Weather modifies the spatial extent of carbohydrate transfers from CO.sub.2-supplied broad-leaved trees to ectomycorrhizal fungi

Background Weather conditions and fungal morphology may influence spatial and temporal dynamics of carbon transfer to ectomycorrhizal fungi from their tree hosts but field studies of these dynamics are challenging. Methods Broad-leaved trees in a Swiss forest were labeled with .sup.13C-depleted CO.sub.2 from 2001-2005 and .sup.13C traced into ectomycorrhizal sporocarps collected at different distances from those trees. We then correlated distance (zone), year, fungal genus, fungal morphology, and solar radiation against sporocarp [delta].sup.13C and log.sub.e C/N in stepwise regressions. Results CO.sub.2-supplied trees contributed 71 ± 5%, 32 ± 6%, and 19 ± 7% of sporocarp carbon at 0-6, 6-12, and 12-18 m from these trees, respectively. Fungal morphology (rhizomorph presence and ectomycorrhizae hydrophobicity) was uncorrelated with fungal carbon acquisition. After drought in 2003, CO.sub.2-supplied trees contributed less carbon in 2004 than in other years to sporocarps and sporocarp log.sub.e C/N decreased. In contrast, sporocarp log.sub.e C/N peaked in 2005 and contributions from CO.sub.2-labeled trees to the 6-12 and 12-18 m zones increased. Solar radiation with a 14-21 day lag correlated negatively with log.sub.e C/N. Sporocarp log.sub.e C/N correlated positively with [delta].sup.13C at 0-6 m and negatively elsewhere. Conclusions These patterns indicated: 1) fungal morphology did not affect carbon transfers, 2) carbohydrates rather than amino acids were transferred across zones to sporocarps (inferred from spatial correlations of log.sub.e C/N and [delta].sup.13C), 3) sporocarp [delta].sup.13C and log.sub.e C/N reflected the last 2-3 weeks of photosynthesis; 4) summer drought and plant allocation influenced the quantity and spatial extent of belowground transfer.