Ectomycorrhizal fungal composition shows structured spatial variation around Sarcodes sanguinea and Corallorhiza...

Fully mycoheterotrophic plants obtain carbon from ectomycorrhizal (ECM) fungi, but whether these interactions are strictly parasitic or more context-dependent remains unresolved. Previous work on Sarcodes sanguinea suggested its fungal symbiont, Rhizopogon ellenae, achieves local dominance near the plant, raising the possibility of reciprocal benefit. Here, we asked whether two unrelated mycoheterotrophs exhibit similar patterns of localized fungal dominance and shifts in ECM community composition.

We sampled soils and pooled ECM root tips from S. sanguinea and Corallorhiza striata rootballs and along transects up to 500 cm away in Sierra Nevada forests (California, USA). Fungal communities were characterized with ITS metabarcoding and analyzed for proximity-related changes in symbiont abundance, ECM richness, and community turnover.

S. sanguinea rootballs were highly enriched with Rhizopogon ellenae sequences, but the enrichment declined steeply with distance. More than half of ECM operational taxonomic units (OTUs) present outside rootballs were absent inside. For C. striata, Tomentella fuscocinerea showed a weaker distance-related gradient in soil samples but was most abundant on root tips inside rootballs, with relative abundance declining outward. Both species were associated with reduced ECM richness inside rootballs compared to surrounding soils.

These results replicate earlier findings for S. sanguinea and extend them to an orchid lineage using DNA metabarcoding. The co-occurrence of focal symbiont enrichment and reduced ECM richness suggests that mycoheterotrophs are not simply parasites but contribute to localized community restructuring. This work underscores the need for experimental approaches to test whether these interactions yield reciprocal benefits for fungal partners.