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Background To assess the extent of belowground carbon transfer to ectomycorrhizal fungi in natural forests, we used δ13C and loge C/N measurements to calculate spatial dynamics of carbon movement into ectomycorrhizal sporocarps. Methods Fourteen broad-leaved trees were labeled with 13C-depleted CO2 from 2001–2005 in Switzerland and 13C traced into ectomycorrhizal sporocarps collected at different distances. We then used stepwise regressions on patterns of δ13C and loge C/N in ectomycorrhizal sporocarps as a function of distance (zone), solar radiation, fungal genus, and association type. Results CO2-labeled trees contributed 76 ± 5%, 36 ± 6%, and 19 ± 7% of sporocarp carbon at 0–6 m, 6–12 m, and 12–18 m from labeled trees, respectively. Literature estimates of hyphal development in different taxa did not correlate with carbon acquisition patterns. After drought in 2003, sporocarp loge C/N was low in 2004 and 13C-depleted carbon from elevated CO2 trees contributed less than in other years to sporocarps. In contrast, sporocarp loge C/N peaked in 2005 and contributions from elevated CO2 trees to the 6–12 m zone increased. Therefore, carbohydrate transport belowground decreased in 2004, reflecting plant allocation priorities, and increased in 2005. Sporocarp loge C/N varied less among years under elevated CO2 than elsewhere. Conclusions These patterns indicated that 1) belowground transport was influenced by climate and plant allocation, 2) root transport rather than ectomycorrhizal transport drove carbon spatial dynamics of ectomycorrhizal fungi, and 3) elevated CO2 decreased the sensitivity of belowground allocation to climatic fluctuations, suggesting improved drought resistance in a high-CO2 world.

Background. The spatial extent and timing of carbon fluxes from mature trees to ectomycorrhizal fungi associated with different hosts is challenging to assess in natural forests but could provide insights into carbon dynamics of fungi differing in exploration capabilities. Methods. We analyzed carbon movement into ectomycorrhizal sporocarps at the Swiss Forest Free-Air CO2 Enrichment (FACE) site in 2010 and 2011 during continuous labeling of five mature Picea abies (L.) H. Karst. with 13C-depleted CO2. Sporocarps were collected 0-6 m, 6-12 m, 12-18 m, and > 18 m from 13C-labeled trees. We then used stepwise regressions on patterns of 13C:12C ratios (δ13C) in ectomycorrhizal sporocarps as a function of distance (zone), solar radiation, fungal genus, and association type. Results. 18%, 10%, 40%, and 32% of ectomycorrhizal sporocarps were associated with conifers, broad-leaved trees, both, or of unknown association, respectively. Conifer-associated sporocarps derived 58 ± 7%, 41 ± 10%, and 20 ± 7% of their carbon from labeled trees in the 0-6 m, 6-12 m and 12-18 m zones, respectively, whereas other sporocarps in the 0-6 m zone only derived 34 ± 4% of their carbon from labeled trees. Sporocarp δ13C correlated positively with solar radiation for the 16-19 days prior to sporocarp harvest. Conclusions. These patterns indicated that (1) carbon spread further through conifer-associated mycorrhizal networks than through other fungal networks, and (2) carbon accumulation, storage, and transfer dynamics for sporocarp formation can take several weeks. Mature conifers supplied carbon to mycorrhizal fungi up to ~15 m from host trees.