Responses of Northern U.S. Forests
to Environmental Change
Chapter 14: Forest Responses to Changing Climate;
Lessons from the Past and Uncertainty for the Future
Donald H. DeHayes, George L. Jacobson Jr., Paul
G. Schaberg, Bruce Bongarten, Louis Iverson, and Ann C. Dieffenbacher-Krall
Changes in the distribution of vegetation are strongly correlated
with climatic change over long time periods, and it is reasonable
to expect that future climate changes will affect forest composition
and distribution. Specific biological factors that affect species
response to environmental changes include survival, reproductive
capacity, rate of migration, and response variation between or within
species. Anthropogenic stressors are a relatively new set of factors
that affect evolutionary responses to changing environment.
Most modern vegetation assemblages have been in their present configuration
for no more than 6,000 to 8,000 years. Present vegetation communities
are transitory combinations of species that have responded individually
to environmental changes and competition. Eastern white pine is
an example of a species whose distribution has closely followed
climate changes of the last 10,000 years.
Regression tree analysis has been used to indicate prospective
responses of individual species which are then aggregated to examine
potential changes in community dynamics and biodiversity. Under
climate change scenarios, Balsam fir and quaking aspen were mostly
lost from northern U.S. forests, and maples were greatly reduced.
Pine species, primarily loblolly pine and white pine, expanded their
dominance in the southern and northern parts of the region, respectively.
Oaks also expanded in some areas.
Both paleoecological studies and modeling efforts have clearly
shown that vegetation communities are unlikely to move together
as intact communities. Consideration of additional factors that
contribute to the uncertainty of prediction only strengthens this
general conclusion. For example, the determinants of the boundaries
of the current distribution of any particular species are poorly
known. And anthropogenic factors such as exotic species introductions,
changes in land use, and forest management also play a role in conjunction
with changing environmental factors.
There are differential responses to environmental change between
species and within species, as shown by numerous experimental studies.
Eastern white pine may be the most sensitive eastern tree species
to air pollution, with many of the sensitive individuals already
lost from natural populations.
Most natural populations of temperate and boreal species seem to
be quite tolerant of climate differences as indicated by a long
history of provenance testing. In the northern U.S., most species
should tolerate climates as much as 5oC warmer than present, as
long as other factors such as moisture availability dont change
simultaneously. However, regeneration in the face of climate change
is likely to be more difficult than survival, because the most sensitive
stage of a trees life is the beginning, when warmth and drought
can have strong effects.
Species migration is dependent on regeneration success. Other factors
are also important, such as nonnative tree species and their competition
effect, or forest fragmentation which may comprise a barrier to
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