The secret life of the trees

Recent research shows that the creatures that inhabit forests such as plants and fungi form a wholesome organism after some time which is the forest itself. There are more trees than people in Canada, no less than 8,953 per inhabitant as well as more forests than cities. 

Suzanne Simard grew up next to one of the lush Pacific coasts, where there are trees over 100 metres tall and a thousand years old. The forest engineer from the University of British Columbia has found strong scientific evidence that trees are social beings that cooperate and communicate.

"As a child, when I walked with my grandfather in the wild, I had the feeling that the forest was a living thing in itself, that all the people in the forest were a harmonious unit," she recalls in one of her TED talks.

Today we know from her experiments that the forest behaves like an organism interconnected on a microscopic scale in a complex underground web. The key to it all lies beyond where the eye can see, under the ground. Tree roots - which can expand two to four times the diameter of their canopy - intertwine with mycelium, the mass of thin underground filaments of fungi, to form gigantic information networks that carry not only water and nutrients but also messages of encouragement or danger.

"They communicate through their own system. They are not individuals who grow up on their own in order to be the most successful. Rather, they are part of a network that is constantly interacting, where collaboration is paramount," says Simard, who, before turning to research, had to work in the timber industry, planning logging and production plantations, to understand that something was wrong. She confesses, "It didn't fit with my understanding of the forest.

Forests vs. plantations

When compared to the wild forests, the plantations were lifeless, no more than rows of trees of the same species that grew sadly, more slowly and less vigorously than their wild brothers. "Just like children in an orphanage, deprived of their parents' affection," says Peter Wohlleben, a forestry technician who manages the communal forest of Hümmel, Germany, and author of the book The Secret Life of Trees (2017), in the documentary Intelligent Trees.

Simard discovered that in the plantations, the tree community does not interact freely. "I saw that if you take away some species and separate them from their neighbours, they get sick and become more vulnerable to insect attacks. I wanted to understand why, and it occurred to me that the answer might be underground," she explains. And not just in the roots of the trees, but in the association of mutual aid, they form with mycorrhizae or intertwined networks of the myceliums of fungi and the roots of plants.

Teodoro Marañón, a forest ecologist and researcher at CSIC, explains to the Muy Interesante team, that a mycorrhiza is the "symbiosis of a plant's root with a fungus”. 

“The fungus colonizes the root and receives compounds that the plant produces through photosynthesis. The relationship is mutually beneficial, because the fungus, through its extensive network of myceliums, captures water and minerals that it transfers to the plant". The fungi become entangled like a tangle of filaments along miles below the surface.

 "Mushrooms are just the fruit, just like apples are the fruit of the apple tree. The body of the fungus is in the soil," Wohlleben adds.

In exchange for being able to use their efficient information highway to communicate with the entire forest, the trees share sugars and nutrients with them. In her first experiment, collected by Nature magazine twenty-five years ago, an imaginative young Simard came up with the idea of studying the relationship between two species that are genetically distant but used to occur together in nature. She injected carbon isotopes (13C and 14C) into the leaves of birch trees (Betula papyrifera) and found that in spring these molecules reached the Firs (Pseudotsuga menziesii) growing in the shade of their dense foliage. In winter, the tables were turned, and it was the Firs that passed the isotopes to the birches, which are deciduous.

Thus she knew that the trees communicate and that the direction of their transactions is determined by their needs. The leafless birches live more precariously in winter, while the Firs suffer more the rest of the year because in the shade they cannot photosynthesize as effectively as their neighbours.

To understand how the pathways of this exchange worked, Simard isolated the outside of several small birch trees in plastic bags, into which he injected a carbon isotope. He then did the same with the roots. In some cases, the plastic prevented them from communicating with the mycorrhizome. In others, the bags were porous and allowed the mycelium to pass through and connect to the roots. Thus they saw that those seedlings in which the filaments of the fungi could cross the barrier continued to exchange substances and grew stronger and more luxuriant than those that had their roots isolated from the fungi. Also that the more vigorous and older trees provided nutrients to the smaller and younger seedlings.

It made sense, then, that in timber plantations the trees would grow more hearty. "The heavy machinery for logging and transport compacts the soil with its weight and damages the mycorrhizae, hinders exchanges and prevents the compacted soil from storing winter water for the summer," says Wohlleben.

The power of mycorrhiza

Not only do nutrients circulate between the trees through the underground mycorrhizal networks, but also biochemical signals warn of dangers. In 2010, biologist Ren Sen of the University of South China in Guangzhou planted pairs of potted tomatoes. Some of them could form mycorrhizae and others could not. He then sprayed the leaves of one of each pair with the Alternaria solani fungus, which causes late blight in tomatoes. After sixty-five hours, Zeng tried to infect the second plant of each pair and saw that those that had established mycorrhizal networks to communicate with their infected partner were much more resistant to the pathogen and less ill.

The same was recently found at the University of Aberdeen in Scotland with aphid-infested bean plants - insects that destroy them. Unlike those that were isolated, the beans that remained united underground with congeners that had been attacked before seemed to be prevented and already had their anti-aphid chemicals ready to defend themselves. In their own way, their sick companions had sent them a warning signal.

Simard found that Firs that had lost their leaves to insect attack transmitted stress signals through the mycorrhizal network to neighbouring trees, not just those of the same species. In response, they activated the genes that set off the defensive enzymes. According to the University of British Columbia researcher, "trees send chemical messages to each other to protect themselves from insect attack or disease by making defensive toxins or resins, or by fattening their bark.

In another study, Simard analyzed a forest attacked by the mountain pine beetle (Dendroctonus ponderosae). The dying trees passed on the legacy to new generations, with information on how to optimize their defence system, so that the new ones grew stronger. A finding that, in the opinion of this scientist, should change the way forests are managed: "We have to be careful and stop cutting down sick trees as soon as possible to sell them before their wood deteriorates because then we prevent them from passing on their wisdom to the younger ones," she warned.

Along with her team from the University of British Columbia's Department of Forestry and Conservation Sciences, Simard set out to map the underground connections of a spruce forest. Using molecular biology techniques, they analyzed the DNA and identified genotypes of trees and fungi. Thus, they recorded an average length of 20 meters for the myceliums - small filaments that serve as the roots of the fungi - and found "a 94-year-old Fir connected to 47 trees, through different genotypes of the fungus Rhizopogon sp," according to Marañón. 

The architecture of the Wood Wide Web

This study was published in the New Phytologist magazine with the illustrative title Wood Wide Web Architecture - a word game with the world wide web (the famous www) - which compares the underground forest ecosystem with the internet, a web of many overlapping networks with busier central nodes acting as switches and smaller satellite nodes. "It would be a clonal super-organism, a symbiotic tree-fungus network that shares the forest's resources," Marañón describes.

It's not much different from how our mind works: "The root system is the brain of the forest. It's connected just like the neural networks through which information circulates," reflects Simard. 

Looking at this map, the Canadian scientist could identify which trees were most important and discovered "that they were the biggest and oldest. We called them mother trees because we discovered that the younger ones growing around them fed on nutrients that the older ones passed through the mycorrhizal network.

It's a bit like a female breast-feeding," Simard continues. And not just the offspring of his own species. In another experiment, his team tagged mother trees with carbon isotopes so they could track these molecules and find out who they were feeding. The biologists observed that the relatives received more, but also sent food to the rest of the neighbours. "They try to create a favourable and healthy environment for the community to grow, so they provide it with nutrients," explains the scientist, who also leads the Mother Tree Project at her university. The huge canopies of these specimens rise above the others and collect the sun's energy to produce large amounts of glucose in their leaves and share it with those who have less access to sunlight and with the small newborn seedlings.

Do plants know how to recognize their neighbours? A recent experiment by Amanda Asay, a researcher on Simard's team at the University of British Columbia, suggests that they do. 

Asay planted three trees in a single pot, one larger and two young shoots, one of which came from a seed from the same mother as the large tree and the other from another region. Months later, there were more underground connections between the big one and his brother, to whom he transferred more nutrients than between the big one and the stranger. And what about those that are already just a patchy trunk without branches or leaves to make photosynthesis, and yet they are still alive? "We discovered that, in some cases, a neighbouring tree was passing nutrients and carbon to them. The forest takes care of its elders," says Asay.

Listening to these experts, could it be said that there are similarities between plant life and the emotional life of people? It seems like an exaggerated metaphor, but to some extent, yes: "There is a friendship between trees. It does not happen often, because they cannot choose beside whom they grow. Perhaps it happens to one in fifty. Look at these two, their roots are firmly intertwined, their branches do not overlap, so as not to take away the light," says Wohlleben in the documentary Intelligent Trees as he shows the camera two beautiful cedars. "If one dies, the one that is left suffers, gets sick and dies soon after.

The green friendship

Simard smiles when asked about green friendship: "In ecology, we call it interaction, although it's a rather clinical term. Plants collaborate, help each other, maintain mutualistic relationships. It's a question of language. In human terms, we would call that phenomenon friendship," he reflects. In any case, the idea of the forest as a family of trees changes our perception and our way of treating them, according to Simard.

Among her research projects is investigating how to strengthen forests to deal with climate change. In an article published in 2018 in the New Phytologist magazine, he highlights the importance of so-called host receptivity, that is, the ability of a tree species to associate with a large number of mycorrhizal fungi species. According to Marañón, "the most promiscuous trees had the highest rate of northward expansion after the last glaciation.

Simard continues to search for ways of collaboration between humans and trees from which we both benefit. He has found a new dimension to the word ecosystem and has taught us that, in its own way, the forest has a huge brain that functions underground. For his part, Wohlleben calls attention to the problem of uncontrolled logging. In his opinion, the alternative is to make friendly management of the forest, which allows the trees to satisfy their social needs and pass on "their knowledge to the next generation. At least some should be allowed to age with dignity and die a natural death.