Soil secrets and tree tales: an in-depth comparison of carbon storage in mixed and pure stands of pine and birch

With climate change intensifying droughts, pest outbreaks and fire risks, forest management practices are increasingly focused on stabilizing soil carbon (C) stocks, which are essential for long-term ecosystem productivity and climate change mitigation. Planting more species-rich forests has been proposed as a potential solution, but the impact of species selection and planting density on carbon stocks remains largely unknown, particularly in mixed-species plantations, where local water availability plays a key role. In this study, our objective was to examine the effects of tree diversity, water availability, and tree density on carbon stocks and fluxes. Using a 10-year-old tree diversity experiment, we investigated how pure monocultures of pine (Pinus pinaster) and birch (Betula pendula), as well as mixed pine-birch stands, influence carbon dynamics under contrasting water conditions and different tree densities. Our results indicated that tree species mixtures slightly increased total C stocks, primarily through enhanced soil C storage due to niche partitioning and greater root turnover. However, pine monocultures showed higher aboveground biomass productivity than birch monocultures and mixed stands, regardless of water availability. Overall, increased soil moisture enhanced both tree biomass and soil carbon stocks, especially in mixed stands, likely by alleviating drought stress for birch. In contrast, higher water availability accelerated litter decomposition, reducing C stocks in the litter layer. Tree density was a key driver of C storage, with denser stands of pine monocultures showing increased aboveground biomass but reduced understory C stocks. These findings highlight the context-dependent benefits of mixed stands: while species mixtures can enhance soil C storage and adaptability to drought, they may be inferior to pine monocultures in aboveground C storage, at least under the specific environmental conditions and temporal scale covered by our study. This study underscores the need for site-specific forest management strategies that balance productivity and C sequestration goals, offering guidance for climate change mitigation through alternative planting schemes adjusting tree density and species composition while maintaining ecosystem services.