Species compositions determine the ecosystem services of alternative forest transitions in the boreal-temperate ecotone

Boreal forests are integral for the maintenance of multiple ecosystem services to humanity, yet climate change, wildfire regimes and land-use intensification are rapidly driving these conifer-dominated systems towards a boreal-temperate ecotone, characterized by a mixture of conifers and broadleaves, with uncertain implications for sustaining multiple ecosystem functions (i.e. multifunctionality). Here, we conducted a standardized 34-year field manipulative experiment in a boreal-temperate ecotone region of Northeast Asia, including four monocultures (conifers and broadleaves) and four two-species mixed forests (each combining coexisting broadleaf and conifer trees). We aimed to assess the ecological impacts of these forest transitions in the boreal-temperate ecotone. Our results showed that ecosystem multifunctionality depended strongly on tree species composition. Crucially, specific compositions generated distinct functional trade-offs rather than uniform changes in ecosystem multifunctionality. For example, transitioning to incompatible mixed-species stands maintained provisioning functions like wood production but resulted in significant reductions in critical regulating services, such as soil carbon sequestration, nutrient cycling and water regulation. We further provide evidence that these functional declines were mechanistically driven by selection effects that superseded niche complementarity. Specific tree compositions imposed constraints on soil resources and microbial properties, strongly influencing ecosystem services in the boreal-temperate ecotone. Synthesis: This research provides new insights into identifying species compositions that enhance ecosystem services in the boreal-temperate ecotone, thereby guiding climate change mitigation and forest management efforts in these sensitive regions.