Three populations of Cypripedium fasciculatum, the clustered lady's slipper orchid, and its associated mycorrhizal fungi were the subject of our study in southern Oregon. These orchids maintain a mycorrhizal relationship into maturity. Although most fungi-forming orchid mycorrhizas are saprophytes or necrotrophic parasites, some non-photosynthetic, achlorophyllous orchids are mycoheterotrophs. These orchids derive all their carbon through association with fungi that obtain carbon via ectomycorrhizal relationships with trees. By analysis of fungal DNA with PCR-RFLP and DNA sequencing, we found that C. fasciculatum associates with several fungal species, at least one of which belongs to the family Russulaceae, including common ectomycorrhizal fungi in coniferous forests. The same fungi also were found in rhizomes of the non-photosynthetic orchid Corallorhiza. Stable isotope analysis of orchid and non-orchid tissues indicates that digestion of fungal biomass in root cells supplies C. fasciculatum with substantial proportions of both carbon and nitrogen. Although C. fasciculatum is green and presumably photosynthetic under favorable conditions, our results indicate that the species also has the ability to parasitize fungi as an intermediate between the trophic patterns of non-photosynthetic, mycoheterotrophic orchids and photosynthetic non-orchids. These results, which elucidate the ecological connections of C. fasciculatum, have implications for managing and conserving the species and its accompanying fungi.
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