Ectomycorrhizal fungi of Douglas‐fir retain newly assimilated carbon derived from neighboring European beech

Summary Ectomycorrhizal (ECM) fungi distribute tree‐derived carbon (C) via belowground hyphal networks in forest ecosystems. Here, we asked the following: (1) Is C transferred belowground to a neighboring tree retained in fungal structures or transported within the recipient tree? (2) Is the overlap of ectomycorrhizal fungi in mycorrhizal networks related to the amount of belowground C transfer? We used potted sapling pairs of European beech ( Fagus sylvatica ) and North‐American Douglas‐fir ( Pseudotsuga menziesii ) for 13 CO 2 pulse‐labeling. We compared 13 C transfer from beech (donor) to either beech or Douglas‐fir (recipient) and identified the ECM species. We measured the 13 C enrichment in soil, plant tissues, and ECM fractions of fungal‐containing parts and plant transport tissues. In recipients, only fungal‐containing tissue of ectomycorrhizas was significantly enriched in 13 C and not the plant tissue. Douglas‐fir recipients shared on average one ECM species with donors and had a lower 13 C enrichment than beech recipients, which shared on average three species with donors. Our results support that recently assimilated C transferred belowground is shared among fungi colonizing tree roots but not among trees. In mixed forests with beech and Douglas‐fir, the links for C movement might be hampered due to low mycorrhizal overlap with consequences for soil C cycling.