Addressing barriers to assisted population migration during reforestation in the Western United States

Climate change is likely to continue dramatically altering the growing conditions for Pacific Northwest trees, creating challenges for maintaining healthy forests. Trees are adapted to their local climate as a result of natural selection at a location. Their response to climate is largely determined by their genetic composition and genetic diversity. Because of this, we expect that local populations will become increasingly maladapted as the climate changes. However, we can make forests more resilient to climate change by using assisted population migration.

These 12-year-old Douglas-fir trees near Soda Creek, Oregon, are part of an assisted migration study. Photo by Rich Cronn, USDA Forest Service.

There is increasing interest in using assisted population migration for operational reforestation as a climate adaptation strategy. Before this strategy can be widely implemented, however, we need more information. For example, what is the optimal climate transfer distance that balances short term reductions in tree productivity with longer-term adaptation to future climate?

Will moving tree species outside of their local conditions make them more susceptible to damage from insects or disease?

Will forest health or soils be affected by the use of nonlocal seed?

This project will make use of a long-established reciprocal transplant study of 60 Douglas-fir populations—the Seed Source Movement Trial—to evaluate and address those trade-offs. It will also take advantage of advancements in genomics technologies that have enabled researchers to decode the Douglas-fir genome. This makes it possible to identify the genes and genomic regions responsible for climate adaptation in assisted migration trials.

Finally, this project ties into an existing experimental network made up of beneficial partnerships between researchers and landowners called the Experimental Network for Assisted Migration and Establishment Silviculture (ENAMES). ENAMES includes more than 30 sites across national forests in Washington, Oregon, and California and multiple ownerships. The ENAMES network is being used to test the effect of four climate scenarios and a suite of establishment practices on the survival and growth of important western tree species at a much larger scale than has ever been attempted for related studies.

Objectives

1. Identify Douglas-fir seed sources that are adapted to future climate conditions. We will use the Seed Source Movement Trial, a reciprocal transplant study of 60 Douglas-fir populations, to model a universal climate transfer function—a scalable relationship that optimizes the trade-off between the near-term productivity of a seedling and its long-term health in a changing climate.
2. Assess potential consequences of using nonlocal seed sources on soil properties and insect and disease occurrence. We will use the same reciprocal transplant study to conduct forest health assessments and analyze soils that have been influenced by the different seed sources for the past 15 years.
3. Identify combinations of genes and genomics regions that control important traits for climate adaptation and develop a genomic assay that predicts the suitability of seed sources for assisted population migration.
4. Enhance and expand a new experimental network focused on assisted population migration by deploying climate and soil monitoring sensors, initiating new network sites that use genetically improved seed, and conducting the first remeasurement of the network 3 years after initiation.
5. Evaluate where reforestation is likely to occur and identify the future-adapted seed sources for those locations to ensure that suitable seed is available for assisted population migration.

Expected Products

• We will write peer-reviewed papers and summary reports for each of the objectives. We will also disseminate our findings via workshops and webinars coordinated with study partners and the Western Forestry and Conservation Association.
• Experimental sites associated with ENAMES provide opportunities for demonstration tours for managers to see the impact and value of assisted population migration.
• Spatial assessments of seed source demand and supply will be hosted on the ENAMES website as a GIS layer, and we will develop an interactive map that outlines the challenges associated with seed collections and availability.
• We will work with partners to ensure that the assessment is useful to prioritize seed collection across the Western United States by the Forest Service, and other federal and state agencies.
• Summary climate and soil moisture data and initial survival and growth estimates will be summarized and hosted on the ENAMES website. Raw data made available for download.
• Genes associated with tree fitness will be translated into an inexpensive genetic (SNP) assay that land managers can use to evaluate climate suitability of existing stands and restoration sources for assisted population migration, as well as the origin of “off-site” plantings. We will offer training in the use of this SNP ARRAY to geneticists in USDA Forest Service Regions 5 and 6 and the National Forest Genetics Laboratory.

Expected Outcomes

• Spatially referenced products and other datasets can be used to predict where forest disturbance is most likely to occur in the future, allowing users to determine potential seed sources that will be best adapted to future climate conditions by using existing transfer functions.
• Discoveries from this project will provide managers with immediate information on seed sources that are adapted to future climate conditions, thus ensuring long-term resilience of functional forest ecosystems.
• Findings related to optimal climate transfer distance and seed source effects on forest health and soils will be incorporated into the Seedlot Selection Tool, which allows users to determine appropriate seed sources for assisted population migration at a given site. The Seedlot Selection Tool is used extensively throughout the USDA Forest Service Pacific Northwest Region (Region 6) across all ownerships. Updates to the tool will facilitate immediate and direct incorporation of project findings into operational practice.

Metrics of Success

• Wider implementation of assisted population migration as a climate adaptation strategy.
• Greater long-term reforestation success with the use of seed sources that are able to maintain desirable growth rates and characteristics while also being best suited to future climates.