Geoscience Reference
In-Depth Information
A question that has for a long time affected paleoclimate
work using pollen data is the ability of vegetation to respond
rapidly to abrupt climate changes and, therefore, the ability
of pollen records to be used for the study of abrupt climate
changes. Since many trees are long-lived, it is argued that
vegetation responds very slowly to climate changes with a
long lag time [Davis, 1986; Wright, 1976]. However, Webb
[1986] and Huntley and Webb [1989] have argued that veg-
etation changes are in equilibrium with climate changes at
the Milankovitch scale [Webb, 1986]. Studies have shown
that vegetation responds to perturbation at decadal to millen-
nial scales [Wendland and Bryson, 1974; Gajewski, 1987,
1993, 2008; Grimm and Jacobson, 1991; Shuman et al., 2002a,
2002b; Williams et al., 2002; Jackson and Williams, 2004;
Peros et al., 2008], and tree ring and ecological anal-
ysis has extensively documented the response of trees to
climate variations at annual to multidecadal scales [Intergov-
ernmental Panel on Climate Change, 2007]. In part, the
response time depends on the type of vegetation; herbaceous
or shrubby vegetation responds more rapidly, and indeed,
changes in shrub growth at tree line are being observed in
Alaska, presumably in response to global warming [Tape et
al., 2006]. However, even in forests, structural changes in the
vegetation can occur rapidly, after a
2006]. Although the impact of European settlement on
the vegetation of North America is well known, that of
Native Americans is under debate. Several local studies have
shown indications of Native American agriculture in pollen
records from eastern North American, and using pollen and
archeological databases, Munoz and Gajewski [2010, and
references therein] showed this at a regional scale. There
is ongoing research into these questions, as geographers,
archaeologists, and paleobotanists continue to debate ques-
tions such as the size of Native American populations and
their impact on the vegetation. But it seems that climate
change impacts on the vegetation can still be identi
ed in
spite of human impacts on the landscape.
Among other aspects of the paleoenvironmental record
that are not well understood include the importance of CO
2
fertilization on plant production and biodiversity, where it
has been suggested that this may contribute to the so-called
“
communities of the full glacial [Cowling and
Sykes, 1999; Williams et al., 2000]. Nonanalogue conditions
during the late glacial, when CO
2
reached preindustrial lev-
els, are also not entirely explained, although Northern
Hemisphere summer insolation changes may be at cause
[Webb, 1986; Williams and Jackson, 2007]. In addition, the
impact of changes in seasonality on plants and vegetation
needs further study, and there appears to be a fundamental
difficulty in reconstructing seasonal climates using the MAT
[Viau et al., 2008]. At the moment, these effects seem to be
secondary and the broad-scale reconstructions are in general
agreement with independent proxy records.
nonanalogue
”
fire, for example. In an
uneven aged forest, subcanopy trees can quickly start to
grow faster following a climatic or local perturbation, caus-
ing a change in the relative pollen production of the species
in the forest. Finally, it is important to note that the fossil
indicator used in sediments is pollen and not plants [Webb
and Bryson, 1972]. Changes in pollen production of different
plant taxa can respond rapidly following rapid climatic tran-
sitions (see below) and can be registered in sediment with a
high enough sedimentation rate. Thus, abrupt changes can be
interpreted from pollen records [Gajewski, 1987, 1993; Shu-
man et al., 2002b; Gajewski et al., 2007].
Another concern when using pollen records is the potential
importance of human impact on the landscape, especially as
it may influence paleoclimate reconstructions. If human ac-
tivities, such as agriculture or urbanization affect the land-
scape, this may make paleoclimate impacts more dif
3. RECORDS OF ABRUPT CLIMATE CHANGE IN
NORTH AMERICA DURING THE HOLOCENE
3.1. Regional Holocene Climate Reconstructions
In North America, the most studied area is the deciduous
forest region of eastern North America. Owing in large part
to the concentration of data from the region, a long research
program has shown conclusively the importance of climate
change in affecting the vegetation of the region [e.g., Bernabo
and Webb, 1977; Webb, 1986; Williams et al., 2004]. This
work has provided key data supporting the importance of
Milankovitch-scale climate variations in affecting the cli-
mate [COHMAP Members, 1988; Wright et al., 1993]. More
recent work has illustrated the importance of abrupt climate
changes [Gajewski, 1987; Shuman et al., 2002a, 2002b] on
vegetation composition.
In areas with less dense site networks, however, many sites
are being studied at high temporal resolution and are show-
ing abrupt climate changes. As an example of a recent multi-
proxy synthesis from one site in the Canadian Arctic, Lake
cult to
discern. Ruddiman [2003] has recently argued that human
activities have had a signi
cant impact on atmospheric com-
position, suggesting such a large human impact on the global
landscape that it should be discernible in pollen records. In
Europe, palynologists have identi
ed the impact of human
activities on the landscape during the past several millennia,
and these impacts are so extensive that much of the subconti-
nent is considered a
However, millennial-
scale climate changes still impacted the vegetation, as shown
by the coherence of major transitions in pollen diagrams
with those occurring in North America [Gajewski et al.,
“
cultural landscape.
”
Search WWH ::
Custom Search