When we see a tree, we tend to think of it as a singular unit – just as we think of ourselves as individuals. But biologists have discovered that it’s not quite so simple. They have come to understand that trees depend on certain kinds of fungi in the soil: hair-thin structures called hyphae that interlace with cells in the roots of trees to form mycorrhiza. The fungi benefit by receiving someof the sugar that plants produce through photosynthesis (which it cannot otherwise make), while the trees benefit in turn by receiving elements like phosphorous and nitrogen that they cannot produce for themselves, and without which they cannot survive.
But this reciprocity is not confined to just two parties in this ancient relationship. Invisible fungal networks also connect the roots of different trees to one another, sometimes over great distances, forming an underground internet that allows them to communicate, and even to share energy, nutrients and medicine. The ecologist Robert Macfarlane explains how this works: A dying tree might divest itself of its resources to the benefit of the community, for example, or a young seedling in a heavily shaded understory might be supported with extra resources by its stronger neighbours. Even more remarkably, the network also allows plants to send one another warnings. A plant under attack from aphids can indicate to a nearby plant that it should raise its defensive response before the aphids reach it.
It has been known for some time that plants communicate above ground in comparable ways, by means of airborne hormones. But such warnings are more precise in terms of source and recipient when sent by means of the myco-net. 16 Trees co-operate. They communicate. They share. Not only among members of the same species, but across species barriers: Douglas firs and birches feed each other. And it’s not just trees; we now know that all plants – except for a handful of species – have this same relationship with mycorrhiza. Just as with our gut bacteria, these findings challenge how we think about the boundaries between species. Is a tree really an individual? Can it really be conceived as a separate unit? Or is it an aspect of a broader, multi-species organism?
There’s also something else going on here – something perhaps even more revolutionary. Dr Suzanne Simard, a professor in the department of forest & conservation at the University of British Columbia, has argued that mycorrhizal networks among plants operate like neural networks in humans and other animals; they function in remarkably similar ways, passing information between nodes. And just as the structure of neural networks enables cognition and intelligence in animals, mycorrhizal networks provide similar capacities to plants. Recent research shows that the network not only facilitates transmission, communication and co-operation – just like our neurons do – it also facilitates problem-solving, learning, memory and decision-making.
These words are not just metaphorical. The ecologist Monica Gagliano has published groundbreaking research on plant intelligence, showing that plants remember things that happen to them, and change their behaviour accordingly. In other words, they learn. In a recent interview with Forbes, she insisted: ‘My work is not about metaphors at all; when I talk about learning, I mean learning. When I talk about memory, I mean memory.’ Indeed, plants actively change their behaviour as they encounter new challenges and receive messages about the changing world around them. Plants sense: they see, hear, feel and smell, and they respond accordingly. If you’ve ever seen time-lapse footage of a vine growing up a tree, you’ll have an idea of what this looks like in action: that vine is no automaton – it’s sensing, moving, balancing, solving problems, trying to figure out how to navigate new terrain. The more we learn, the stranger (or perhaps more familiar?) it all becomes. Simard’s work shows that trees can recognise their own relatives through mycorrhizal networks. Older ‘mother’ trees can identify nearby saplings that came from their own seeds, and they use this information to decide how to allocate resources in times of stress. Simard also describes how trees seem to have ‘emotional’ responses to trauma in a way that’s not dissimilar to animals. After a machete whack or during an aphid attack, their serotonin levels change (yes, they have serotonin, along with a number of neurochemicals that are common in animal nervous systems), and they start pumping out emergency messages to their neighbours.
Of course, none of this is to say that plant intelligence is exactly like that of animals. In fact, scientists warn that our urge to constantly compare the intelligence of some species with that of others is exactly the problem: it ends up blinding us to how other kinds of intelligence might work. Set out in search of a brain and you’ll never even notice the mycorrhiza that have been pulsing through the earth, evolving right under our feet, for 450 million years. This research is just taking off, and we have no idea where it might lead. But Simard is careful to point out that it’s not exactly new: If you listen to some of the early teachings of the Coast Salish and the Indigenous people along the western coast of North America, they knew [about these insights] already. It’s in the writings and in the oral history.
The idea of the mother tree has long been there. The fungal networks, the below-ground networks that keep the whole forest healthy and alive, that’s also there. That these plants interact and communicate with each other, that’s all there. They used to call the trees the tree people … Western science shut that down for a while and now we’re getting back to it.
Trees aren’t only connected with each other. They are also connected with us. Over the past few years, research into human–tree relationships has yielded some truly striking findings. A team of scientists in Japan conducted an experiment with hundreds of people around the country. They asked half of the participants to walk for fifteen minutes through a forest, and the other half to walk through an urban setting, and then they tested their emotional states. In every case, the forest walkers experienced significant mood improvements when compared to the urban walkers, plus a decline in tension, anxiety, anger, hostility, depression and fatigue. The benefits were immediate and effective. Trees also have an impact on our behaviour. Researchers have found that spending time around trees makes people more co-operative, kinder and more generous. It increases our sense of awe and wonder at the world, which in turn changes how we interact with others. It reduces aggression and incivility. Studies in Chicago, Baltimore and Vancouver have all discovered that neighbourhoods with higher tree cover have significantly fewer crimes, including assault, robbery and drug use – even when controlling for socioeconomic status and other confounding factors.
It’s almost as though being with trees makes us more human. We don’t know quite why this happens. Is it just that green environments are somehow more pleasant and calming? A study in Poland suggests that doesn’t explain it. They had people spend fifteen minutes standing in a wintertime urban forest: no leaves, no green, no shrubbery; just straight, bare trees. One might think such an environment would have minimal if any positive impact on people’s mood, but not so: participants standing in the bare forest reported significant improvements in their psychological and emotional states when compared to a control group that spent those fifteen minutes hanging out in an urban landscape. And it’s not just mood and behaviour. It turns out that trees have an impact on our physical health too – in concrete, material terms. Living near trees has been found to reduce cardiovascular risk. Walking in forests has been found to lower blood pressure, cortisol levels, pulse rates and other indicators of stress and anxiety.
Even more intriguingly, a team of scientists in China found that elderly patients with chronic health conditions demonstrated significant improvements in immune function after spending time in forests. We don’t know for sure, but this may have something to do with the chemical compounds that trees exhale into the air. The aromatic vapours released by cypress, for example, have been found to enhance the activity of a number of human immune cells, while reducing stress hormone levels. In an attempt to quantify the overall benefit of trees, scientists in Canada found that trees have a more powerful impact on our health and well-being than even large sums of money. Having just ten more trees on a city block decreases cardio-metabolic conditions in ways comparable to earning an extra $20,000. And it improves one’s sense of well-being as much as earning an extra $10,000, moving to a neighbourhood with $10,000 higher median income, or being seven years younger. These results are astonishing. There’s a real mystery here, which scientists still do not yet understand. But perhaps we shouldn’t be so surprised. After all, we have co-evolved with trees for millions of years. We even share DNA with trees. After countless generations, we’ve come to depend on them for our health and happiness just as we depend on other humans. We are, in a very real sense, relatives.
- Jason Hickel, Less is More
