The wood wide web – an ancient relationship

The wood wide web – an ancient relationship

What is the wood wide web?

Fungi form a large underground network in our forest soils that has important ecological functions for the growth and health of the forest ecosystem. This network has been given the name “the wood wide web”, in analogy to the world-wide-web that connects us humans.

In this solo-episode with Judith Lundberg-Felten, we take a deep dive underground and explore
– How old beneficial fungus-plant interactions, also called mycorrhiza, are.
– How trees and fungi can benefit from entering into a symbiotic (beneficial) relationship.
– Why they matter for the forest even today.
– What the risk of deforestation is for the hidden underground life.

A myco….what please?

Judith is not only one of the founders of Flora-L Design, she also conducts research on the interaction of trees with soil fungi at Umeå Plant Science Centre, Swedish University of Agricultural Sciences. Judith’s research group is studying the molecular dialogue between tree roots and their beneficial fungal partners in the wood wide web. Roots and fungi form what is called a mycorrhiza. This is Greek and stands for myco=fungus and rhiza=root, so a “fungus-root”.  Below you find a microscopy image of a cross-cut through a root colonized by a mycorrhizal fungus. In green you see the fungus, and in red you see the plant cells. Can you distinguish how the fungus forms an entire ring (the mantle) around the root, and how it has pushed the outermost root cells apart to grow in between them? It’s in the area, where the fungus grows in between the root cells, that the magic of this partnership happens: root and fungus start to exchange nutrients with each other and foster their respective growth.

Older than Metusalem

The symbiosis with plants and fungi is very very veeeeery old. As we know today, fungi have helped plants colonize the land. Already 450 million years ago, fungi interacted with premitive plants such as liverwort, as researchers from CNRS recently found out. This suggests that the fungi helped their plant friends to get a foothold on the rather rocky land and to start colonizing Earthes land masses.  Back in the days where there was hardly any soil, the fungi were however more aboveground and formed a green crust on the rather rocky surface of our continents together with the first plants, algae and bacteria. Today, where different layers of soil exist, the wood wide web has moved below-ground together with the roots. We usually see mostly their reproductive structures, the mushrooms or fruiting-bodies, popping up aboveground.

The oldest tree on Earth

Trees are suggested to have evolved stepwise from these basic, primitive plants. It is amazing that there are still traces on earth from the times of the first trees. The oldest trace of a tree is a huge tree root fossil that was found in New York state and is dated back to 385 million years ago. Isn’t this just amazing? The evolution of trees created the green lung of Earth. That again made carbon dioxide levels in the air, which were high at that time, drop and oxygen levels raise. Imagine all the life that forest made possible on our wonderful planet!

The wood wide web below the trees

Whether the wood wide web existed already then below the tree hasn’t for sure been answered yet. The evolution of the Pinacea family, marks the beginning of trees as we know them today and this happened “only” about 200 million years ago. Research suggests that these trees were capable of establishing mycorrhizal symbiotic relationships, as our trees today. This fungal relationship with trees is specifically called the ecto-mycorrhiza, to describe that the fungus never grows into, just around and in between the root cells. Ectomycorrhizal symbiosis developed many times independently. This relationship originates from fungi that otherwise degraded dead biomass. Over time they lost some of the machinery required for dead biomass decay and they acquired new mechanisms in order to live a shared life with tree roots. As compared to the very first fungus-plant relationship that helped plants colonize land, ectomycorrhizal symbiosis are much much younger. However, the mycorrhizal mechanisms that were at the origin of land-colonization are conserved in many other plants (non-trees/shrubs) still today.

Why fungi matter

The wood wide web has many functions. It can help trees gain access to nutrients such as nitrogen and phosphorous. By surrounding the roots, the fungi can also protect roots from drought and soil-living herbivores. The fungi have also often a great capacity to absorb and detoxify contaminants, which can be chemicals or heavy metals. Therefore on old industrial sites, phytoremediation, which is cleaning the soil using plants, can benefit from fungal contribution. However, whether the wood wide web really shares its nutrients with the tree partners can depend on the soil quality. As our collaborators recently discovered, the nutrient exchange between trees and fungi in the forest soil is under control under a complex market strategy, that still haven’t understood in depth.

A clearcut in the forest with 3 retention trees
A small clearcut in Norther Sweden with three retention trees.

The impact of forestry

An aspect to consider is, that the wood wide web is a network to which both plant and fungal partners contribute. Which also means disturbing one partner will have consequences for the other. Clear cutting is a common forestry practice in Sweden, which results in the removal of trees on large surface areas. These are usually replanted with nursery seedlings two years after the harvest. The problematic part of this is, that during this phase the soil environment changes. Temperature, moisture, ground vegetation, erosion can also contribute to this. Furthermore, by removing the trees, the fungal source to sugars are removed as they largely benefit from the photosynthesis happening in the trees’ leaves or needles. This is why alternative harvesting strategies are today considered, such as keeping (retention) trees once in a while on clear-cuts. This can help conserve fungi in the soil in proximity to the retained trees, yet it drops further beyond the radius of the roots. In her own research Judith is investigating whether gap-cutting instead of clear-cutting could be a solution to do less harm to the fungal communities in the soil. While we already know much about the wood wide web, there is definitely more to be explored!

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