The Social Lives of Trees: Part 3 Underground Partnerships

By Emma Ewert, graduate student in the Institute’s 15th cohort. Take a look at part one and part two of her series on the social lives of trees!
While the partnerships between trees and fungi that I have been discussing in my previous posts are fascinating, what really intrigues and excites me is how these mycorrhizal networks we have been talking so much about help trees connect to each other, and how they form the basis for almost every aspect of the forest ecosystem. The relationships between trees allow them to survive and adapt to the world around them. It certainly helps explain why a tree that relies so much on access to the sun would choose to live so closely together that their ability to photosynthesize might be compromised.
At the moment, what we know is that mycorrhizal fungi not only play a huge part in keeping trees alive, but they also connect trees and plants, creating a forest ecosystem where almost every plant in any one square mile is directly connected to every other plant. This underlying mycelial structure allows trees and plants to share resources and warn each other of dangers.

Looking at this system as a network, trees act as the hubs, providing the energy that keeps it going and acting as points of connection. While up to 200 different types of fungi, can be attached to an individual tree, over 8000 individual species of fungi can be found in the forest ecosystem overall. The mat of hyphae in the soil of a forest is so thick that according to Suzanne Simard, a researcher at UBC, there are about 300 miles of hyphae under every footstep.
Scientists long thought that ecosystems were based on competition, that each organism existed because it was the best suited to survive in a particular area and had been able to outcompete any competitors. Mycorrhizal networks in forests have helped to dispel this theory. Trees communicate and share resources with each other, both within their own species and even in between separate species. A forest ecosystem is based not on competition, but on cooperation. When it comes to trees, you could say that it is not the fittest that survive in a forest, but the friendliest.
Old growth and maturing forests are mostly multigenerational, meaning that there are both large older trees that create the canopy and middle-aged to young trees that take advantage of disturbances that give them access to light. This is especially the case in the Pacific Northwest where we have a history of mosaic-like fires that provide many open areas where younger trees can flourish. However, even when they can find a small opening, and get some light, these saplings are often at least partially shaded, meaning they struggle to produce enough carbon and other nutrients. So it seems that the smaller trees in a forest are at a disadvantage, particularly when they are growing in an old growth forest.
Luckily, the larger trees with access to the sun produce more than they can use. Using the mycorrhizal networks as intermediaries, the older trees, known as mother or grandfather trees actually send carbon and other nutrients to the saplings in their network that may not be getting enough. How and why they do this is still unknown, but so far we have seen this in Douglas-firs, Western hemlocks and many other conifers. Large mother trees provide nutrients to trees of many different species, and even to other plants, including the Indian Pipe, seemingly without any compensation. A likely reason for why Western Hemlocks flourish so well in the shade is that they are particularly adept at this type of exchange. Taking this even further, a study by a graduate student at the University of British Columbia established that these older trees do not blindly or indiscriminately nourish all the trees in their network. Instead, using their hyphal links, they can somehow identify the saplings that came from their own seeds, and preferentially feed their own “children”.
This transfer of carbon and other nutrients is not limited to the passing of nutrients from older trees to saplings. Trees can also trade nutrients with other species when they to take advantage of their specialized abilities or adaptations. In her thesis, Suzanne Simard found that deciduous birch trees and evergreen Douglas-firs exchange varying amounts of carbon throughout the year. The balance of this exchange changes depending on the time of year and the stage these trees are in. The birch trees transferred carbon to Douglas-fir in the spring and summer, when their highly productive leaves were able to photosynthesize more than the darker, thicker needles of the Douglas-fir. However, during the fall and winter, once the birch trees had lost their leaves, the Douglas-firs began to transfer more carbon to the birch.
Another example of intraspecies cooperation comes from the local Red Alder. Red Alder can flourish in nitrogen poor soils thanks to another underground partner: the bacteria Rhizobium japonica. Alder roots grow nodules in their roots for the specific purpose of housing this bacteria. The nitrogen that these bacteria process feeds the Red Alder, but much of it is also released into the soil, creating richer soils, and allowing nitrogen dependent trees and plants, including Douglas-fir or Western Hemlock to grow. Unfortunately, it takes a lot of energy to house and feed these bacteria, meaning Red Alder do not produce enough starch for themselves to grow. Luckily, the spirit of collaboration in the forest ecosystem gives them a solution, and red Alder is able to get 10% of the starch it needs from other plants in the mycorrhizal system.
Trees can also use these incredibly complex networks to warn each-other of incoming pest attacks. As we already know, mycorrhizal fungi can identify intruders and warn a tree of an attack. When this happens, the signal is sent throughout the network, and neighboring trees are able to increase their production of repellents or insecticides in preparation and become more resistant even before they are attacked. Trees can also initiate warnings themselves if the attack come from the air rather than the soil. While the primary and quickest warning system for defoliating insects or other canopy-related pathogens is airborne, through wind borne chemicals that, the subterranean system allows trees who may be up to a mile away or more to get the warning as well.
There are certainly more ways that trees are able to interact with one another. Trees that grow close together often merge their roots and essentially share the same phloem. The ways that trees can harness their mycorrhizal systems are still being discovered, and I have only been able to describe a few different interactions here. This is an area of study that is still growing, and as more and more is understood, I have a feeling that what we think we understand now is going to be only a small sliver of the whole incredible truth.
 
For more information on how these networks function, Professor Suzanne Simard has a two fabulous talks available online:
 
The Science, Art and Meaning of Forest Wisdom, a talk given at the 13th Annual International Bioethics Forum in 2014: https://www.youtube.com/watch?v=pLU9EPo1iwQ
 
The networked beauty of forests, a TED-Ed talk from 2014: https://www.youtube.com/watch?v=dRSPy3ZwpBk