Prevalent theories explaining the outcome of interactions among terrestrial plants suggest that interactions are contingent only upon limiting resources and plant strategies for capture and use of those resources. Ubiquitous plant symbionts such as mycorrhizal fungi may, however, commonly affect plant-plant interactions through mechanisms that are not adequately described by these theories. For example, mycorrhizal fungi in mature forests typically link seedling roots to those of adult trees through a common mycorrhizal network (CMN). These networks are hypothesized to ameliorate overstorey-understorey competition in forests by facilitating the transfer of nutrients among plants, or by altering the carbon or mineral nutrient budgets of plants connected to the network. We used a field manipulation of CMNs and direct root-root interactions between overstorey trees and understorey seedlings in a monodominant native Monterey pine (Pinus radiata) forest to estimate the effects of CMNs on overstorey-understorey interactions. We used experimental combinations of trenching and belowground cylindrical barriers allowing fungal penetration to create three distinct experimental treatments into which pine seedlings were planted: CMN-only, CMN + overstorey roots, and neither CMN nor overstorey roots. We also included a treatment controlling for the effects of our cylindrical barriers.
Results/Conclusions

Eighteen months after the initiation of treatments, survival of understorey seedlings attached only to CMNs was substantially higher compared to seedlings cut off from both CMNs and direct root-root interactions, and compared to seedlings subject to both CMNs and root-root interactions. Among surviving seedlings, those subject to both CMNs and root-root interactions also grew much more slowly and photosynthesized less compared to those in other treatments. Altogether, these results are consistent with the hypothesis that CMNs ameliorate negative interactions between overstorey and understorey trees in the ecological context of this study. Although empirical studies remain rare, evidence is accumulating to suggest that belowground mycorrhizal networks between plants may have substantial impacts on plant-plant interactions.