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

Sarcodes sanguinea is a non-photosynthetic mycoheterotrophic plant which 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. R. 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. The ectomycorrhizal community away from Sarcodes was instead dominated by members of the Russulaceae and Thelephoraceae, which are common dominants in other California pinaceous forests. The highly clumped distribution of Abies-R. ellenae ectomycorrhizas indicates that either Sarcodes plants establish within pre-existing clumps or they stimulate their formation. Several lines of evidence favor the later interpretation. Thus, this system appears to have unexpected mutualistic aspects, but the mechanism by which Sarcodes could stimulate clumping remains unknown.