Nitrogen mediates root–fungal effects on European beech biomass but not adaptability to the environment

Abstract Climate change poses significant challenges to forests, requiring adaptive forest management. In Central Europe, beech ( Fagus sylvatica L.) is a dominant tree species with a wide ecological range. It is widely used for reforestation and the conversion of monospecific conifer into mixed forests. Selecting superior populations for establishing climate‐resilient European beech forests is critical, but whether and to what extent populations differ in their ecological adaptedness and adaptability remains unclear. Beech tree roots host diverse fungal communities, including symbiotrophic and saprotrophic fungi, which are essential for nitrogen acquisition and tree growth. However, it is still unclear whether these fungal associations also contribute to enhanced seedling biomass and adaptability across variable environmental conditions. Our findings indicate that ecological adaptedness in beech seedlings was generally low. In contrast, adaptability to different environments varied among populations, with only one population exhibiting low adaptability while the others demonstrated the potential to perform well across a range of environmental conditions. Root‐associated fungi did not show clear effects on beech adaptability, but they significantly influenced seedling biomass, primarily through their impact on total nitrogen content and nitrogen uptake, which together accounted for over 60% of the observed effect on biomass. Notably, there was a clear contrast in the roles of fungal guilds: symbiotrophic fungi had a positive influence on biomass, whereas saprotrophic fungi exerted a negative effect. Synthesis and applications . The results reveal low adaptedness but high adaptability among beech seedlings, supporting mixed plantings of local and climate‐matched non‐local populations to strengthen forest resilience. Root‐associated fungi strongly influenced seedling growth through nitrogen uptake, with mycorrhizal fungi having positive effects and saprotrophs negative ones. These results emphasise the importance of integrating both genetic diversity and microbial associations into climate‐smart forest management strategies.