The spatial contraction, expansion, and persistence of tree ranges in response to climate change is species-specific and can be collectively referred to as tree migration. Past migration of tree species across long time scales is well-documented, especially since the last ice age (1). However, given recent changes in climate and projected future conditions, the ability of tree species to migrate at the pace of rapid climate change is unlikely (2). Since tree species will respond individually to climate change, changes in the spatial co-occurrence of some species are expected (3, 4, 5), potentially resulting in future North American biomes for which there is no contemporary analog (6). The rapid rate of climate change combined with the response of individual tree species could lead to extirpation, or regional loss of certain tree species. This in turn could lead to a loss in biodiversity (7), loss or gain of forest area, or all of these. For example, forests may be converted to a different ecosystem such as grassland, or alpine tundra could be converted to forest.

It is important to recognize that under future climates there will be species-specific zones of contraction, expansion, and persistence across the landscape. Management practices that are developed for certain species under certain conditions cannot necessarily be extrapolated to different species under disparate conditions.

Expected shifts in tree ranges mean that the healthy forests of the future may not look exactly like the healthy forests of today. The following considerations may help managers retain forests and the benefits they provide even as the areas they manage become more or less suitable for specific species.

• Consider rotation periods that match the time trees growing on a site will likely remain genetically attuned to the environment. This will often require the use of shorter rotations.
• For currently growing trees of a given species, introduce trees of the same species that are more climatically-adapted to projected future climate (e.g., from a different subpopulation).
• Establish species and seed imported from a wide array of locations to increase the genetic diversity at a location, with the expectation that some portion of the trees will be viable at the site as the climate changes.

Finally, it is important to remember that tree regeneration is a complex dynamic, mediated by numerous processes in addition to climate. Tree regeneration and mortality are affected by site factors (e.g., soils and elevation), climate (e.g., precipitation and temperature regimes), herbivory (i.e., deer browse), mast periodicity, management actions, stochastic disturbances, forest succession, and competing vegetation (e.g., invasives). This complicates both tree range monitoring and the development of adaptive management responses to climate. Managing tree ranges, whether through assisted migration or species selection during silvicultural operations, will need to take into account the fact that more than climate will be affecting tree ranges in any given region of the US.

Woodall, C.; Moisen, G.; Iverson, L.; Crookston, N. (February, 2014). Using forest inventory analysis to detect tree migration in response to climate change. U.S. Department of Agriculture, Forest Service, Climate Change Resource Center. www.fs.usda.gov/ccrc/topics/inventory-analysis