Another form of plant communication occurs through their root networks. Through roots, plants can share many different resources including carbon, nitrogen, and other nutrients. This transfer of below ground carbon is examined in Philip et al. 2011. The goals of this paper were to test if carbon transfer was bi-directional, if one species had a net gain in carbon, and if more carbon was transferred through the soil pathway or common mycorrhizal network (CMN). CMNs occur when fungal mycelia link roots of plants together. The researchers followed seedlings of paper birch and Douglas-fir in a greenhouse for 8 months, where hyphal linkages that crossed their roots were either severed or left intact. The experiment measured amounts of labeled carbon exchanged between seedlings. It was discovered that there was indeed a bi-directional sharing of carbon between the two tree species, with the Douglas-fir receiving a slight net gain in carbon. Also, the carbon was transferred through both soil and the CMN pathways, as transfer occurred when the CMN linkages were interrupted, but much more transfer occurred when the CMN's were left unbroken.
This experiment showed that through fungal mycelia linkage of the roots of two plants, plants are able to communicate with one another and transfer nutrients as well as other resources through below ground root networks. Further studies go on to argue that this underground “tree talk” is crucial in the adaptation of forest ecosystems. Plant genotypes have shown that mycorrhizal fungal traits are heritable and play a role in plant behavior. These relationships with fungal networks can be mutualistic, commensal, or even parasitic. It has been shown that plants can rapidly change behavior such as root growth, shoot growth, photosynthetic rate, and defense mechanisms in response to mycorrhizal colonization. Through root systems and common mycorrhizal networks, plants are able to communicate with one another below ground and alter behaviors or even share nutrients depending on different environmental cues.