Attempts to understand forest dynamics have focused primarily on resource competition among trees, resulting in poor predictions of forest regeneration, succession and productivity in changing environments. Mycorrhizal networks, or connections between two or more plants through a single fungal mycelium, are increasingly recognized as mediators of interactions among trees through their effects on survival, growth, and competitive ability. However, the effects of mycorrhizal networks on forest dynamics remain hindered by the elusiveness of their underlying spatial structure. We addressed this problem using microsatellite DNA markers to identify individual tree roots and fungal genets isolated from mycorrhizal root tips and to map mycorrhizal networks in the field. Our objectives were to identify the belowground spatial patterns of the ectomycorrhizal fungi Rhizopogon vesiculosus and R. vinicolor, to determine their associations with different interior Douglas-fir cohorts, and to identify the basic structure of mycorrhizal networks formed between these trees and fungi. 

Results/Conclusions We found that R. vesiculosus and R. vinicolor each had unique horizontal and vertical spatial patterning belowground, with implications for niche differentiation. Both species of Rhizopogon were found to connect multiple Douglas-fir cohorts, with young saplings established within the mycorrhizal network of old veteran trees. The networks were expansive yet easily traversed through trees highly interconnected by Rhizopogon fungi, and a strong positive association between tree size and connectivity resulted in a scale-free network structure where large trees acted as hubs. Our results, together with evidence that mycorrhizal networks can facilitate regeneration, suggest that large hub trees play a foundational role in mixed-aged interior Douglas-fir forests and are critical for maintaining forest stability and the functional continuity of mycorrhizal networks.