High root concentration and uneven ectomycorrhizal diversity near Sarcodes sanguinea (Ericaceae): a cheater that stimulates its victims?

Sarcodes sanguinea is a nonphotosynthetic mycoheterotrophic plant that obtains all of its fixed carbon from neighboring trees through a shared ectomycorrhizal fungus. We studied the spatial structuring of this tripartite symbiosis in a forest where Sarcodes is abundant, and its only fungal and photosynthetic plant associates are Rhizopogon ellenae and Abies magnifica, respectively. We found disproportionately high concentrations of Abies roots adjacent to Sarcodes roots compared to the surrounding soil. Rhizopogon ellenae colonizes the vast majority of those Abies roots (86-98%), and its abundance tends to decrease with increasing distance from Sarcodes plants. At 500 cm from Sarcodes plants we did not detect R. ellenae, and the ectomycorrhizal community instead was dominated by members of the Russulaceae and Thelephoraceae, which are commonly dominant in other California pinaceous forests. The highly clumped distribution of Abies-R. ellenae ectomycorrhizas indicates that Sarcodes plants either establish within pre-existing clumps, or they stimulate clump formation. Several lines of evidence favor the latter interpretation, suggesting an unexpected mutualistic aspect to the symbiosis. However, the mechanism involved remains unknown.