EDUCATION
The words that trees tell each other
- SUDHAMAHI REGUNATHAN*
The words that trees tell each other



A few weeks ago we heard a talk which told us that trees engineer their own clouds by releasing vapours into the atmosphere. This week Suzanne Simard, a professor of forest ecology at the University of British Columbia’s Department of Forest and Conservation Sciences, tells us that trees talk to each other!

This fascinating revelation comes with a story which begins with Simard telling us of how she helped her grandfather rescue their pet dog who had fallen into a pit. In that process, Simard found that under the ground lay, "…white mycelium and under that the red and yellow mineral horizons. I realized that that palette of roots and soil was really the foundation of the forest… trees are the foundation of forests, but a forest is much more than what you see…underground there is this other world, a world of infinite biological pathways that connect trees and allow them to communicate and allow the forest to behave as though it's a single organism. It might remind you of a sort of intelligence.”

As Simard took it up for study she says, "Scientists had just discovered in the laboratory in vitro that one pine seedling root could transmit carbon to another pine seedling root and I wondered, could this happen in real forests? Trees in real forests might also share information below ground. ..some people thought I was crazy. But I persevered, and eventually conducted some experiments deep in the forest, 25 years ago. I grew 80 replicates of three species: paper birch and Douglas fir…”

How did the experiment work? Simard put plastic bags over the trees. She then injected the bags with tracer isotope carbon dioxide gases. She injected carbon-14, the radioactive gas, into the bag of birch. And then for fir, she injected the stable isotope carbon-13 carbon dioxide gas. She used two isotopes, because, "I was wondering whether there was two-way communication going on between these species… I waited an hour. I figured it would take this long…I went to my first bag with the birch. I pulled the bag off. I ran my Geiger counter over its leaves. The birch had taken up the radioactive gas. Then the moment of truth… I went over to the fir tree. I pulled off its bag. I ran the Geiger counter up its needles, and I heard the most beautiful sound. .. the sound of birch talking to fir, the sound that it had taken up the radio active gas too!”

Continues Simard, "I knew I had found something big, something that would change the way we look at how trees interact in forests, from not just competitors but to cooperators. And I had found solid evidence of this massive below ground communications network, the other world. How were paper birch and Douglas fir communicating? They were conversing not only in the language of carbon but also nitrogen and phosphorus and water and defence signals and the chemicals and hormones -- information. Before me, scientists had thought that this below ground mutualistic symbiosis called a mycorrhiza was involved. Mycorrhiza literally means "fungus root”. You see their reproductive organs when you walk through the forest. They are the mushrooms. The mushrooms, though, are just the tip of the iceberg, because coming out of those stems are fungal threads that form a mycelium, and that mycelium infects and colonizes the roots of all the trees and plants. And where the fungal cells interact with the root cells, there's a trade of carbon for nutrients, and that fungus gets those nutrients by growing through the soil and coating every soil particle. The web is so dense that there can be hundreds of kilometers of mycelium under a single footstep. And not only that, that mycelium connects different individuals in the forest, individuals not only of the same species but between species, like birch and fir, and it works kind of like the Internet…You see, like all networks, mycorrhizal networks have nodes and links,” so saying she shows an illustration. "We made this map by examining the short sequences of DNA of every tree and every fungal individual in a patch of Douglas fir forest.”

Simard identifies on the map what she calls hub trees or mother trees because they nurture the young ones by passing them nutrients. "In a single forest, a mother tree can be connected to hundreds of other trees. And using our isotope tracers, we have found that mother trees will send their excess carbon through the mycorrhizal network to the understory seedlings, and we’ve associated this with increased seedling survival by four times…”

Simard’s experiment turns into a beautiful story when she says, "And it turns out they recognize their kin. Mother trees colonize their kin with bigger mycorrhizal networks. They send them more carbon below ground. They even reduce their own root competition to make elbow room for their kids. When mother trees are injured or dying, they also send messages of wisdom on to the next generation of seedlings. So we've used isotope tracing to trace carbon moving from an injured mother tree down her trunk into the mycorrhizal network and into her neighbouring seedlings, not only carbon but also defence signals. And these two compounds have increased the resistance of those seedlings to future stresses.”

Indeed truth is stranger than fiction, more beautiful than poetry, to coin a new comparison.
 
 
(Courtesy: *The Hindu)






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