The most memorable moments of a stroll through the forest often include interactions
between flora and fauna that can be observed at eye-level, such as birds nesting in trees,
caterpillars consuming leaves, and chipmunks burrowing under logs. While we are inclined to
pay closer attention to these interactions, we often fail to acknowledge some of the most complex
relationships in forests, including the hidden means by which almost all land-rooted trees
“communicate”. Almost all healthy forests possess a vast and complex microscopic web of
fungus referred to as a “common mycorrhizal network” or CMN, that remains hidden beneath
the soil. This network connects the vast majority of trees rooted on land, helping to transfer
water, nitrogen, carbon, and other crucial minerals [1]. Many have described this process as a
means of “tree communication,” depicting trees with attributes of sentience and arguing that they
are actively responding to each other’s signals. Tree activists have used this concept of
communication to attempt to gain protective legal statuses for threatened tree species as
thought-possessing beings. However, some scientists claim there is a lack of evidence proving
that mycorrhizal networks truly facilitate communication and cooperation among trees.
In 2023, Justine Karst, an ecologist at the University of Alberta’s Department of
Renewable Resources, asserted that suggestive lines of evidence have been taken as more
conclusive than they really are [2]. While it is true that fungi are capable of transferring essential
sustenance between trees, Karth and her team of researchers argued that it is incredibly difficult
to determine if this is actually taking place under the soil. If fungi connecting two trees are split
apart, each segment of fungi will continue to grow as an individual. This makes it difficult for
scientists to determine if fungi between two trees remain connected, or have simply begun to
grow independently after segmentation. Scientists can therefore only treat genetic samples of
mycelium as a single individual, preventing inferences about whether fungi between two trees
are connected.
These claims are challenged by a 2016 study in a Swiss forest, in which researchers
sprayed some trees’ leaves with a particular isotope of carbon and found that isotope showed up
in unsprayed neighbors [3]. While the researchers concluded that carbon transfer between trees
was mediated by plant-associated fungi, Karst and her team argue it’s not clear that fungi are
necessarily responsible for this transfer. While it is tempting to infer that mycorrhizal networks
are actively responsible for this movement of carbon between a treated and untreated tree,
nutrients can also move directly from root to root and through pores in the soil.
One attempt at mitigating the presence of other transfer sources was attempted in a 2008
study that utilized mesh to allow fungi, but not roots, to connect pine seedlings to older pines [4].
After applying a dye to the older pines and discovering its presence in the seedlings, researchers
concluded that water transfer was occurring via mycorrhizal networks. Karst and her colleagues
have agreed that this was perhaps the strongest evidence for trees sending resources through
fungal pathways. However, they still struggle to accept the idea of true “communication,” or the
cooperation of trees in responding to one another’s signals through a mycorrhizal network. This
is because the seedlings that were connected to older trees through fungus performed
equivalently or worse than those cut off from the fungal network [2]. The main argument behind
communication and cooperation within forests revolves around the transfer of resources through
call-and-response interactions between two trees. In other words, while the 2008 study
demonstrates the ability of fungal pathways to transfer resources, the research fails to address
whether this is truly a means of “communication” through call-and-response interactions or
simply the movement of resources along concentration gradients.
While Karst and her co-authors speak to the narrow focus of previous experiments that
have been used to make broader claims about mycorrhizal networks, they agree that there is at
least some involvement of mycorrhizal networks in tree-to-tree networking. There is still much
to be discovered regarding the capabilities of mycorrhizal networks and how much sentience is
involved in resource transfers between individual trees. While there have been calls to apply
personhood to trees after recent discussions of sentient mycorrhizal networks, the research is
inconclusive as to whether such networks are truly means of “communication”. For those who
strive to establish protective rights for threatened tree species, using mycorrhizal networks to
depict trees as sentient is likely alluring, yet not based on conclusive evidence. Perhaps in the
future, clear evidence indicating purposeful communication between trees will allow for the
protection of tree species that breathe life into our forests, creating the eye-level beauty we enjoy.
References:
[1] “Underground Networking: The Amazing Connections Beneath Your Feet.” National Forest
Foundation, https://www.nationalforests.org/blog/underground-mycorrhizal- network. Accessed
19 Nov. 2024.
[2] Karst, Justine, et al. “Positive Citation Bias and Overinterpreted Results Lead to
Misinformation on Common Mycorrhizal Networks in Forests.” Nature Ecology & Evolution,
vol. 7, no. 4, Apr. 2023, pp. 501–11. www.nature.com, https://doi.org/10.1038/s41559-023-
01986-1.
[3] Klein, Tamir, et al. “Belowground Carbon Trade among Tall Trees in a Temperate Forest.”
Science, vol. 352, no. 6283, Apr. 2016, pp. 342–44. DOI.org (Crossref),
https://doi.org/10.1126/science.aad6188.
[4] Warren, Jeffrey M., et al. “Hydraulic Redistribution of Water from Pinus Ponderosa Trees to
Seedlings: Evidence for an Ectomycorrhizal Pathway.” New Phytologist, vol. 178, no. 2, Apr.
2008, pp. 382–94. DOI.org (Crossref), https://doi.org/10.1111/j.1469- 8137.2008.02377.x.