How trees talk to each other Suzanne Simard
Imagine you’re walking through a forest.
I’m guessing you’re thinking
of a collection of trees,
what we foresters call a stand,
with their rugged stems
and their beautiful crowns.
Yes, trees are the foundation of forests,
but a forest is much more
than what you see,
and today I want to change
the way you think about forests.
You see, underground
there is this other world,
a world of infinite biological pathways
that connect trees
and allow them to communicate
and allow the forest to behave
as though it’s a single organism.
It might remind you
of a sort of intelligence.
How do I know this?
Here’s my story.
I grew up in the forests
of British Columbia.
I used to lay on the forest floor
and stare up at the tree crowns.
They were giants.
My grandfather was a giant, too.
He was a horse logger,
and he used to selectively cut
cedar poles from the inland rainforest.
Grandpa taught me about the quiet
and cohesive ways of the woods,
and how my family was knit into it.
So I followed in grandpa’s footsteps.
He and I had this curiosity about forests,
and my first big “aha” moment
was at the outhouse by our lake.
Our poor dog Jigs
had slipped and fallen into the pit.
So grandpa ran up with his shovel
to rescue the poor dog.
He was down there, swimming in the muck.
But as grandpa dug
through that forest floor,
I became fascinated with the roots,
and under that, what I learned later
was the white mycelium
and under that the red
and yellow mineral horizons.
Eventually, grandpa and I
rescued the poor dog,
but it was at that moment that I realized
that that palette of roots and soil
was really the foundation of the forest.
And I wanted to know more.
So I studied forestry.
But soon I found myself working
alongside the powerful people
in charge of the commercial harvest.
The extent of the clear-cutting
was alarming,
and I soon found myself
conflicted by my part in it.
Not only that, the spraying
and hacking of the aspens and birches
to make way for the more commercially
valuable planted pines and firs
was astounding.
It seemed that nothing could stop
this relentless industrial machine.
So I went back to school,
and I studied my other world.
You see, scientists had just discovered
in the laboratory in vitro
that one pine seedling root
could transmit carbon
to another pine seedling root.
But this was in the laboratory,
and I wondered,
could this happen in real forests?
I thought yes.
Trees in real forests might also
share information below ground.
But this was really controversial,
and some people thought I was crazy,
and I had a really hard time
getting research funding.
But I persevered,
and I eventually conducted
some experiments deep in the forest,
25 years ago.
I grew 80 replicates of three species:
paper birch, Douglas fir,
and western red cedar.
I figured the birch and the fir
would be connected in a belowground web,
but not the cedar.
It was in its own other world.
And I gathered my apparatus,
and I had no money,
so I had to do it on the cheap.
So I went to Canadian Tire –
(Laughter)
and I bought some plastic bags
and duct tape and shade cloth,
a timer, a paper suit, a respirator.
And then I borrowed some
high-tech stuff from my university:
a Geiger counter, a scintillation counter,
a mass spectrometer, microscopes.
And then I got some
really dangerous stuff:
syringes full of radioactive
carbon-14 carbon dioxide gas
and some high pressure bottles
of the stable isotope
carbon-13 carbon dioxide gas.
But I was legally permitted.
(Laughter)
Oh, and I forgot some stuff,
important stuff: the bug spray,
the bear spray,
the filters for my respirator.
Oh well.
The first day of the experiment,
we got out to our plot
and a grizzly bear and her cub
chased us off.
And I had no bear spray.
But you know, this is how
forest research in Canada goes.
(Laughter)
So I came back the next day,
and mama grizzly and her cub were gone.
So this time, we really got started,
and I pulled on my white paper suit,
I put on my respirator,
and then
I put the plastic bags over my trees.
I got my giant syringes,
and I injected the bags
with my tracer isotope
carbon dioxide gases,
first the birch.
I injected carbon-14, the radioactive gas,
into the bag of birch.
And then for fir,
I injected the stable isotope
carbon-13 carbon dioxide gas.
I used two isotopes,
because I was wondering
whether there was two-way communication
going on between these species.
I got to the final bag,
the 80th replicate,
and all of a sudden
mama grizzly showed up again.
And she started to chase me,
and I had my syringes above my head,
and I was swatting the mosquitos,
and I jumped into the truck,
and I thought,
“This is why people do lab studies.”
(Laughter)
I waited an hour.
I figured it would take this long
for the trees to suck up
the CO2 through photosynthesis,
turn it into sugars,
send it down into their roots,
and maybe, I hypothesized,
shuttle that carbon belowground
to their neighbors.
After the hour was up,
I rolled down my window,
and I checked for mama grizzly.
Oh good, she’s over there
eating her huckleberries.
So I got out of the truck
and I got to work.
I went to my first bag with the birch.
I pulled the bag off.
I ran my Geiger counter over its leaves.
Kkhh!
Perfect.
The birch had taken up
the radioactive gas.
Then the moment of truth.
I went over to the fir tree.
I pulled off its bag.
I ran the Geiger counter up its needles,
and I heard the most beautiful sound.
Kkhh!
It was the sound of birch talking to fir,
and birch was saying,
“Hey, can I help you?”
And fir was saying, “Yeah,
can you send me some of your carbon?
Because somebody
threw a shade cloth over me.”
I went up to cedar, and I ran
the Geiger counter over its leaves,
and as I suspected,
silence.
Cedar was in its own world.
It was not connected into the web
interlinking birch and fir.
I was so excited,
I ran from plot to plot
and I checked all 80 replicates.
The evidence was clear.
The C-13 and C-14 was showing me
that paper birch and Douglas fir
were in a lively two-way conversation.
It turns out at that time of the year,
in the summer,
that birch was sending more carbon to fir
than fir was sending back to birch,
especially when the fir was shaded.
And then in later experiments,
we found the opposite,
that fir was sending more carbon to birch
than birch was sending to fir,
and this was because the fir was still
growing while the birch was leafless.
So it turns out the two species
were interdependent,
like yin and yang.
And at that moment,
everything came into focus for me.
I knew I had found something big,
something that would change the way
we look at how trees interact in forests,
from not just competitors
but to cooperators.
And I had found solid evidence
of this massive belowground
communications network,
the other world.
Now, I truly hoped and believed
that my discovery would change
how we practice forestry,
from clear-cutting and herbiciding
to more holistic and sustainable methods,
methods that were less expensive
and more practical.
What was I thinking?
I’ll come back to that.
So how do we do science
in complex systems like forests?
Well, as forest scientists,
we have to do our research in the forests,
and that’s really tough,
as I’ve shown you.
And we have to be really good
at running from bears.
But mostly, we have to persevere
in spite of all the stuff
stacked against us.
And we have to follow our intuition
and our experiences
and ask really good questions.
And then we’ve got to gather our data
and then go verify.
For me, I’ve conducted and published
hundreds of experiments in the forest.
Some of my oldest experimental plantations
are now over 30 years old.
You can check them out.
That’s how forest science works.
So now I want to talk about the science.
How were paper birch
and Douglas fir communicating?
Well, it turns out they were conversing
not only in the language of carbon
but also nitrogen and phosphorus
and water and defense signals
and allele chemicals and hormones –
information.
And you know, I have to tell you,
before me, scientists had thought
that this belowground
mutualistic symbiosis called a mycorrhiza
was involved.
Mycorrhiza literally means “fungus root.”
You see their reproductive organs
when you walk through the forest.
They’re the mushrooms.
The mushrooms, though,
are just the tip of the iceberg,
because coming out of those stems
are fungal threads that form a mycelium,
and that mycelium
infects and colonizes the roots
of all the trees and plants.
And where the fungal cells
interact with the root cells,
there’s a trade of carbon for nutrients,
and that fungus gets those nutrients
by growing through the soil
and coating every soil particle.
The web is so dense that there can be
hundreds of kilometers of mycelium
under a single footstep.
And not only that, that mycelium connects
different individuals in the forest,
individuals not only of the same species
but between species, like birch and fir,
and it works kind of like the Internet.
You see, like all networks,
mycorrhizal networks have nodes and links.
We made this map by examining
the short sequences of DNA
of every tree and every fungal individual
in a patch of Douglas fir forest.
In this picture, the circles represent
the Douglas fir, or the nodes,
and the lines represent the interlinking
fungal highways, or the links.
The biggest, darkest nodes
are the busiest nodes.
We call those hub trees,
or more fondly, mother trees,
because it turns out
that those hub trees nurture their young,
the ones growing in the understory.
And if you can see those yellow dots,
those are the young seedlings
that have established within the network
of the old mother trees.
In a single forest, a mother tree can be
connected to hundreds of other trees.
And using our isotope tracers,
we have found that mother trees
will send their excess carbon
through the mycorrhizal network
to the understory seedlings,
and we’ve associated this
with increased seedling survival
by four times.
Now, we know we all
favor our own children,
and I wondered, could Douglas fir
recognize its own kin,
like mama grizzly and her cub?
So we set about an experiment,
and we grew mother trees
with kin and stranger’s seedlings.
And it turns out
they do recognize their kin.
Mother trees colonize their kin
with bigger mycorrhizal networks.
They send them more carbon below ground.
They even reduce
their own root competition
to make elbow room for their kids.
When mother trees are injured or dying,
they also send messages of wisdom
on to the next generation of seedlings.
So we’ve used isotope tracing
to trace carbon moving
from an injured mother tree
down her trunk
into the mycorrhizal network
and into her neighboring seedlings,
not only carbon but also defense signals.
And these two compounds
have increased the resistance
of those seedlings to future stresses.
So trees talk.
(Applause)
Thank you.
Through back and forth conversations,
they increase the resilience
of the whole community.
It probably reminds you
of our own social communities,
and our families,
well, at least some families.
(Laughter)
So let’s come back to the initial point.
Forests aren’t simply
collections of trees,
they’re complex systems
with hubs and networks
that overlap and connect trees
and allow them to communicate,
and they provide avenues
for feedbacks and adaptation,
and this makes the forest resilient.
That’s because there are many hub trees
and many overlapping networks.
But they’re also vulnerable,
vulnerable not only
to natural disturbances
like bark beetles that preferentially
attack big old trees
but high-grade logging
and clear-cut logging.
You see, you can take out
one or two hub trees,
but there comes a tipping point,
because hub trees are not
unlike rivets in an airplane.
You can take out one or two
and the plane still flies,
but you take out one too many,
or maybe that one holding on the wings,
and the whole system collapses.
So now how are you thinking
about forests? Differently?
(Audience) Yes.
Cool.
I’m glad.
So, remember I said earlier
that I hoped that my research,
my discoveries would change
the way we practice forestry.
Well, I want to take a check on that
30 years later here in western Canada.
This is about 100 kilometers
to the west of us,
just on the border of Banff National Park.
That’s a lot of clear-cuts.
It’s not so pristine.
In 2014, the World Resources Institute
reported that Canada in the past decade
has had the highest forest disturbance
rate of any country worldwide,
and I bet you thought it was Brazil.
In Canada, it’s 3.6 percent per year.
Now, by my estimation, that’s about
four times the rate that is sustainable.
Now, massive disturbance at this scale
is known to affect hydrological cycles,
degrade wildlife habitat,
and emit greenhouse gases
back into the atmosphere,
which creates more disturbance
and more tree diebacks.
Not only that, we’re continuing
to plant one or two species
and weed out the aspens and birches.
These simplified forests lack complexity,
and they’re really vulnerable
to infections and bugs.
And as climate changes,
this is creating a perfect storm
for extreme events, like the massive
mountain pine beetle outbreak
that just swept across North America,
or that megafire in the last
couple months in Alberta.
So I want to come back
to my final question:
instead of weakening our forests,
how can we reinforce them
and help them deal with climate change?
Well, you know, the great thing
about forests as complex systems
is they have enormous
capacity to self-heal.
In our recent experiments,
we found with patch-cutting
and retention of hub trees
and regeneration to a diversity
of species and genes and genotypes
that these mycorrhizal networks,
they recover really rapidly.
So with this in mind, I want to leave you
with four simple solutions.
And we can’t kid ourselves
that these are too complicated to act on.
First, we all need
to get out in the forest.
We need to reestablish
local involvement in our own forests.
You see, most of our forests now
are managed using
a one-size-fits-all approach,
but good forest stewardship
requires knowledge of local conditions.
Second, we need to save
our old-growth forests.
These are the repositories of genes
and mother trees and mycorrhizal networks.
So this means less cutting.
I don’t mean no cutting, but less cutting.
And third, when we do cut,
we need to save the legacies,
the mother trees and networks,
and the wood, the genes,
so they can pass their wisdom
onto the next generation of trees
so they can withstand
the future stresses coming down the road.
We need to be conservationists.
And finally, fourthly and finally,
we need to regenerate our forests
with a diversity of species
and genotypes and structures
by planting and allowing
natural regeneration.
We have to give Mother Nature
the tools she needs
to use her intelligence to self-heal.
And we need to remember
that forests aren’t just a bunch of trees
competing with each other,
they’re supercooperators.
So back to Jigs.
Jigs’s fall into the outhouse
showed me this other world,
and it changed my view of forests.
I hope today to have changed
how you think about forests.
Thank you.
(Applause)