early Holocene.

Vegetation and environment in Eastern North America during the Last Glacial Maximum

Abstract Knowledge of the vegetation and environment of eastern North America during the Last Glacial Maximum (LGM) is important to understanding postglacial vegetational and biogeographic dynamics, assessing climate sensitivity, and constraining and evaluating earth-system models. Our understanding of LGM conditions in the region has been hampered by low site density, problems of data quality (particularly dating), and the possibility that LGM vegetation and climate lacked modern analogs. In order to generate improved reconstructions of LGM vegetation and environment, we assembled pollen and plant macrofossil data from 21 and 17 well-dated LGM sites, respectively. All sites have assemblages within the LGM timespan of 21,000±1500 calendar yr BP. Based on these data, we prepared maps of isopolls, macrofossil presence/absence, pollen-analogs, biomes, inferred mean January and July temperatures and mean annual precipitation for the LGM. Tundra and open Picea -dominated forest grew along the Laurentide ice sheet, with tundra predominantly in the west. In the east, Pinus -dominated vegetation (mainly P. banksiana with local P. resinosa and P. strobus ) occurred extensively to 34°N and possibly as far south as 30°N. Picea glauca and a now-extinct species, P. critchfieldii , occurred locally. Picea -dominated forest grew in the continental interior, with temperate hardwoods ( Quercus , Carya , Juglans , Liriodendron , Fagus , Ulmus ) growing locally near the Lower Mississippi Valley at least as far north as 35°N. Picea critchfieldii was the dominant species in this region. The Florida peninsula was occupied by open vegetation with warm-temperate species of Pinus . Eastern Texas was occupied by open vegetation with at least local Quercus and Picea . Extensive areas of peninsular Florida and the continental interior had vegetation unmatched by any modern pollen samples. The paleovegetational data indicate more extensive cooling in eastern North America at the LGM than simulated by either the NCAR CCM0 or CCM1 climate models. The occurrence of cool-temperate conifers and hardwoods as far north as 34-35°N, however, indicates less severe cooling than some previous reconstructions. Paleoclimate inferences for the LGM are complicated by lowered atmospheric CO 2 concentrations, which may be responsible for the open nature and dominance of conifers in LGM vegetation.

Relationships between Contemporary Pollen and Vegetation Data from Wisconsin and Michigan, USA

Scatter diagrams and regression analysis of paired pollen and tree—inventory data show how pollen percentages represent the percent basal area for the major arboreal genera in Michigan and Wisconsin. We show that the relationship between pollen and tree percentages for each taxon is generally similar for two states of comparable size and similar vegetation (Wisconsin and Michigan), but that the relationship is influenced by the size of the pollen—collecting site and the size of the area surveyed for trees around each site. These results provide information concerning the relative size of the pollen—source area for seven arboreal pollen types: Pinus, Quercus, Betula, Tsuga, Ulmus, Fagus, and Acer, listed in descending order of pollen—source area. Moderate—sized lakes (30—150 ha) accumulate significant quantities of Pinus and Quercus pollen produced farther than 30 km away, but accumulate relatively few Fagus grains from >4.5 km, and even fewer Acer grains from >2.3 km. The source areas for Betula, Tsuga, and Ulmus pollen lie within 30 km of each lake, and significant quantities of these grains travel farther than 4.5 km. Regression analysis of data from basins of different size supports the hypothesis that small basins collect their pollen from a smaller area of the surrounding vegetation than do large basins.

Mapping eastern North American vegetation change of the past 18 ka: No-analogs and the future

The method of modern analogs and an extensive data base of modern and fossil pollen data were used to generate a new series of paleovegetation maps for eastern North America spanning the past 18 ka. The maps illustrate the continuous nature of climate-induced vegetation change and the development, after about 10 ka, of modern regional vegetation patterns. Before the Holocene, vegetation biomes without modern analogs were widespread in response to climate conditions without modern analogs and, to a lesser extent, to the rapidity of climate change over the last glacial-interglacial transition. This geological perspective suggests that possible future climate changes could force similarly complex changes in natural vegetation, including the development of biomes without modern analogs.

Holocene climatic change in the northern Midwest: Pollen-derived estimates

Mapping of Holocene pollen data in the midwestern United States has revealed several broadscale vegetational changes that can be interpreted in climatic terms. These changes include (1) the early Holocene northward movement of the spruce-dominated forest and its later southward movement after 3000 yr B.P. and (2) the eastward movement of the prairie/forest border into southwestern Wisconsin by 8000 yr B.P. and its subsequent westward retreat after 6000 yr B.P. When certain basic assumptions are met, multiple regression models can be derived from modern pollen and climate data and used to transform the pollen record of these vegetational changes into quantitative estimates of temperature or precipitation. To maximize the reliability of the regression equations, we followed a sequence of procedures that minimize violations of the assumptions that underlie regression analysis. Reconstructions of precipitation during the Holocene indicated that from 9000 to 6000 yr B.P. precipitation decreased by 10 to 25% over much of the Midwest, while mean July temperature increased by 0.5° to 2.0°C. At 6000 yr B.P. precipitation was less than 80% of its modern values over parts of Wisconsin and Minnesota. After 6000 yr B.P. precipitation generally increased, while mean July temperature decreased in the north, and increased in the south. The time of the maximum temperature varies within the Midwest and is earlier in the north and later in the south.