Mixed Forest plantations for climate Change mitigation (MixForChange)

The forests provide services to us such as food and timber but also collecting carbon dioxide from the air decreasing the greenhouse effects. Naturally most forests are mixtures of many different tree species which together with thousands of other organisms forms an ecosystem. These divers ecosystems have been shaped by evolution creating stable forests. This means that they can withstand disturbances such as droughts, fires and insects outbreaks and recover fast from damages as those things formed the forest. Many forests of today have been formed by man and often they are monocultures as those are easy to manage and harvest. With only one tree species we also get fewer other organisms and when a disturbance happens the few organisms that are there might not manage so well. In this project we want to look at how growing trees in mixtures can help make forest more stable and store more carbon in the soil. To decrease the greenhouse effect you need to get the carbon from the air via the photosynthesis and to store it. The best way to store it for a long time is to put it in the soil. The soil in the forest often contains more carbon than the trees themselves! So how does the carbon get into the soil? The carbon in the soil comes from trees and other plants when they die, their leaves and roots but also their mycorrhiza which is a fungi that live on their roots. The mycorrhiza gives nutrients to the tree and gets carbon in return, carbon that can end up and be stored in the soil. Depending on if trees grow in monocultures or in mixtures we think that the roots and the mycorrhiza may grow in different amounts, shapes, depths and when it comes to mycorrhiza different species of fungi. We also think that the growth of roots and mycorrhiza will be affected differently by the disturbance drought if they are in monocultures or mixtures. To better understand how this works we will look how the roots and mycorrhiza are growing and what species of mycorrhiza we have in mixtures and monocultures. We will see how fast the roots grow using root-traps but also cameras in the soil taking photos of the roots growing and decomposing. We will have mycorrhiza-traps to see how much and what kind of mycorrhiza grows in the different stands. Finally, we will try to see how much of the carbon from the roots and mycorrhiza that stays in the soil. Together with our partners who look at how the trees above ground are affected by growing in mixture we hope to be able to better understand how we can use mixtures to create productive, stable forests that can help us to store carbon to mitigate climate change.

The project highlights how tree diversity significantly influences forest productivity, individual tree adaptation, and vital carbon sequestration, offering valuable insights for optimising future forest management practices. We explored how individual tree species respond and adapt when grown in mixed communities versus monocultures. This research revealed that while aboveground traits showed limited flexibility, belowground traits-particularly roots and their associated mycorrhizal fungi-exhibited considerable plasticity. Less competitive species demonstrated pronounced shifts in their trait mean and increased trait variability, adapting to the competitive environment. This underscores the importance of considering both above and belowground dynamics, as well as competitive hierarchies, to fully understand how mixed forests function and thrive. This adaptability, it turns out, significantly boosts overall forest productivity, even during early growth stages. We found in the early dynamics of mixed deciduous plantations within just six to eight years post-planting, mixtures, particularly those incorporating fast-growing acquisitive species like Acer, significantly outperformed monocultures in terms of biomass accumulation. This "overyielding"-where mixed stands produce more than the sum of monocultures-was observed both above and below ground, demonstrating a more efficient utilisation of resources. Key insights showed that biomass allocation varied by species and tree size, highlighting the complex interplay between active acclimatisation and passive growth-related changes. The absence of root segregation suggested that this increased productivity was driven by complementary resource use rather than simple spatial separation. These findings underscore the substantial benefits of species diversity in enhancing growth and resource allocation within young mixed forests. Furthermore, a comprehensive pan-European study investigated the critical role of tree diversity in soil carbon sequestration. It confirmed that stands with higher tree species richness consistently exhibited greater topsoil carbon stocks compared to monocultures. This positive effect was particularly evident under specific environmental conditions, such as lower soil fertility and reduced temperature variability. While tree diversity didn't directly influence fungal diversity, a positive correlation was found between fungal species richness and topsoil carbon stocks, suggesting diverse fungal communities may contribute to the context-dependency of carbon sequestration benefits. Collectively, these studies underscore the significant ecological and climate-mitigation benefits of incorporating tree species diversity into forest management. By enhancing individual tree adaptability, boosting overall productivity, and increasing soil carbon sequestration, mixed forests represent a robust and resilient approach to sustainable forestry.