WEBVTT

00:00:10.724 --> 00:00:13.616
Most of the forest lives in the 
shadow of the giants

00:00:13.616 --> 00:00:15.757
that make up the highest canopy.

00:00:15.757 --> 00:00:17.376
These are the oldest trees,

00:00:17.376 --> 00:00:21.003
with hundreds of children and 
thousands of grandchildren.

00:00:21.003 --> 00:00:24.325
They check in with their neighbors, 
sharing food, supplies,

00:00:24.325 --> 00:00:27.468
and wisdom gained over their long lives.

NOTE Paragraph

00:00:27.468 --> 00:00:33.564
They do all this rooted in place, unable 
to speak, reach out, or move around.

00:00:33.564 --> 00:00:37.132
The secret to their success lies 
under the forest floor,

00:00:37.132 --> 00:00:41.134
where vast root systems support the 
towering trunks above.

00:00:41.134 --> 00:00:45.937
Partnering with these roots are symbiotic 
fungi called mycorrhizae.

NOTE Paragraph

00:00:45.937 --> 00:00:49.965
These fungi have countless branching, 
thread-like hyphae

00:00:49.965 --> 00:00:52.965
that together make up the mycelium.

00:00:52.965 --> 00:00:57.329
The mycelium spreads across a much larger 
area than the tree root system

00:00:57.329 --> 00:01:00.425
and connect the roots of different 
trees together.

00:01:00.425 --> 00:01:03.517
These connections form 
mycorrhizal networks.

00:01:03.517 --> 00:01:05.158
Through mycorrhizal networks,

00:01:05.158 --> 00:01:10.648
fungi can pass resources and signaling 
molecules between trees.

NOTE Paragraph

00:01:10.648 --> 00:01:13.834
We know the oldest trees have the 
largest mycorrhizal networks

00:01:13.834 --> 00:01:16.056
with the most connections to other trees,

00:01:16.056 --> 00:01:19.438
but these connections are incredibly 
complicated to trace.

00:01:19.438 --> 00:01:23.782
That’s because there are about a hundred 
species of mycorrhizal fungi–

00:01:23.782 --> 00:01:28.482
and an individual tree might be colonized 
by dozens of different fungal organisms,

00:01:28.482 --> 00:01:31.675
each of which connects to a 
unique set of other trees,

00:01:31.675 --> 00:01:36.101
which in turn each have their own unique 
set of fungal associations.

00:01:36.101 --> 00:01:39.293
To get a sense of how substances flow 
through this network,

00:01:39.293 --> 00:01:41.142
let’s zoom in on sugars,

00:01:41.142 --> 00:01:45.007
as they travel from a mature tree to a 
neighboring seedling.

NOTE Paragraph

00:01:45.007 --> 00:01:47.518
Sugar’s journey starts high above 
the ground,

00:01:47.518 --> 00:01:50.553
in the leaves of the tallest trees 
above the canopy.

00:01:50.553 --> 00:01:55.433
The leaves use the ample sunlight up there
to create sugars through photosynthesis.

00:01:55.433 --> 00:01:58.016
This essential fuel then travels 
through the tree

00:01:58.016 --> 00:02:00.749
to the base of the trunk in the thick sap.

NOTE Paragraph

00:02:00.749 --> 00:02:04.069
From there, sugar flows down to the roots.

00:02:04.069 --> 00:02:06.717
Mycorrhizal fungi encounter the 
tips of the roots

00:02:06.717 --> 00:02:10.096
and either surround or penetrate 
the outer root cells,

00:02:10.096 --> 00:02:12.628
depending on the type of fungi.

00:02:12.628 --> 00:02:17.468
Fungi cannot produce sugars, though they 
need them for fuel just like trees do.

00:02:17.468 --> 00:02:18.477
They can, however,

00:02:18.477 --> 00:02:22.226
collect nutrients from the soil much 
more efficiently than tree roots—

00:02:22.226 --> 00:02:25.765
and pass these nutrients into 
the tree roots.

00:02:25.765 --> 00:02:26.571
In general,

00:02:26.571 --> 00:02:30.425
substances flow from where they are more 
abundant to where they are less abundant,

00:02:30.425 --> 00:02:32.322
or from source to sink.

00:02:32.322 --> 00:02:36.234
That means that the sugars flow from the 
tree roots into the fungal hyphae.

00:02:36.234 --> 00:02:38.040
Once the sugars enter the fungus,

00:02:38.040 --> 00:02:41.329
they travel along the hyphae through 
pores between cells

00:02:41.329 --> 00:02:44.412
or through special hollow 
transporter hyphae.

00:02:44.412 --> 00:02:46.592
The fungus absorbs some of the sugars,

00:02:46.592 --> 00:02:50.113
but some travels on and enters 
the roots of a neighboring tree,

00:02:50.113 --> 00:02:53.403
a seedling that grows in the shade and 
has less opportunity

00:02:53.403 --> 00:02:56.047
to photosynthesize sugars.

NOTE Paragraph

00:02:56.047 --> 00:02:59.536
But why does fungus transport resources 
from tree to tree?

00:02:59.536 --> 00:03:02.946
This is one of the mysteries of the 
mycorrhizal networks.

00:03:02.946 --> 00:03:07.255
It makes sense for fungus to exchange soil
nutrients and sugar with a tree—

00:03:07.255 --> 00:03:09.346
both parties benefit.

00:03:09.346 --> 00:03:13.348
The fungus likely benefits in less obvious
ways from being part of a network

00:03:13.348 --> 00:03:17.758
between trees, but the exact ways 
aren’t totally clear.

00:03:17.761 --> 00:03:20.048
Maybe the fungus benefits from 
having connections

00:03:20.048 --> 00:03:22.458
with as many different trees as possible,

00:03:22.458 --> 00:03:26.380
and maximizes its connections by shuttling
molecules between trees.

00:03:26.380 --> 00:03:29.212
Or maybe plants reduce their 
contributions to fungi

00:03:29.212 --> 00:03:32.778
if the fungi don’t facilitate exchanges 
between trees.

NOTE Paragraph

00:03:32.778 --> 00:03:34.126
Whatever the reasons,

00:03:34.126 --> 00:03:38.576
these fungi pass an incredible amount 
of information between trees.

00:03:38.576 --> 00:03:42.964
Through the mycorrhizae, trees can tell 
when nutrients or signaling molecules

00:03:42.964 --> 00:03:46.305
are coming from a member of their 
own species or not.

00:03:46.305 --> 00:03:49.512
They can even tell when information is 
coming from a close relative

00:03:49.512 --> 00:03:51.722
like a sibling or parent.

00:03:51.722 --> 00:03:54.977
Trees can also share information 
about events like drought

00:03:54.977 --> 00:03:57.524
or insect attacks through their 
fungal networks,

00:03:57.524 --> 00:04:00.817
causing their neighbors to increase 
production of protective enzymes

00:04:00.817 --> 00:04:03.347
in anticipation of threats.

NOTE Paragraph

00:04:03.347 --> 00:04:08.037
The forest’s health relies on these 
intricate communications and exchanges.

00:04:08.040 --> 00:04:10.699
With everything so deeply interconnected,

00:04:10.699 --> 00:04:14.949
what impacts one species is bound 
to impact others.