Assisted migration is the human movement of species from one region to another where the likelihood of this movement happening naturally is low. Currently, assisted migration of forest trees is proposed as an adaptation strategy to climate change and increasingly hotter droughts in western North America. We argue that managers should proceed with extreme caution regarding assisted migration specifically to protect native forests, and look within a region for adaptive traits in forest trees. Within any given biogeoclimatic region there is variation in adaptive traits in tree populations. Tree improvement programs throughout western North America have selected for desired traits while staying within seed zones.
Results/Conclusions
Assisted migration has the potential to negatively affect native forest diversity (e.g., species and genetic) and health by: 1) introduction of non-native pathogens and insect pests in soils and on plant parts, 2) introduction of weedy plant material, 3) introduction of trees that are maladapted to native insect pests and pathogens (climate envelope models rarely consider trophic issues), and 4) potential for biodiversity changes due to shifts in forest composition, foliage and litter chemistry, and competitive relationships among trees. Drought tolerance is known to vary across the range of widespread species such as Douglas-fir (Pseudotsuga menziesii). Moving drought tolerant races in anticipation of climate change is an attractive management response. However, recent studies of foliage diseases in Douglas-fir using a reciprocal planting framework indicate that moving seed sources from drier regions into wetter environments leads to increased foliage disease incidence. Therefore, planting drought tolerant species or seed sources in wetter regions in anticipation of climate change may introduce maladapted trees into new environments. Some principles for performing assisted migration in native forests include: 1) protect native biodiversity, 2) avoid transporting soil, debris, and plant material, 3) interpret species distribution models with extreme caution, 4) look regionally for adaptive traits, 5) use only regionally tested plant materials for deployment on large scales to prevent introductions of maladapted seed sources, 6) control pests in plant materials by growing in the region and inspecting nurseries, 7) limit the movement of a species to the obvious needs of the species to survive. Although assisted migration of forest trees will continue to be important in commercial global plantation forestry, native forest ecosystems should be managed for maintenance of ecological function and biodiversity.