Evolutionary and operational trade-offs in assisted gene flow for climate-adaptive forestry

Camilla Stefanini , Jannis Bolzern, Katalin Csilléry

Assisted gene flow (AGF) is an adaptive forest management strategy to increase forests' resilience to climate change, yet little is known about how management decisions interact with the strength of natural selection and introgression dynamics that co-determine relative stand productivity. We used individual-based, spatially explicit simulations to investigate how spatial configuration (ranging from clustered to dispersed) and introduction intensity influence introgression and relative stand productivity in the case of AGF of Oriental beech (Fagus orientalis) to European beech (Fagus sylvatica), two closely related species that hybridize, across a range of realistic selection scenarios. AGF scenarios were obtained based on questionnaires completed by forest researchers and practitioners at an international workshop. We found that clustered introductions consistently promoted hybrid formation and maintained higher levels of introgression, reaching 50% hybrid proportions 50-120 years earlier than dispersed configurations across selection scenarios. In contrast, when selection favoured either the introduced or the resident species, dispersed introductions achieved up to 15% higher relative stand productivity after 100 years, while rapidly diluting hybrids. Transect introductions generally produced intermediate outcomes for both introgression and relative stand productivity. To further evaluate trade-offs among introgression, relative stand productivity, and implementation cost, we conducted a Pareto frontier analysis. As expected, no single AGF strategy simultaneously maximized relative stand productivity and minimized implementation costs, and strategies that performed best after 100 years were not necessarily performing best after 500 years. Since AGF outcomes emerge from interactions between uncertain future selection regimes and controllable management decisions, our results highlight the importance of integrating evolutionary, demographic, and operational trade-offs when designing climate-adaptive forest management strategies.