Forest Atlas Of The United States

Forests on the Move

Global climate change will influence the distribution of tree species. Past glaciation and warming events caused changes in the distribution and abundance of forests, and rapid changes in climate predicted during the 21st century have the potential to transform forest ecosystems. We need to understand how trees may adapt to these changes; a critical component of adaption will be species migration.

Tree species migration is closely related to how climate, soil, and landscape characteristics create suitable habitats for different tree species. While generalist species respond to broad ecological conditions, species with narrow ranges of environmental conditions are constrained by specific site characteristics.

Ancient pollen trapped in soil provides evidence of historical tree species migrations. The occurrence of pollen in different layers indicates species presence on the landscape and allows us to see how trees recolonized North America after the last glaciation. These maps of pollen abundance (percent of total) include pollen in areas currently underwater because sea levels were lower in the past.

Understanding species’ responses to habitat changes will be critical if newly suitable habitat does not occur near where the species presently exists and colonization across the landscape becomes necessary. Because trees can live for decades, and many years pass before they begin to reproduce, there will be lag time between when the location of suitable habitat changes and when the shift in distribution occurs. In some cases, shifting distributions of mature trees and seedlings of the same species are already evident. In other cases we are yet to see such shifts at species range edges, making it important to monitor how these changes unfold as climate changes continue. In the end, it is difficult to predict the timing and direction of range shifts, but computer models and multi- layered approaches integrating climate predictions, disturbance patterns, and habitat needs allow us to evaluate forest vulnerabilities to climate change.

Climate models are used to examine how suitable habitats may change in the future. Tree species with projected declines in suitable habitat will likely face greater stress under new conditions and may have reduced reproductive capacity or be less able to respond to other stressors. Species with projected increases in suitable habitat will likely be in a better position to grow and reproduce.

Like the past, the future shows the potential for large changes in suitable habitat for tree species; suitable habitats may shrink, expand, move north, remain stable, or even move south. The concern is that the climate is changing—and suitable habitat is moving—more quickly than in the past. Can tree species migrate just as fast? Models suggest “no,” especially given the fragmented state of today’s forests. Some scientists and managers advocate facilitating migration by creating landscape corridors or directly assist migration by physically moving plant materials.

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tree biomass diagram

TREE MIGRATION. Mature trees can’t migrate like animals, but tree seedlings may colonize locations beyond the range of established trees. For example, sugar maple seedlings (open circles) occur farther north than mature sugar maples (solid circles), foretelling a future northward shift in sugar maple.
10,000 Years Ago
5,000 Years Ago
Year 2000
Forest Type Groups
Shortleaf Pine
Sugar Maple
Harsh Scenario
Moderate Scenario
Current Range
PREDICTED FUTURE RANGES. Various predictions exist for how climate will change in the by 2100. The predicted effects of climate change on Sugar maple (Acer saccharum), Shortleaf pine (Pinus echinata), and forest type groups have been mapped. Sugar maple is likely to lose habitat, while Shortleaf pine will gain. Forest type group distribution will change because each tree species will respond differently to climate change. These results suggest northern forest types (spruce/fir and aspen/birch) will become uncommon, while the oak-hickory types will increase in abundance.