Quantifying the capacity for assisted migration to achieve conservation and forestry goals under climate change

Many tree species might be threatened with extinction because they cannot disperse or adapt quickly enough to keep pace with climate change. One potential, and potentially risky, strategy to mitigate this threat is assisted migration, the intentional movement of species to facilitate population range shifts to more climatically suitable locations under climate change. The ability for assisted migration to minimize risk and maximize conservation and forestry outcomes depends on a multi-faceted decision process for determining, what, where, and how much to move. To quantify how the benefits and risks of assisted migration could affect the decision-making process, we used a dynamical vegetation model parameterized with 23 tree species in the western United States. We found that most of the modeled species are likely to experience a substantial decline in biomass, potentially facing regional extinction by 2100 under the high-emission SSP5-85 climate-change scenario. Though simulations show assisted migration had little effect on the forestry goal of total biomass across all species, its effects on the conservation goal of promoting individual species persistence were far more substantial. Among eight assisted migration strategies we tested that differ in terms of life cycle stage of movement and target destination selection criteria, the approach that conserved the highest biomass for individual species involved relocating target seedlings to areas with the highest canopy openness. Although this strategy significantly reduced extinction risk for six at-risk species compared to no action, it also slightly reduced biomass of four species, due to increasing competition. Species with relatively weak tolerance to drought, fire or high temperature were the most likely candidate groups for assisted migration. This model framework could be applied to other forest ecosystems to evaluate the efficacy of assisted migration globally.