Trees and their Fungal Friends

The relationship between plants and fungi is a very old story. Plants were able to move from water to land about 400 million years ago because of their relationship with fungi which served as their root systems until they evolved to develop their own roots. Millions of different fungal species inhabit the earth and a majority of these dwell in the soil.  Soil fungi are grouped into 3 categories: decomposers, mutualists, and pathogens. In this article, we are only looking at the mutualists; specifically, the fungal species that have adapted to live communally with trees where both tree and fungi benefit from establishing a relationship.

DEFINITIONS:

• Hyphae — individual fungal threads, usually between 1-10 thousandths of a millimeter in diameter. A single hyphae can grow up many meters long.
• Rhizomorphs –—an aggregation of hyphae intertwining like strands of a rope making a “root-like” structure.
• Mycelium — The thallus, or vegetative part, of a fungus made up of a mass of branched hyphae. Mycelial networks can extend over tens or hundreds of meters.
• Mycorrhiza — a mutual, symbiotic relationship between a fungus and a plant, unlike either fungi or roots alone.
• Symbiotic relationships — Broadly defined as relationships occurring between living entities. Although there are several types of symbiotic relationships, for this story, we are looking at one subtype: mutualistic symbiosis.
• Ecosystem — A community of living organisms in conjunction with the nonliving components of their environment, interacting as a system.

Trees thrive when they live together in a forest. A single tree will struggle to survive on its own, however many trees together create an ecosystem that produces a protective environment in which the trees can live to be very old. Since every tree is a valuable member of the community, trees have developed several ways to support and nourish each other. This is true for trees of the same or different species. There are several ways trees communicate and nourish each other above ground, however we are going to focus on how they do this underground.

Indigenous peoples have long understood that trees communicate with and nourish each other. Only recently have scientists used modern tools and techniques to explain just how this amazing communal network occurs. The most important means of underground communication is by participation in a mycelia fungal network — the relationship between tree roots and the fungal hyphae present in the soil. These hyphae form networks known as mycelium which infiltrate and connect tree roots of the same or different species. Over centuries, a single fungus can cover many square miles and network an entire forest, enabling the sharing of water and nutrients. The mycelial network can also transmit very low voltage electrical impulses to trees which communicates information about insects, drought, and other dangers. This vast underground mycorrhizal network can be thought of as the “internet” of the soil and is often referred to as the “wood wide web”.

Another service fungi provide trees is the filtering out of any heavy metals in soil. These diverted pollutants turn up in the fungi’s fruiting body (i.e. mushrooms). Fungi will ward off bacteria or other destructive fungi that are trying to invade the tree. This tree/fungi relationship can go on for hundreds of years, however if conditions in the environment become unhealthy, the fungi may die out. At that point the tree may hook up with a different fungal species that settles in at its feet. Every tree species has multiple options for mutualistic fungal partners.

I’ve focused above on the benefits a tree derives from its relationship with a fungal partner however, as I mentioned, this is a mutualistic partnership. What is the benefit for the fungi?

Payment for the services the fungi provide to the trees is in the form a nutrition – sugar and other carbohydrates – which the fungi cannot produce on their own. The fungi retain about 30% of the carbohydrates the tree produces, thank you very much.

Some species of fungi are considered “host specific” and will partner only with a specific tree type (e.g. birches or larches). Others, like chanterelles, get along with many different tree types. Underground competition is fierce which works to the benefit of the trees; it is only when all the fungal species die out that the tree becomes vulnerable. Because fungi are dependent on stable conditions, they support a variety of species in order to ensure that one tree species doesn’t manage to dominate. Fungi can store and later share resources (particularly nitrogen and phosphorous) when the soil becomes depleted. In some tree/fungal relationships where the soil becomes depleted of nitrogen, the fungi will release a deadly toxin into the soil which causes minute organisms such as springtails (tiny insects that live in leaf litter, compost piles, and soils) to die and release nitrogen tied up in their bodies, forcing them to become fertilizer for both the tree and the fungi.

Saplings (young trees) growing in a shady area that do not receive enough sunlight to perform adequate photosynthesis often receive assistance from older established trees called “hub trees” or “mother trees” to provide water and nutrients via the mycorrhizal network. Studies have shown that trees can recognize the root tips of their relatives and favor them when sending nutrients. Through the mycorrhizal network, hub trees detect distress from their neighbors and send them needed nutrients. During a recent walk in the coastal forest south of Coos Bay, Oregon I came upon the tree pictured below. I was reflecting on the tortured life this tree must have lived when I look up at the canopy and was amazed to see branches that still had green fir needles — it was still living! Further inspection led me to realize that tree next to it (in the far left of the picture) had a huge root leading right to the crippled tree, most likely providing the nutrients needed for the older tree to remain alive. Quite possibly this younger tree is the offspring of the older tree and demonstrating that both older and younger trees take care of each other as needed.

MAKING THE CONNECTION: People are generally aware of the many benefits trees provide to the natural world including removal of carbon dioxide from the atmosphere, production of oxygen, provision of shade and habitat for numerous species, wood and fruit production, to name just a few. It’s easy for humans to take for granted the gifts that trees provide to our ecosystems. Having a deeper understanding of how trees are able to grow and thrive in community with the help of their fungal friends helps foster greater respect and gratitude for both of these species. Although the mycelial network is largely invisible to human awareness, knowing of its existence and the important role it plays in nurturing our forests is an important connection to make in understanding how symbiotic relationships between species are crucial to maintaining balance in natures ecological processes.  Try to imagine a world without trees. It would be a world vastly different from the one we live in — one devoid of most, if not all, terrestrial life forms.

FURTHER READING AND REFERENCES:

Wohlleben, Peter. The Hidden Life of Trees: What they Feel, How they Communicate. 2015. Germany: Random House GmbH.

Kimmerer, Robin Wall. Braiding Sweetgrass. 2013. Canada: Milkweed Editions.

https://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/soils/health/biology/?cid=nrcs142p2_053864

https://www.arboristnow.com/news/mycorrhizae-my-favorite-kind-of-fungi

https://preservationtree.com/blog/how-mycorrhizal-fungi-help-trees-communicate

https://sciencing.com/symbiotic-relationship-8794702.html

https://www.nationalforests.org/blog/underground-mycorrhizal-networkhttps://www.scientificamerican.com/article/a-poetic-mind-bending-tour-of-the-fungal-world/