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Responses of Northern U.S. Forests to Environmental Change
ISBN 0-387-98900-5

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 don’t 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 tree’s 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 migration.

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