The Weekend Wonk: How Trees Talk to Each Other
September 10, 2016
Two decades ago, while researching her doctoral thesis, ecologist Suzanne Simard discovered that trees communicate their needs and send each other nutrients via a network of latticed fungi buried in the soil – in other words, she found, they “talk” to each other. Since then, Simard, now at the University of British Columbia, has pioneered further research into how trees converse, including how these fungal filigrees help trees send warning signals about environmental change, search for kin, and transfer their nutrients to neighboring plants before they die.
By using phrases like “forest wisdom” and “mother trees” when she speaks about this elaborate system, which she compares to neural networks in human brains, Simard’s work has helped change how scientists define interactions between plants. “A forest is a cooperative system,” she said in an interview with Yale Environment 360. “To me, using the language of ‘communication’ made more sense because we were looking at not just resource transfers, but things like defense signaling and kin recognition signaling. We as human beings can relate to this better. If we can relate to it, then we’re going to care about it more. If we care about it more, then we’re going to do a better job of stewarding our landscapes.”
Yale Environment 360: Not all PhD theses are published in the journal Nature. But back in 1997, part of yours was. You used radioactive isotopes of carbon to determine that paper birch and Douglas fir trees were using an underground network to interact with each other. Tell me about these interactions.
Suzanne Simard: All trees all over the world, including paper birch and Douglas fir, form a symbiotic association with below-ground fungi. These are fungi that are beneficial to the plants and through this association, the fungus, which can’t photosynthesize of course, explores the soil. Basically, it sends mycelium, or threads, all through the soil, picks up nutrients and water, especially phosphorous and nitrogen, brings it back to the plant, and exchanges those nutrients and water for photosynthate [a sugar or other substance made by photosynthesis] from the plant. The plant is fixing carbon and then trading it for the nutrients that it needs for its metabolism. It works out for both of them.
It’s this network, sort of like a below-ground pipeline, that connects one tree root system to another tree root system, so that nutrients and carbon and water can exchange between the trees. In a natural forest of British Columbia, paper birch and Douglas fir grow together in early successional forest communities. They compete with each other, but our work shows that they also cooperate with each other by sending nutrients and carbon back and forth through their mycorrhizal networks.
e360: And they can tell when one needs some extra help versus the other, is that correct?
Simard: That’s right. We’ve done a bunch of experiments trying to figure out what drives the exchange. Keep in mind that it’s a back and forth exchange, so sometimes the birch will get more and sometimes the fir will get more. It depends on the ecological factors that are going on at the time.
One of the important things that we tested in that particular experiment was shading. The more Douglas fir became shaded in the summertime, the more excess carbon the birch had went to the fir. Then later in the fall, when the birch was losing its leaves and the fir had excess carbon because it was still photosynthesizing, the net transfer of this exchange went back to the birch.
There are also probably fungal factors involved. For example, fungus that is linking the network is going to be looking to secure its carbon sources. Even though we don’t understand a whole lot about that, it makes sense from an evolutionary point of view. The fungus is in it for its own livelihood, to make sure that it’s got a secure food base in the future, so it will help direct that carbon transfer to the different plants.
e360: Through molecular tools, you and one of your graduate students discovered what you call hub, or mother, trees. What are they, and what’s their role in the forest?
Simard: Kevin Beiler, who was a PhD student, did really elegant work where he used DNA analysis to look at the short sequences of DNA in trees and fungal individuals in patches of Douglas fir forest. He was able to map the network of two related sister specials of mycorrhizal fungi and how they link Douglas fir trees in that forest.
Just by creating that map, he was able to show that all of the trees essentially, with a few isolated [exceptions], were linked together. He found that the biggest, oldest trees in the network were the most highly linked, whereas smaller trees were not linked to as many other trees. Big old trees have got bigger root systems and associate with bigger mycorrhizal networks. They’ve got more carbon that’s flowing into the network, they’ve got more root tips. So it makes sense that they would have more connections to other trees all around them.
In later experiments, we’ve been pursuing whether these older trees can recognize kin, whether the seedling that are regenerating around them are of the same kin, whether they’re offspring or not, and whether they can favor those seedlings — and we found that they can. That’s how we came up with the term “mother tree,” because they’re the biggest, oldest trees, and we know that they can nurture their own kin.