Assisted migration of forest tree populations through reforestation and restoration is a climate change adaptation strategy under consideration in many jurisdictions. Matching climates in which seed sources evolved with near future climates projected for plantation sites should help reduce maladaptation and increase plantation health and productivity. For threatened tree species, assisted migration outside of the species range could help avert extinction. Here, we examine lessons, limitations, and challenges of assisted migration through the lens of three assisted migration field trials of conifers in Canada and Mexico: Pinus albicaulis Engelm., an endangered subalpine tree species in the mountains of western North America; the Picea glauca (Moench) Voss × P. engelmannii Parry ex Engelm hybrid complex, of great economic and ecological importance in western Canada, and Abies religiosa (Kunth) Schltdl. & Cham., a tree species that provides overwintering sites for the monarch butterfly. We conclude that: (a) negative impacts of climate change on productivity of Picea glauca × P. engelmannii may be mitigated by planting seed sources from locations that are 3 °C mean coldest month temperature warmer than the plantation; (b) it is possible to establish Pinus albicaulis outside of its current natural distribution at sites that have climates that are within the species’ modelled historic climatic niche, although developing disease-resistant trees through selective breeding is a higher priority in the short term; (c) Abies religiosa performs well when moved 400 m upward in elevation and local shrubs (such as Baccharis conferta Kunth) are used as nurse plants; (d) new assisted migration field trials that contain populations from a wide range of climates tested in multiple disparate climates are needed, despite the costs; and (e) where naturalization of a migrated tree species in recipient ecosystem is viewed as undesirable, the invasive potential of the tree species should be assessed prior to large scale establishment, and stands should be monitored regularly following establishment.

As climate change accelerates, foresters are looking to ever warmer climates to secure sources of climatically adapted tree seed with which to establish healthy and productive plantations. However, as seed procurement areas approach jurisdictional boundaries (states, provinces, nations), across which seed and seed transfer systems are not typically shared, innovative approaches are required to identify those plantation areas for which suitable domestic provenances will be lacking, as well as areas in neighbouring jurisdictions with matching warmer, future climates that could fill domestic seed supply gaps. We describe a straightforward, climate envelope approach to locate these areas, using British Columbia (BC), Canada, and the Pacific Northwest (PNW), USA, to illustrate the analysis. We find that 21% of BC’s ecosystems (seed zones) will be at moderate or high risk of lacking adapted domestic provenances for plantation establishment by 2040. Importantly, however, we find large areas in the PNW that should be able to fill most of BC’s domestic seed supply gaps. Spatial analyses of this type will inform seed suppliers, managers, and policymakers where alternative seed procurement arrangements are needed and underscore the operational and policy barriers to acquiring seed from warmer jurisdictions. More broadly, they also highlight the need for interjurisdictional cooperation in matters pertaining to resource management.

Key message: Forest assisted migration applied in combination with a climate-based seed transfer system to two North American tree species: lodgepole pine (Pinus contorta var. latifolia Dougl.) and interior spruce (Picea engelmanii x glauca (Moench) Voss and their hybrids), resulted in increased height growth and a substantially expanded seed deployment area, compared to a geographic-based seed transfer system in British Columbia, Canada. Context: Forest assisted migration and climate-based seed transfer (CBST) are two recent innovations that have received significant attention as climate change adaptation strategies, but claims regarding their merits have not been well evaluated. Aims: We aim to test the claim that CBST, combined with assisted migration, can provide closer matching of seed source and plantation climate, greater height growth, and wider seedlot deployment area than a conventional geographic-based seed transfer system (GBST). Methods: Using transfer functions developed from two comprehensive, wide-ranging provenance trials of lodgepole pine and interior spruce, with populations from across western Canada, we estimated relative tree height growth at rotation and seed deployment area for a large set of simulated seed transfers in a CBST system with and without assisted migration and in a GBST system. Results: When assisted migration and CBST were used in combination, volume growth was 13% (lodgepole pine) or 6% (interior spruce) greater at rotation age and deployment area was 2.2 times (lodgepole pine) or 7.3 times (interior spruce) greater than was expected in a GBST system. Height growth increases were primarily associated with assisted migration, whereas increases in seed deployment area were primarily associated with the use of CBST rather than GBST. Conclusion: Converting from GBST to CBST in conjunction with assisted migration should substantially improve adaptation of lodgepole pine and interior spruce in British Columbia. This approach will also significantly offset the impacts of climate change on growth rates, increase deployment area, reduce seed collection costs, and provide greater flexibility to seed users.