High root concentration and uneven ectomycorrhizal diversity near Sarcodessanguinea (Ericaceae): A cheater that stimulates its victims?

Sarcodes sanguinea is a nonphotosynthetic mycoheterotrophic plant that obta ins all of its fixed carbon from neighboring trees through a shared ectomyc orrhizal fungus. We studied the spatial structuring of this tripartite symb iosis in a forest where Sa,codes is abundant, and its only fungal and photo synthetic plant associates are Rhizopogon ellenae and Abies magnifica, resp ectively. We found disproportionately high concentrations of Abies roots ad jacent to Sarcodes roots compared to the surrounding soil. Rhizopogon ellen ae colonizes the vast majority of those Abies roots (86-98%), and its abund ance tends to decrease with increasing distance from Sarcodes plants. At 50 0 cm from Sarcodes plants we did not detect R, ellenae, and the ectomycorrh izal community instead was dominated by members of the Russulaceae and Thel ephoraceae, which are commonly dominant in other California pinaceous fores ts. The highly clumped distribution of Abies-R. ellenae ectomycorrhizas ind icates 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 symbios is. However, the mechanism involved remains unknown.