Abrupt Climate Changes During the Holocene Across North America From Pollen and Paleolimnological Records - Abrupt Climate Change: Mechanisms, Patterns, and Impacts - page 175

Recently, paleoclimate reconstructions using pollen data
quantified Holocene climate variability on several timescales
across the Canadian Boreal and Alaskan regions [Viau and
Gajewski, 2009; Viau et al., 2008]. Regional reconstruction
of the past 12,000 years show a time-transgressive pattern of
climate change on orbital scales where eastern Canada was
relatively cool compared to western Canada and Alaska
during the early Holocene due to the remnants of the Lau-
rentide Ice Sheet in the east [Viau and Gajewski, 2009]. A
similar time-transgressive pattern is seen at the millennial
scale. Results for the past 2 kyr identified multicentennial-
scale climate variability (i.e., MWP and LIA) across the boreal
region [Viau and Gajewski, 2009], which shows that even
relatively brief climate variations cause rapid responses in
regional pollen production and vegetation patterns [Gajewski,
1987; Gajewski et al., 2007; Williams et al., 2002].

3.2. Continental Paleoclimate Records of Abrupt
Climate Changes

Peros et al. [2008] illustrated the impacts of rapid climate
change, especially during the Younger Dryas, on the popula-
tions of poplar (Populus) across North America. They found
that poplar pollen increased at both the beginning and end of
the Younger Dryas. At the beginning of the Younger Dryas,
spruce (Picea) pollen decreased in many sites across North
America, whereas poplar increased in abundance, probably
as the forest opened in many areas, and poplar could tolerate
the conditions that were too cold for spruce. Today, poplar is
found at or near tree line and forms tree line in a few small
areas of North America. Because it is a pioneer species, it
could quickly invade the landscape as conditions ameliorated.
During the entire period, alder (Alnus) pollen changed little.

Figure 3. Mean July temperature anomaly for North America estimated using the MAT and the pollen data shown in
Figure 1. (a) Mean July temperature anomalies for the past 14 kyr. (b) As in Figure 3a, detrended using 10th-order
polynomial. (c) Band-pass-filtered curve of Figure 3b. (d) Curve from Figure 3a with x axis expanded. Gray lines are
confidence limits, and gray shading shows general subdivisions of the curve for the Holocene. After the work of Viau et al.
[2006].