0:00:10.724,0:00:13.616
Most of the forest lives in the [br]shadow of the giants

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that make up the highest canopy.

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These are the oldest trees,

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with hundreds of children and [br]thousands of grandchildren.

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They check in with their neighbors, [br]sharing food, supplies,

0:00:24.325,0:00:27.468
and wisdom gained over their long lives.

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They do all this rooted in place, unable [br]to speak, reach out, or move around.

0:00:33.564,0:00:37.132
The secret to their success lies [br]under the forest floor,

0:00:37.132,0:00:41.134
where vast root systems support the [br]towering trunks above.

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Partnering with these roots are symbiotic [br]fungi called mycorrhizae.

0:00:45.937,0:00:49.965
These fungi have countless branching, [br]thread-like hyphae

0:00:49.965,0:00:52.965
that together make up the mycelium.

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The mycelium spreads across a much larger [br]area than the tree root system

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and connect the roots of different [br]trees together.

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These connections form [br]mycorrhizal networks.

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Through mycorrhizal networks,

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fungi can pass resources and signaling [br]molecules between trees.

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We know the oldest trees have the [br]largest mycorrhizal networks

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with the most connections to other trees,

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but these connections are incredibly [br]complicated to trace.

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That’s because there are about a hundred [br]species of mycorrhizal fungi–

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and an individual tree might be colonized [br]by dozens of different fungal organisms,

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each of which connects to a [br]unique set of other trees,

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which in turn each have their own unique [br]set of fungal associations.

0:01:36.101,0:01:39.293
To get a sense of how substances flow [br]through this network,

0:01:39.293,0:01:41.142
let’s zoom in on sugars,

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as they travel from a mature tree to a [br]neighboring seedling.

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Sugar’s journey starts high above [br]the ground,

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in the leaves of the tallest trees [br]above the canopy.

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The leaves use the ample sunlight up there[br]to create sugars through photosynthesis.

0:01:55.433,0:01:58.016
This essential fuel then travels [br]through the tree

0:01:58.016,0:02:00.749
to the base of the trunk in the thick sap.

0:02:00.749,0:02:04.069
From there, sugar flows down to the roots.

0:02:04.069,0:02:06.717
Mycorrhizal fungi encounter the [br]tips of the roots

0:02:06.717,0:02:10.096
and either surround or penetrate [br]the outer root cells,

0:02:10.096,0:02:12.628
depending on the type of fungi.

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Fungi cannot produce sugars, though they [br]need them for fuel just like trees do.

0:02:17.468,0:02:18.477
They can, however,

0:02:18.477,0:02:22.226
collect nutrients from the soil much [br]more efficiently than tree roots—

0:02:22.226,0:02:25.765
and pass these nutrients into [br]the tree roots.

0:02:25.765,0:02:26.571
In general,

0:02:26.571,0:02:30.425
substances flow from where they are more [br]abundant to where they are less abundant,

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or from source to sink.

0:02:32.322,0:02:36.234
That means that the sugars flow from the [br]tree roots into the fungal hyphae.

0:02:36.234,0:02:38.040
Once the sugars enter the fungus,

0:02:38.040,0:02:41.329
they travel along the hyphae through [br]pores between cells

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or through special hollow [br]transporter hyphae.

0:02:44.412,0:02:46.592
The fungus absorbs some of the sugars,

0:02:46.592,0:02:50.113
but some travels on and enters [br]the roots of a neighboring tree,

0:02:50.113,0:02:53.403
a seedling that grows in the shade and [br]has less opportunity

0:02:53.403,0:02:56.047
to photosynthesize sugars.

0:02:56.047,0:02:59.536
But why does fungus transport resources [br]from tree to tree?

0:02:59.536,0:03:02.946
This is one of the mysteries of the [br]mycorrhizal networks.

0:03:02.946,0:03:07.255
It makes sense for fungus to exchange soil[br]nutrients and sugar with a tree—

0:03:07.255,0:03:09.346
both parties benefit.

0:03:09.346,0:03:13.348
The fungus likely benefits in less obvious[br]ways from being part of a network

0:03:13.348,0:03:17.758
between trees, but the exact ways [br]aren’t totally clear.

0:03:17.761,0:03:20.048
Maybe the fungus benefits from [br]having connections

0:03:20.048,0:03:22.458
with as many different trees as possible,

0:03:22.458,0:03:26.380
and maximizes its connections by shuttling[br]molecules between trees.

0:03:26.380,0:03:29.212
Or maybe plants reduce their [br]contributions to fungi

0:03:29.212,0:03:32.778
if the fungi don’t facilitate exchanges [br]between trees.

0:03:32.778,0:03:34.126
Whatever the reasons,

0:03:34.126,0:03:38.576
these fungi pass an incredible amount [br]of information between trees.

0:03:38.576,0:03:42.964
Through the mycorrhizae, trees can tell [br]when nutrients or signaling molecules

0:03:42.964,0:03:46.305
are coming from a member of their [br]own species or not.

0:03:46.305,0:03:49.512
They can even tell when information is [br]coming from a close relative

0:03:49.512,0:03:51.722
like a sibling or parent.

0:03:51.722,0:03:54.977
Trees can also share information [br]about events like drought

0:03:54.977,0:03:57.524
or insect attacks through their [br]fungal networks,

0:03:57.524,0:04:00.817
causing their neighbors to increase [br]production of protective enzymes

0:04:00.817,0:04:03.347
in anticipation of threats.

0:04:03.347,0:04:08.037
The forest’s health relies on these [br]intricate communications and exchanges.

0:04:08.040,0:04:10.699
With everything so deeply interconnected,

0:04:10.699,0:04:14.949
what impacts one species is bound [br]to impact others.