Climatically
forced vegetation dynamics in eastern North America during the
late-Quaternary
T. Webb III, B. N. Shuman, and J. W. Williams
2003 The Quaternary Period in the United States
Chapter 21
Introduction
— Vegetation
dynamics span multiple spatial and temporal scales, and the changes
involved
manifest themselves in a variety of ways depending upon the ecological
unit
(from individuals to biomes) and/or taxonomic level (e.g., species ,
genera, families
and orders) of description. Many biotic phenomena contribute to
vegetation
change including 1) the establishment, growth, and death of individual
plants
within stands, 2) changes in the frequency, size, and genetic make-up
of
populations within landscapes, 3) changes in the distribution of
species,
genera, and plant functional types across regions and continents, and
4) the
evolution and extinction of species. These biotic phenomena cause the
vegetation to change in structure, density, extent, and composition,
and they
lead to and result from a variety of biospheric dynamics (such as
variations in
net primary production and carbon sequestration). Depending on scale,
vegetation changes are caused by some combination of external (i.e.,
environmental) forcing and the biotic phenomena themselves. The
multiple
competing forcings (at work at different scales) and many nonlinear
linkages
(including feedbacks) can make the cause-and-effect explanations
difficult to
sort out at certain temporal and spatial scales. Across long-time
spans,
however, such as the late Quaternary, environmental variations are
large and
well known and their effect on vegetation history is relatively easy to
recognize.
In this chapter, we consider
vegetation dynamics at regional to continental scales and across
millennia,
scales at which vegetation change is primarily forced by centennial to
orbital
scales of climate change. The vegetation changes show up as the
changing
abundance, geographic extent, location, and association of plant taxon
populations, which we record as changing pollen percentages. Only by
linking
the forces and induced responses can we convert the study of vegetation
change
and history into an analysis of vegetation dynamics, because to do so
we must
relate the apparent “motion’ in these taxon populations to underlying
forces,
which is the very definition of dynamics. Motion by definition is
temporal
change in location, which requires temporal sequences of maps,
difference maps,
and/or isochrone maps to illustrate. Mapping temporal change in the
vegetation
is therefore central to studies of climatically forced vegetation
dynamics.
Here, we map fossil pollen data, as a proxy for vegetation data, from
eastern
and northern North America and compare both continental-scale and local
records
of the pollen-recorded vegetation change to maps and time series of
independently observed or estimated paleoclimate data. These
comparisons are
key to our empirical understanding of late-Quaternary vegetation
dynamics. We
admit that the “motion” of taxon populations shown on our maps is an
epiphenomenon of the differential carbon sequestration in the different
taxa in
different locations, but we focus here on the motion apparent in the
time
series of pollen maps and use it and other pollen-recorded changes to
represent
how the vegetation changed. Many studies show how well pollen data from
surficial sediments represent plant taxon abundances today and thus
underpin
our interpretative step here (Webb, 1974; Bradshaw and Webb, 1985;
Jackson,
1994; Webb, 1995).
Datasets of lake-level variations,
chironomid-inferred temperatures, and stable isotope ratios, as well as
climate
model output, help us to show the “forces” behind vegetation changes
and to
identify dynamics. We therefore take advantage of advances in
paleoclimate
data, analysis, and modeling that are providing an increasingly
detailed
picture of late-Quaternary climate changes. Just as radiocarbon dating
freed
pollen data from a correlation-based time frame, newly developed
paleoclimate
datasets now allow pollen data to be interpreted within an
independently
derived climate framework. We can therefore describe how the vegetation
responded to multivariate changes in climate involving temperature,
moisture,
and seasonality.
We use both time series and maps of pollen data and
climate estimates 1) to illustrate a strong connection between climate
and
vegetation change, 2) to document continental- and regional-scale
vegetation
dynamics that result from millennial- and orbital-scale climate
forcing, and 3)
to demonstrate that the conditions held for dynamic equilibrium between
vegetation and climate at orbital time scales and possibly at
millennial
scales. By mapping both individual taxa and assemblages of taxa, we
describe
vegetation responses to independently documented climatic forcing at
several
levels of ecological organization from taxon movements to shifts in
biome
position, extent, and composition. Our chapter focuses on examples from
North
American vegetation history that illustrate key climatically forced
vegetation
dynamics. In doing so, we aim to complement the discussion of
vegetation
history by Grimm and Jacobson (this vol.), Thompson (this vol.), and
Anderson
(this vol.), and build on the critical reviews written by Cushing
(1965), Davis
(1965), and Whitehead (1965) that Grimm and Jacobson (this vol.) so
ably review
in their introduction. Too few pollen diagrams with radiocarbon dates
existed
for mapping the data on an independent time frame in 1965. Since then
palynologists have published over 500 pollen diagrams with radiocarbon
dates in
eastern and northern North America. Other researchers have generated
data
independent of pollen data for estimating past changes in climate, and
climate
modeling has yielded valuable simulations of late Quaternary climates
and
climate change (Wright et al., 1993; Webb, 1998). These developments
allow a
fresh understanding of vegetation dynamics
and testing of many of the hypotheses posed by Cushing (1965),
Davis
(1965), and Whitehead (1965).