Whispering Leaves: How Trees Communicate

Originally published August 2017

When you stand in a forest what do you see? The first thing you notice is the trees; either the stately old growth, scrubby new growth, or the regimented rows of a monoculture plantation. They stand individually and they don’t look like they are interconnected, but there is much more to the forest than easily meets the eye. Believe it or not, trees communicate with each other. Above ground trees use hormones, whereas deep in the soil trees communicate using an underground web of roots and fungi and the eldest trees, called mother trees, have a very important role in keeping the forest healthy day to day.

An older stand in the Four Mile Stillwater area of the Medway Lakes Wilderness Area

Above ground Communication:

Trees can communicate above ground through hormones. One example of this was demonstrated in the 1980s on the Limpopo Savanna in South Africa. At that time, there was a terrible ongoing drought and the only plant that was doing well enough to support animal populations was the Acacia tree. One of the species of animals that relied heavily on the Acacia tree was a deer-like creature called the Kudu. At some point during the drought Kudus suddenly began to drop dead for no apparent reason. Following research, it was discovered that the Kudus were dying due to excess tannin, a plant poison produced as a defence response by Acacia trees being eaten by the Kudus. However, it was also discovered that Acacia plants that had not yet been affected by Kudu grazing were also producing excess tannins. This led scientists to conclude that the trees being grazed on by the Kudus were releasing a defence signal which was carried downwind to other trees, causing them to increase their tannin production. This transport of defence signals by the wind is a form of tree communication through air (Hughes, 1990).
Since this experience in the 1980s other scientists have carried out experiments on trees to test their ability to communicate through hormones. Studies done in the 2000s on various types of trees and plants also showed evidence of wind borne hormones used to create a defensive response in other trees and plants during insect outbreaks. These chemical signals are not just linked to one species of plant either. The hormone emissions of a corn plant can be received by a tomato plant for example, and will elicit a response (McGowan, 2013).

[caption id="attachment_853" align="aligncenter" width="451"] The trunk and canopy of a tree compose the above ground section of the communication network[/caption]

Below ground communication:

Trees communicate below ground by exchanging carbon, nutrients, and defence signals through root contact and networks of underground fungi. The ability of trees to use the fungal network to transmit information greatly exceeds their ability to communicate through roots as it spans the soil underneath the forest floor. Similar to hormonal emissions, the transmission of carbon, nutrients, and defence signals between plants occurs not only between trees of the same species but also in a symbiotic manner to other species of trees. This is exemplified in the co-operation between birch and fir trees. Birch trees lose their leaves in the fall, preventing them from converting sunlight to stored energy through photosynthesis. Since fir trees retain their needles, they are able to photosynthesise during the winter but they photosynthesise less than birch trees in the spring and summer. Birch trees transfer carbon and nutrients to fir trees in the spring and summer, and fir trees transfer carbon and nutrients to the birch trees in fall and winter. This mutual transfer provides both species a better chance of surviving and thriving (Simard, 2017).
Trees of various species also help each other to respond to insect and disease outbreaks by sending defense signals through root contact and the fungal network. A tree affected by the insect or disease will send out a defense signal which prepares other trees for attack, allowing them to defend themselves when the insect or disease reaches them (Simard, 2016).

[caption id="attachment_855" align="aligncenter" width="450"] The above ground section of the vast fungal network that connects trees[/caption]

The Importance of Mother Trees:

Mother trees are the oldest and largest trees in the forest and their extensive networks and experience allow them to exchange the most information with the other trees. Through their interconnections ‘mother’ trees or ‘hub’ trees are able to support younger, more shaded trees with carbon and nutrients essential for growth, and support sick trees until they are able to do well on their own. Also, due to their increased exposure to tree diseases and pests, mother trees can impart their “knowledge” by way of defence signals to younger trees, allowing them to fight the diseases or insects affecting them. Mother trees are also able to distinguish between their own offspring and trees of other species, allowing them to provide more nutrients to their saplings and discourage the growth of the saplings of other species by producing growth inhibitors (Simard, 2016). Because their “knowledge” is so useful, mother trees can be kept alive by the trees around them through carbon and nutrient transfers, even when they are only a stump. Green wood has been found in the long cut stumps of trees that are hundreds of years old (Wohllben, 2017).

Looking up at a hemlock mother tree

Hug a Tree!

Above and belowground communication is an essential adaptation which allows trees to survive and thrive under challenging circumstances. Next time you find yourself in a forest, don’t think of each tree as an individual unit. Remember that you are breathing and standing on a vast communication network that links all the forest around you. Take care on your visit and remember to hug a mother tree!

Giving a mother tree some love in Kejimkujik National Park

Bibliography
Hughes, S. (1990). Antilope activate the acacia's alarm system. New Scientist.

McGowan, K. (2013, December 20). How Plants Secretly Talk To Each Other. Retrieved From:
https://www.wired.com/2013/12/secret-language-of-plants/

Speaker, Simard, S. (2016, July 22). How Trees Communicate [Video File]. Retrieved from: https://www.ted.com/talks/suzanne_simard_how_trees_talk_to_each_other

Speaker, Simard, S. (2017, February 2). Nature's Internet: How Trees Talk To Each Other in a Healthy Forest [Video File) Retrieved from:
http://tedxseattle.com/talks/natures-internet-how-trees-talk-to-each-other-in-a-healthy-forest/

Wohllben, P. (2015, May 25). The Hidden Life of Trees, Greystone Books

Photos Curtousy of: Lachlan Riehl and Mary Jane Rodger