What did one tree say to another?

Time in the trees seems to have a slower pace than we perceive, because before agitated eyes they appear silent, motionless and perennial. Their life as plants is the subject of such intrigue that the idea of personifying them—endowing them with human speech, movement and senses—has been the subject of popular fictional stories: wise, talking trees, capable of creating alliances, protecting their community, defending themselves and attacking in combat.

Perhaps this design is the one that has guided arboreal research, since there is a scientific approach that explores that 'animated' and hidden side of trees, in particular, their communication through plant connections and symbiotic networks with other organisms. The mission has been, in a way, to infiltrate its channels and study them to know the signals that are sent; in other words, what trees are said to be. But it's not as simple as it reads. To understand their interaction and language, we can go through them from the root.

Global forest network

Most plant communication is underground. Beneath our feet, there is an alliance between plant roots and some fungi to exchange resources: trees offer carbon that they obtain from photosynthesis and fungi transfer nutrients that they obtain from the soil. This symbiosis, called “mycorrhiza”, is one of the most impressive terrestrial adaptations because between 85% and 90% of the plants in all the Earth's ecosystems are present. It is not a simple association, but rather a complex network with which plants have evolved to adapt to different environmental conditions, spread over different territories and communicate to maintain a benefit.

Fungi, which may belong to the Glomeromycota (Glomus, Gigaspora, Acaulospora), Ascomycota (Peziza, Tuber) and Basidiomycota (Amanita, Cantharellus) phyla, connect to tree roots through fungal hyphae or fibers that form the mycelium. Its appearance is that of a fine mesh of filaments, usually white; at first glance it looks like a thick cobweb. In mushrooms with mushrooms - the little hat with which certain fungi are identified - the mycelium can be seen underneath them, and in fungi without mushrooms, the mycelium is part of their filamentous structure. These fibers cover plant roots with a mantle and penetrate their cells, or they enter directly into their cells without forming a mantle. With this cellular connection, communication with plants and with other organisms is possible.

Mycelium grows even longer than tree roots; it extends so far that it forms a kilometer-long system of roots and mycelium that interconnect trees of the same or different species. Some scientists call this network colloquially Wood Wide Web (“global forest network”), in analogy with World Wide Web (“global computer network” with which the Internet works), since information and nutrients flow through nodes, or large trees, and links, multiple connections (thousands) to other smaller trees and from distant geographical locations.

Chemical bartering

Between trees and fungi, it is a matter of access to different poles, an exchange of substances that trees obtain above, in their canopies; and fungi, below, several meters below the ground. The exchange occurs, however, under the ground: the trees give the fungi carbon in the form of sugar and, in return, receive minerals. Plants absorb carbon from the air through their leaves high in their foliage, process it through photosynthesis and carry it down to their branches, trunks and roots, to promote their growth and respiration. With fungi this happens the other way around. They share nutrients with trees that are usually poorly available in the soil, especially nitrogen and phosphorus. They also transfer water, from the underground soil to the roots, with which trees acquire another source of collection of it, in addition to the one they obtain through their leaves and bark.

However, the exchange is not only of nutrients, but also of defense or alarm chemicals, depending on the type of fungus with which one lives. The result of this “mutualism-parasitism” (because fungi parasitize and invade roots) is a healthy tree with these additional benefits:

- Protection against the attack of parasites, pathogenic fungi and nematodes, since the mycorrhizal fungi with which the tree is allied are responsible for scaring away or annihilating such pathogens by secreting proteins and enzymes without affecting plants.

- Increased resistance to herbivores, particularly leaf-eating insects Thanks to mycorrhizal symbiosis with fungi, just as with the attack of pathogenic microorganisms, fungi promote the production of defensive substances in the tree, such as chitinase and glucanase enzymes, to ward off insects or other herbivorous animals.

- Limitation of the absorption of toxic heavy metals such as zinc and cadmium, since they are 'trapped' in the mycelium of fungi.

- Improvement of the physical and chemical properties of the soil through the enrichment of organic matter. This reduces erosion and increases water retention.

- Exploration of underground territory. The hyphae of the fungi, when they spread, find spots rich in minerals and water, so that trees can provide themselves with nutrients without having to move.

Forest Detective

Although the study of this symbiosis began in the 19th century, it has been more explored in recent decades. One of the researchers who has achieved the most publicity and controversy, unintentionally, is the ecologist Suzanne Simard, from the University of British Columbia. It was she who discovered, after analyzing the roots of Canadian forests for 25 years, that symbiosis also worked as a giant communication network. In his doctoral thesis, from 1997, he emphasized that cooperation between trees is the quality that drives the underground system of roots and fungi, since he observed that there are “mother trees” that recognize their saplings and, through the transmission of substances with fungi, send nutrients to them and to allied species that are disadvantaged in light or obtaining resources to support them in their survival. If mothers die, they inherit these nutrients to their peers. The same assistance occurs in the face of an attack; when mother trees detect pathogen infections or herbivore contact, they send signals to their neighbors to warn of the danger and activate their defense. For Simard, this is evidence that trees' talk 'and that they are not isolated individuals, but perhaps a single organism.

Some members of the academic community rejected such statements, despite the fact that Simard's work was peer-reviewed, endorsed, and published in Nature -one of the most prestigious global scientific journals- and reproduced by biologists and ecologists from other research centers. The counterargument is that competition and not cooperation induces symbiosis and communication. However, Simard and other specialists were already taking this phenomenon into account: there are trees that take advantage of the underground network to steal nutrients from their neighbors or to use toxic substances to eliminate them, as does the American black walnut tree.

Under the counterargument, ecologists speak out against the personification of trees -attributing human behavior to them- by spreading their communicative and symbiotic system, because according to them, it does not accurately represent ecosystems and can cause damage to their conservation. Kathryn Flinn, from Baldwin Wallace University, is one of them. He confirms that trees, in a certain way, 'talk' because they emit and receive signals from others, and there is a physiological reaction from them, but he denies that the network of trees works as a single organism to stay alive. The reason is that, in evolutionary terms, individuals survive more successfully than a group that supports themselves, since individuals are more numerous and adapt more quickly to changing circumstances. The interaction between trees can be positive or negative, depending on the environmental conditions they face; that is, they do not always behave the same way. Over long periods, an individual tree could affect a neighbor so as not to die.

Flinn says that placing too much emphasis on cooperation between trees is misleading, because there is fierce competition, even before plants mature: of the millions of seeds that a plant produces, only a few germinate and others less grow into trees. A large number of seeds or seedlings die, either with or without the help of their parent plant. Flinn explains that altruism can exist among plants, but, again, in the evolutionary field; however, cooperation tends to occur more frequently between relative trees and less so between different species. The distribution of resources, he suggests, is asymmetric, but the exact pattern is unknown.

Symbiotic fungi may not even be cooperating with trees, but are taking advantage of their carbon and conquering them to also ensure their survival and dominance of the territory. Flinn joins this hypothesis to propose that personifying trees takes us away from their understanding as plants. Perceiving them as' mothers', although it was a principle to sensitize us, causes them to be endowed with moral values; this is why the ecologist suggests knowing trees and spreading their life on their terms.

There is no consensus, therefore, between whether cooperation or competition predominate in plant communication or if there is an objective behind them beyond the natural one. Humanizing trees may not be the most accurate way to disclose the life of a tree; however, as the debate continues, imagining them that way has in certain cases worked to promote protection and respect for forests. As in other living beings, plant intimacy may persist incognito, but the glimpses suggest that, even in the face of pernicious coexistence, trees can be considered social creatures.

References

CAMARGO-RICALDE, Sara Lucía; Noé Manuel Montaño; Claudia Janette De la Rosa-Mera and Susana Adriana Montaño Arias. “Mycorrhiza: a great union under the ground.” University Digital Magazine. Vol. 13, No. 7. July 1, 2012. Available in: http://www.revista.unam.mx/vol.13/num7/art72/index.html

FLINN, Kathryn. “The Idea That Trees Talk to Cooperate Is Misleading”. Scientific American. July 19, 2021. Available in: https://www.scientificamerican.com/article/the-idea-that-trees-talk-to-cooperate-is-misleading/

MARRIS, Emma. “It Takes a Wood to Raise a Tree: A Memoir”. Nature. June 7, 2021. Available in: https://www.nature.com/articles/d41586-021-01512-y

RAMÍREZ GOMEZ, Margarita and Alia Rodríguez. “Plant defense mechanisms and responses in mycorrhizal interaction: a review”. Colombian Journal of Biotechnology. Vol. XIV, No. 1, July 1, 2012. 271-284. Available in: http://www.scielo.org.co/pdf/biote/v14n1/v14n1a25.pdf

SIMARD, Suzanne. “Mycorrhizal Networks Facilitate Tree Communication, Learning, and Memory”. Memory and Learning in Plants. 2019. 191-213