The abundance and reproductive activity of orchids have been linked to variations in weather conditions, but few investigators have examined the relationships between orchid flowering dynamics and the distribution and abundance of mycorrhizal fungi. We quantified the abundance of flowering individuals of Corallorhiza odontorhiza, a mycoheterotrophic orchid, over a 14-year period and mapped the distribution of individuals in six of the 14 years. For two seasons, we conducted intensive and extensive studies of the mycorrhizal fungi that were associated with C. odontorhiza. The annual abundance of flowering plants was statistically related to growing-season precipitation and winter temperature, and the distribution of individuals within the study plot was related to the abundance and distribution of appropriate host fungi. We used DNA sequencing to identify ectomycorrhizal root tips that hosted Tomentella fungi that could potentially support C. odontorhiza. We found that Tomentella spp. were distributed throughout the study plot and on all ectomycorrhizal tree species, including in areas that have historically supported few or no orchids. However, there were fewer ectomycorrhizal trees, total ectomycorrhizal root tips, and root tips hosting Tomentella spp. in areas with few or no orchids compared to areas with abundant orchids. Furthermore, one Tomentella taxon dominated in C. odontorhiza rhizomes but was never found except immediately adjacent to C. odontorhiza plants. This suggests that abundance of flowering C. odontorhiza reflects both the presence of "preferred" taxa and abundance of appropriate host fungi associated with ectomycorrhizal roots. Results of this research provide the first indication that the relationship between plants and mycorrhizal fungi may be influenced by both the relative abundance of fungi that are sufficient to support orchid growth and by the presence of particular fungal types that are especially good at supporting orchid growth.
