Hypogeous Ectomycorrhizal Fungal Species on Roots and in Small Mammal Diet in a Mixed-Conifer Forest

The purpose of this study was to estimate the portion of an ectomycorrhizal (ECM) fungi root community with a hypogeous fruiting habit. We used molecular methods (DNA sequence analysis of the internally transcribed spacer [ITS] region of rDNA) to compare three viewpoints: ECM fungi on the roots in a southern Sierra Nevada Abies-dominated old-growth forest, fungi in scat samples collected from small mammals in the same forest, and hypogeous sporocarps found throughout the Sierra Nevada. We found that hypogeous taxa accounted for a minimum of 21% of the species and 25–40% of the dry root biomass of all samples. This estimate is two to three times greater than estimates from previous studies. This difference may be due to methodological advantages of this study, but may also be related to conditions in dry forests typical of western North America where prolonged drought may favor this form of fruiting. Although molecular analysis of scat samples did not add to our view of the ECM roots, we readily isolated sequences from Rhizopogon species. From these results we inferred that two species, R. occidentalis and R. olivaceotinctus, are represented primarily in the spore bank and may be dependent on substantial disturbance to become abundant on roots. FOR. SCI. 51(3): 243–254.