Carbon isotopes in sporocarps from the Swiss conifer web-FACE, 2010-2011

Background. The Swiss Forest Free-Air CO2 Enrichment (FACE) study provided a unique opportunity to study the spatial extent and timing of 13C movement from mature trees to ectomycorrhizal fungi associated with different hosts. Methods. We analyzed carbon transport dynamics of ectomycorrhizal networks at this site in 2010 and 2011 during continuous labeling of five mature Picea abies with 13C-depleted CO2. We traced movement of that carbon for two years into ectomycorrhizal and saprotrophic sporocarps 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 (expressed as d13C) 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 d13C 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.