Have you ever heard a tree talk?
If you said yes to that question, we might suggest professional help.
There are many forms of communication, like, verbal communication, body language, and the written word. Now, while trees may not have conversations like you and me, they do communicate.
Scientists have argued for decades about whether saying trees communicate is misleading or not. However, if the definition of communication is the passing of information, then trees most definitely communicate, but how?
In trees, information is passed back and forth throughout the mycorrhizal network. The information exchanged is in the form of chemical compounds which are received and transmitted similarly to how human neurotransmitters receive and transmit compounds through the central nervous system (figure 1). The mycorrhizal network acts as a pathway for the exchange of compounds which include carbon, nitrogen, phosphorus, nutrient molecules, amino acids and electrochemicals (aka charged chemicals).
Now you might be thinking, how is the exchange of chemicals a form of communication? Well the exchange of chemicals through the mycorrhizal network between trees is driven by an external trigger (figure 2), including biochemical signaling and stress signaling.
Biochemical signaling is essentially the supply and demand of resources back and forth between a host tree and a donor tree. This type of communication or exchange of resources is how the mycorrhizal network improves plant fitness. Trees with a surplus of resources can share with those around them who may be in need. How nice of them to share!
Stress signaling is initiated by extreme chemical concentrations, which trigger a specific emergency response based on the chemical compound in surplus or deficiency. These extreme chemical concentrations are usually caused by an external trigger, such as defoliation, insect infestation, and drought.
In order for communication to be considered communication, the signal must be received and processed. When information is exchanged between trees via the mycorrhizal network, a response will be activated, typically in the form of adaptation. Adaptation can be behavioral or physiological. Suzanne Simard is a well recognized ecologist who was one of the first to suggest trees communicate. Simard studied Douglas Fir insect defoliation and revealed that excess carbon was transmitted through the mycorrhizal network in the area. As a result, Jack Pines that neighbor the Douglas Firs responded to the influx of carbon by producing an insect repelling enzyme. Offspring of these Jack Pines also had additional genetics which helped them defend against insects [1]. Thus, the communication between the Douglas Firs and the Jack Pines resulted in a stronger generation of trees, which is an example of physiological adaptation (figure 3).
Similarly, neighboring nitrogen-fixing plants have been shown to influence host tree offspring to have increased productivity and nitrogen flux capability versus offspring from host trees without nitrogen fixing-neighbor plants [2]. Nitrogen flux capability and increased productivity as a result of exchanged information is a form of behavioral adaptation (figure 4).
So no, trees do not conversate. But they do have a lot to say to each other.
It has been proven that, in addition to behavioral and physiological adaptations, trees are also able to recognize kin (family) via mycorrhizal signaling. Related trees have similar fungal DNA in their mycorrhizal network. Host trees are able to identify this fungal DNA in neighboring trees and determine who is family or not. Researchers have shown that offspring from a host tree are more successful when neighboring trees are related to the host tree [1] (figure 5). Kind of like how mom and dad would leave you at grandma's house for the weekend because they knew grandma would take care of you.
Now what if I told you trees have a memory and are able to share their ancestors history via their mycorrhizal network? Not only can grandma trees help out their grandbabies, they can also tell them stories about “the good ol’ days”. We know the rings of a tree tell its life story, but the DNA from other species, such as ocean salmon, have been found within rings and throughout the mycorrhizal networks of ancient trees [1]. This is an example of a cooperative ecosystem that functions off of experiential learning from all organisms within the ecosystem (figure 6). This concept starts with a typical life cycle, where the fish hatch, grow up and eventually venture to the big sea. Predators, such as an eagle, might take that fish for nutrition, dropping the remains on land. The fish remains are then decomposed into the soil, becoming DNA particles and plant nutrients. The decomposed remains are then diverged into the mycorrhizal network, where they are incorporated into the tree communication system as root exudes. Over time, root exudes either become part of the tree or naturally enter water sources, such as rivers, in an exchange of water to maintain appropriate moisture levels for land plants. Exudes that enter the water system act as a homing signal for fish breeding, restarting the cycle based on the information shared by an ancient tree. Long story short, trees can talk to fish. Kind of.
So yes, trees communicate. They share resources based on the needs of others, they signal for adaptation in neighbouring species, they can identify family members, and tell stories of their vast history with root exudes. Despite the contradictions among tree scientists on this topic, the trees are talking, even if we can't hear them (hopefully).
References
Simard, S.W. (2018). "Mycorrhizal Networks Facilitate Tree Communication, Learning and Memory." Springer Nature, 2018. https://doi.org/10.1007/978-3-319-75596-0_10
Gorzelak, M.A. et al. (2015). "Inter-plant communication through mycorrhizal networks mediates complex adaptive behavior in plant communities." AoB Plants, 7. doi:10.1093/aobpla/plv050
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