- [Mark] As you walk through a forest, trees towering over you, plants winding across the earth, you might be missing something just below your feet.

It's a hidden network of fungi.

These unassuming organisms are a sophisticated communication network that keeps information and resources flowing, and is a key factor in the survival of old-growth forests.

This is "By Nature's Design: Inside the Forest, Fundamental Fungi."

- An old-growth forest is a forest that in many instances is hundreds of years old.

So sometimes we think of old as being a hundred years, but when we're thinking about a hundred-year-old forest, that's really not all that old.

Very often a hundred-year-old forest is developed on former agricultural land.

But an old-growth forest is something that goes back, and probably has never been in a situation where it was cut or farmed.

So the forest is usually several hundred years old, and has a lot of structural characteristics that you wouldn't see in younger forests in terms of the species of trees that are present, the diversity of trees, and some of the structural characteristics as well.

(gentle music) - Old trees that can be characterized by, you know, deeply furrowed older bark with mostly intact canopies, but then these larger openings in the canopies where larger trees have fallen down.

When you look upon the forest floor, you'll see evidence of trees that have fallen.

They will leave behind large holes, or there will actually be rotting logs on the forest floor.

You'll see various stages.

In Ohio, we have, you know, the canopy, the understory, the shrub layer, the herbaceous layer, and the ground cover.

And you'll see that in varying stages of maturity.

And then if you look at the soil, you'll see hopefully, undisturbed soil that's quite thick in its humus layer with very rich, alive soil ecosystems.

And something that I might add to that description is the prevalence of an abundant fauna.

(gentle music) - So all the forests are great, but the old-growth forests are more resilient.

They've lived a really long time, and they've experienced more things, and they know how to adapt to different circumstances, you know, droughts and fires and floods.

There's more diversity in an old-growth forest than a new-growth forest.

And of course the old-growth forest can actually take and store more carbon than a new-growth forest.

(gentle music) So we definitely need our trees to help fix the air and clean the air.

And they're a natural air cleaner, air purifier.

So we need our forests around to be able to do that for us.

(gentle music) The trees in the old-growth forest are very important, even if they're dead.

You may not know this, but walking around an old-growth forest, there could be dead trees standing called a snag.

There could be trees falling over.

And that's actually a natural part of the old-growth forest, is there's lots of different ages and stages.

So if a big tree falls over in an old-growth forest, the root ball is gonna come up too and leave an indentation in the ground, which will fill up with water.

And that will be kind of like a vernal pool for amphibians and salamanders to lay their eggs in.

A vernal pool is like a little isolated wetland in a forest.

So it can actually create more habitat even when something dies, which is pretty cool.

(gentle music) - Dead trees are important.

They're important because they're habitat.

So as that tree dies, it will become a food, really, for fungi and bacteria and insects that will begin to break that wood down.

So they begin to decompose that material and then that becomes food resources for things like woodpeckers, for example.

It also becomes habitat because cavities can be excavated in that wood, that either a woodpecker or a bird can use for a nest, or a squirrel or small mammal can do the same.

So dead trees are really important in forest because they provide habitat.

Sometimes trees when they die also fall down and so they will just exist as a dead tree on the forest floor.

We sometimes refer to that as coarse woody debris.

And coarse woody debris is important as well for the same reasons.

It's habitat for fungi and insects that break that down, for birds that feed on it.

And it becomes, you know, it can become denning sites for mammals and small animals.

So they may be able to tunnel underneath that dead tree, for example, that's laying on the ground and that dead tree becomes sort of the roof, if you will, for their burrow or their habitat.

(gentle music) - [Mark] To the human mind, a dead tree is a nuisance or an obstacle to be removed, but to the forest, to the tree is a nutritious meal.

As dead trees and plants decompose, their nutrients are absorbed into the soil and used to support healthy forest growth.

Fungi are fundamental to this process, serving as an underground engine, passing nutrients from one source to another.

(gentle music) - Trees communicate with each other.

Whether or not we as humans fully comprehend, understand, or acknowledge that communication, trees communicate probably in more ways than I or we fully understand.

But one of the main ways that we know they communicate with one another is through mycelial networks, this underground network of mushroom root systems that support the communication and stabilization of the entire forest ecosystem.

(gentle music) What we really understand is that these mycorrhizal systems, they attach themself onto the roots of trees and other plants, and they spread themselves out, and they sort of extend the life or the reach of those plant roots.

And then they are really good at mining from the soil nutrients and water.

And in exchange, they send those up the tree.

And as the tree photosynthesizes, the tree returns by way of carbohydrates, carbon to the fungal networks.

So it's this really powerful example of mutualism, of symbiosis that creates this reciprocity whereby the mycelial networks are getting what they need by in turn nurturing the trees and the other life forms that they are benefiting.

(gentle music) - So when you think of fungi in a forest, you may think of a mushroom underneath of a tree, growing out of the soil, you may think of a toadstool growing out of the ground, but really, most of the fungi is underground.

So the mushroom that we can find is the fruiting body of the fungi underground.

So think of it like an apple on an apple tree.

So the fungi would just be the apple on the apple tree.

So this is the fungi, this is the mycelium or the rest of the fungi underground.

So the mycelium is made up of lots and lots of thread-like hairs called hyphae.

And these can stretch for miles and miles and miles underground, kind of creating like a super highway of mycelium.

So what happens is the mycelium of the fungi will kind of grow underground, sending out chemical signals and the plant and tree roots will do that as well.

And when they sync up, that will be the mycorrhizal fungi, the mycorrhizal relationship.

So myco means mushroom, rrhizal means roots.

So this symbiotic relationship kind of means mushroom roots, the plants and tree roots connected with the mycelium so that the trees around it can share nutrients and can share carbon and can share water.

So Peter Wohlleben, a German forester, calls this the Wood Wide Web instead of the World Wide Web, giving us an easy to understand example of how the forest is connected.

(gentle music) - So trees can communicate with one another through the production of chemical compounds.

So if a tree's under stress, it could be drought stress, it could be insects or disease, they will change the chemicals that they produce, really, to sort of overcome the stress or resist the insects or the diseases, right?

So those chemical compounds are produced by the plant and they're in the plant body, but they can also be released.

They can be released to the air, or they can be released to the soil.

So as a release to the air, trees nearby can also detect those compounds, and then they will then begin to produce those compounds themselves.

So they may not be under attack by the insect or the disease, but because they detect these compounds, that's their cue to begin producing those substances themselves.

So that may actually help them because it means, well, you know, we're not under attack from this insect, but we're gonna produce these chemicals that deter the insect.

And we're gonna do it before the insect attacks us 'cause it's attacking our neighbor.

So that communication, if you will, that signaling that takes place does take place between plants at least within close proximity to one another.

And it can take place between plants above ground if the compounds, the chemicals are being released by the leaves, or below ground if it's being released by the roots, for example.

And so that's a way that plants can, if you will, communicate with one another or signal to one another that something's up.

(gentle music) So if you're talking about a hemlock tree, for example, then a hemlock tree nearby would be able to pick up those chemical compounds.

Different species produce different types of chemical compounds potentially, and so it really is about communication or signaling between closely related individuals or the same species.

Within soil, it could be excreted by roots into surrounding soil, and then that could be detected by other roots or fungi, for example, in soil.

- If there is an at-risk or a young seedling that is struggling to grow and thrive, this mycelial network sort of taps into that existence and starts to pull nutrients like, you know, they say that the mother tree, the oldest tree will actually forego some of their nutrients in order to sustain the life of this incoming seedling because they know that they won't live forever and if they can support the next generation then the forest will go on.

So they communicate about all sorts of things, whatever a human community would communicate about.

I don't know that they get into gossip, but they seem to talk about a lot of things.

(chuckles) (gentle music) - [Mark] Although we don't know exactly what the trees are saying, scientists have made an exciting discovery of an intricate Wood Wide Web.

Driven by mycelium's ability to exchange nutrients, the benefits of this underground social network are most visible in the resiliency of old-growth forests, where a stronger fungal network means a stronger forest above ground.

(gentle music) - It's very important that they have this mycorrhizal connection between trees and plants in the forest so that trees can actually help each other out.

So the really, really big trees in an old-growth forest, you could call them the hub or the mother trees.

So this is something that if the mother tree detects one of her saplings, or they don't even have to be the same species, they can be connected through the microrhizomes, to hundreds of other trees.

If they detect a disease or a drought event or if a sapling is struggling, they can actually send nutrients and water from the mother tree or the hub tree to the other trees in the forest, even if they're different species.

So it's a very important connection that the forest has.

And about 80 to 90% of plants and trees use the mycorrhizal fungi to survive.

(gentle music) - So these fungi evolved about 450 million years ago.

So right around the time plants were leaving the water and moving onto land.

And some people have suggested that these fungi might have been necessary for plants to actually colonize land 450 million years ago.

So they've been around a really long time.

So they live in partnership with plants.

So they form a mutually beneficial relationship, and they actually live on the root itself.

And so what they can do is they can acquire nutrients from soil and they exchange those with the plant for sugar.

So it's an ancient form of barter.

You give me some sugar and I'm gonna give you some nutrients, right?

And so they both benefit from this relationship.

And we see probably 85 to 90% of all plants form these relationships below ground, so they're extensive.

So we see them in forests.

We see them in agriculture.

If you're planting a garden at home, your tomatoes have them in the roots, your corn have them in the roots.

And of course we see them in forest as well.

So these fungi exist in forests.

They colonize the roots, and their fungal threads go often to explore soil.

And as they explore that soil, they can also contact another root, possibly the same tree or a different tree, and they colonize it as well.

So they can be found on the roots of many different plants and many different trees.

And that, really, web of fungal threads that run through soil is what we refer to as the common mycorrhizal network.

- But this mycelial network is what allows an entire ecosystem to withstand and adapt to change over time in a more cohesive, resilient way.

New-growth forests sort of lack this community structure, this symbiosis that is optimal for adaptation and survival.

(gentle music) - [Mark] The health of old-growth forests relies heavily on the mycorrhizal network passing nutrients from one tree to another.

But when old-growth forests are overtaken by invasive species or cut down for development, the fungal connections keeping a forest strong are lost.

(gentle music) The formation of reliable mycorrhizal networks takes time, and new forests don't offer the same stability as old-growth forests.

- So as always, human development is a threat to old-growth forests, climate stress, fossil fuel extraction, and just not being protected.

Some people don't know that they have an old-growth forest maybe in their woodlot, and may just see their woodlot for the economical value instead of the ecological value.

So if you have a woodlot, knowing what you have, is you have some big trees, measuring them and maybe seeing if you do have an old-growth forest.

So old-growth forests are typically isolated, kind of like an island.

And they really do better when they're connected to other forests to share that information via the mycorrhizal fungi, and even to provide as a wildlife corridor.

It's good to have forests, but it's better if it's connected to other forests.

(gentle music) We definitely need to maintain old-growth forests because there's not a lot of them left, and they are living history museums that we can walk into and see what life in the forest would have been like a long time ago, which is pretty interesting.

They also, of course, sequester a lot of carbon and hold a lot of carbon, and support just so much wildlife.

So it's very important that we keep them on our landscape.

They need a lot of time to grow.

So if we can manage for some forests now to become old-growth forests, I think we should definitely try.

(gentle music) - Increasingly, it seems that we always have a new pest or a new disease coming on the horizon.

So most recently we're dealing with things like emerald ash borer, which affects ash trees; or beech leaf disease in the nematode that probably came from Japan, that is affecting beech trees and really having a devastating effect on beech forests.

So some of our old forests here are affected by beech leaf disease.

So we're seeing beech trees that are hundreds of years old begin to die because of this new pest.

So we have trees that have low symptoms, so not very affected by beech leaf disease, and trees that are very heavily impacted by beech leaf disease.

So we don't really see differences in the community, but we see differences in the mycorrhizal colonization of the roots.

So if the tree is in very poor shape, we don't see very many colonized root tips.

But if the tree's in healthy condition or a healthy condition, we see a lot more fungi on the roots.

And so not having those fungi on your roots makes sense if you're a tree that's under attack because you don't have a lot of leaves, the canopy's thinning out.

You don't have the sugar to give to your partner anymore.

That means your partner's not providing you with the nutrients you need.

So it becomes a little bit of a one-two punch where you've got this pest attacking you and now you can't support your partner, who you need to get you nutrients.

And so those are some of the things we're seeing with fungi on things like beech leaf disease.

(gentle music) - A forest that has this mycorrhizal network that the trees and the other plant species are tapped into this mycorrhizal network are healthier on a larger scale.

So it's not that a forest without these mycorrhizal networks can't be healthy, but the mycorrhizal network acts like this neuronal network.

I mean, imagine a human body without a brain.

We could probably function, you know, sort of like a brainless, what, amoeba or some other organism functions, but a body that has a brain is far more capable of adapting to change, adapting to threats is healthier on the long run.

(gentle music) - Organisms in nature cooperate, right?

Very often when we're being taught biology, especially in high school or college, the focus is on competition.

It's on competition, it's on predation, right?

It's on these interactions where somebody benefits and somebody loses, right?

But mycorrhizal fungi are an example of where both partners can benefit from the relationship.

And I think those sorts of positive relationships where both partners benefit are maybe more common in biology and ecology than we've given it credit in the past.

And I think that's, you know, sort of a positive message that cooperation is just as important to the success of life as competition.

(gentle music) - [Mark] Forests are resilient ecosystems that provide food and habitat to numerous species, and their presence on Earth is critical.

Underground fungal networks serve a foundational role in ensuring our forests' longevity, and the more they can evolve into old-growth forests, the more stable our environment will be for all of its inhabitants.

(gentle music) - When you're in the forest, there's a whole world happening underneath your feet, and we're only beginning to scratch the surface of what's going on, which is really exciting.

I think just knowing, for people, that there is a super highway underneath your feet when you're hiking in the forest is really exciting that something that we've heard in the movies can actually be real, trees talking to each other and sharing.

And we can kind of take that back into our own human lives and remember that humans are kind of like a forest and how we're all connected, and we should all share resources and help each other out, just like the forest community.

(gentle music) I would love for old-growth forests to be more talked about, and for more people to know about them and to experience them, because they're such a treasure and they're so important.

They protect us, and they take out carbon from the atmosphere.

I hope that maybe forestry practices can get a little bit better as they learn a little bit more.

So it's not sustainable to say that we should never cut down trees, because we need wood, but maybe considering things like the hub trees and avoiding them, or at least a couple, so that they can help the forest bounce back after removing trees.

My hope is that people know about old-growth forests and can discover one close to their house, 'cause there's probably one pretty close to your house, and just going out and enjoying them and bringing your kids and your grandkids, because we will protect and love what we have experienced and what we know.

(gentle music) - So one of our goals is to try to keep forests on the landscape and to try to restore forests wherever possible, right?

So I think right now we're at a point where forests have really recovered a great deal, but we see loss of forested lands in some regions.

So we wanna kind of maintain that balance where if we're losing forests, we're actually restoring forests in other locations to try to keep the amount of forest acreage the same.

Forests are important 'cause they provide us with all kinds of resources.

They provide us with clean air and clean water, they're habitat for wildlife.

They also provide us with resources we need like wood, right, and pulp.

So we really have an interest in keeping forests on the landscape so they can provide us with all these benefits that we need as a society.

So trying to find that balance between if we're losing some forests, how do we restore and create forests elsewhere?

So, but I think, you know, the positive take is that forests have recovered dramatically, and as individuals and society, we can have an impact and we can have a very positive impact on our environment just by changing what we do and by changing how we view things and our behavior, so.

(gentle music) - We have this opportunity.

We're sort of at a pivotal moment in Ohio where, again, we have witnessed the recovery of our forests.

And that's extraordinary and something worth celebrating.

And we see that there are still threats, threats of fragmentation.

Many of Ohio's old-growth forests and forests in general are fragmented by private lands, by roads, large highways, industrial development, urban sprawl.

I see the future of Ohio's forests as being more integrated with our human development.

I think that there's so much opportunity for humans to reclaim, here in Ohio especially, this opportunity to reclaim our role as keystone species, as participants within our forest ecosystems.

Rather than seeing our forests as these separate places, integrating them into our lives and our built systems so that they can thrive and we can thrive as a result.

(gentle music) - [Mark] Old-growth forests grow stronger every day, thanks to the underground network of fungi.

Although hidden, these networks enable forest ecosystems to withstand changes and threats by communicating stress signals and exchanging nutrients.

So the next time you walk into a forest, take a glance down at the ground and appreciate all that is happening beneath the forest floor, supporting the trees above by nature's design.

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