Over-interpreted results on common mycorrhizal networks in forests
Karst et al., 2013. Positive citation bias and overinterpreted results lead to misinformation on common mycorrhizal networks in forests. Nature Ecology & Evolution. https://www.nature.com/articles/s41559-023-01986-1 pdf
Common mycorrhizal network (CMN) = Fungal hyphae connect the roots of multiple plants of the same or different species belowground.
- It is well established that the association with mycorrhizal fungi can be beneficial to the performance of plants, including forest trees. What is not as firmly established is that CMNs (i.e. physical, continuous linkages among the roots of at least two different individual plants, by the same genetic individual of mycorrhizal fungus) have the potential to mediate plant-plant interactions and plant performance via the transfer of molecules among plants (e.g. seedling getting stronger because of nutrients being delivered from a nearby mature tree).
Three popular claims about CMNs
Claim 1: CMNs are widespread in forests
- Strong evidence that CMNs exist in general and do transfer nutrients: Mycoheterotrophs, i.e. plants that acquire carbon from mycorrhizal fungi colonizing the roots of other plants.
- With current technology it’s difficult to confirm that continuous, non-transient mycelial connections exist between trees in the field. The permanence of fungal connections is unknown.
- It’s hard to impossible to track a the hyphae originating from a single spore to different trees and root systems.
- Support for this claim is limited, owing to the paucity of information on CMN structure and dynamics. Too few forests have been mapped, and even fewer studies (two so far) demonstrate actual continuity of fungal links among trees.
Claim 2: Resources are transferred through CMNs
- Implicit claim: Fungi forming CMNs are physical extensions of roots, i.e. passive conduits in which the direction of resouce flow is determined by plants. But mycorrhizal fungi don’t always benefit their plant partners, but can e.g. aggrevate nitrogen limitation.
- Unresolved, because CMN-mediated interplant resource transfer has not been conclusively demonstrated.
- Radioactive labelling studies demonstrated the movement of 14C more than 20 cm from one seedling to the roots of another. Autoradiography has definitely demonstrated movement of carbon from one tree seedling to the mycorrhizal roots of another via a CMN. But isoptopes can be transferred through soil (soil solution) as well, not just fungal hyphae.
- A single peer-reviewed study exists that has addressed the phenomenon of signalling among forest tree species in response to insect damage. But the signals could have transferred through root connections as well. However, there is good evidence for CMN-mediated transport of defence signals in a well-controlled investigation on bean plants.
- We shouldn’t draw too many conclusions from pot studies, since the conditions in a mature forest might be quite different. Also, most laboratory studies on resouce transfer lack the full suite of controls required to interpret the results properly and overlook important things, like neighbouring roots.