[light piano music]
- [Joe] It's one of the most breathtaking,
natural spectacles on the planet.
Trillions of leaves across New England
transform into the brilliant hues of fall.
It happens every autumn without fail.
But the reasons why, well, that's still a bit of a mystery.
[flame roaring]
Every year, people flock to New England from far and wide
to witness this fall display.
These leaf peepers,
yes, that's an actual term,
bring in billions of dollars annually
to the New England economy.
But do any of them stop to wonder
why the leaves change color?
- [Aisha] Just Mother Nature rebirth.
- [Sue] Yeah. It's something with um...
- [Carl] I don't have a clue. [laughs]
- [Joe] Well, most people don't.
See, technically, the leaves don't change colors.
I know what you're thinking.
"Joe, when's the last time you got your eyes checked?
How could that possibly be?"
I'll let Josh Halman, a forest health specialist
for the state of Vermont, explain.
- [Josh] The thing that allows the trees to make food,
to photosynthesize, is something called chlorophyll.
And that's what makes the leaves green as well.
In the fall, the leaves are getting the cue
from the environment
that it's it's time to start shutting down for winter.
Once this cold signal and this shortening of day signal
is perceived by the tree,
it's time to start degrading the chlorophyll.
And once that starts breaking down,
these other compounds that have been in the leaves
all along reveal themselves.
So you get these yellows, you get these oranges,
and sometimes some browns as well
that have been in the leaf all along,
but the chlorophyll just masks it with the green.
- [Joe] There's also the reds.
Sugar maples and red maples
are abundant here in New England.
They produce some of the most brilliant reds
of any tree in fall.
And these crimson trees have a secret
that scientists are still trying to crack.
- [Josh] Whereas the the browns and the yellows
have been present all year,
the reds are actually synthesized and generated
once the cold temperatures are upon us.
- [Joe] So why would trees
that are trying to conserve energy for winter
go out of their way to make a special red pigment
in leaves that are dying?
- [Alexandra] The production of red
is a little bit of a mystery in science.
It does seem counterintuitive that a tree would put energy
into leaves that it's just going to drop to the ground.
But what's in that leaf is very valuable to the tree.
- [Joe] Leaves are full of precious nutrients
like nitrogen and phosphorus that trees need to survive.
And the longer trees can hold onto their leaves,
more of those tasty nutrients they can consume.
That's where the red pigment comes in.
- [Alexandra] It's thought that it acts like a sunscreen.
There is also some research that suggests
that it is a deterrent for insects.
So it protects those leaves
as the tree is trying to salvage
as much of the material as possible.
- [Joe] In other words, the red may help the leaves
hang on just a little bit longer.
- [Josh] If you think about it from a tree's perspective,
it's kind of a trade-off, right?
You're using some energy to get the other energy
that's present in the leaf back into the tree.
- [Joe] Scientists aren't 100% sure yet
that's the real story behind the red.
But what's not in dispute is that New England
is one of the best places to see fall colors.
Maple, birch and oak trees exist all across North America.
So what makes these trees so unique?
- [Josh] What makes New England and Vermont really special
in terms of fall foliage and fall color
is really the mix of species that we have here.
- [Alexandra] Red maple turns
this brilliant crimson, dark red.
And Aspen will turn beautiful yellow, the lemon yellow.
And so it creates this pattern on our mountains,
in our landscape.
- [Joe] Scientists in Vermont have been surveying
the changing leaves in these forests for more than 30 years.
And they're noticing some curious trends.
- We do the survey by having our eyes on the trees.
We're not using fancy instrumentation or anything like that.
We're actually coming to a consensus
about what percentage of the tree
is something other than green
and what percentage of the tree actually has the leaves
that are on the ground.
Quite a bit of color still up there.
It's lost leaves, but I wouldn't say it's,
you know, it's not about to drop them all.
- Yeah, it looks pretty decent
considering that we've been in a drought.
- Yeah, exactly.
The 30 years that we have been looking at these trees
and analyzing the data,
what we've seen is that fall is actually
getting a little bit later in Vermont
than when we began taking these records.
- [Joe] What could be driving this?
The days get shorter at the same time of year,
but temperatures are becoming much more unpredictable.
And trees use both of these cues together
to start preparing for winter.
- [Josh] We started to see this trend of this later fall.
It's something that we really just use as an indicator
to say there is likely climate change occurring,
and it's visible within our own backyards.
- [Joe] The Northeast is the fastest warming region
in the lower 48.
- [Alexandra] In New England,
the temperatures will get warmer,
and we are projected to have more wet weather.
- [Joe] Researchers say that regional temperatures
could rise 3 1/2 degrees by 2035
and up to 10 degrees by the 2080s.
That's the wrong recipe for fall colors.
- [Alexandra] Conditions that create good foliage
are certainly bright sunny days, dry weather.
You want temperatures that are cool at night,
but not necessarily below freezing,
because that could actually damage the leaves.
- [Joe] And climate change also brings
more extreme weather events.
- [Josh] The extreme events can definitely impact
the quality of the fall color.
If you had extreme drought
or extreme high temperatures in the fall,
then you could have less attractive fall color.
- [Joe] A changing climate could even lead
to a very different mix of tree species in these forests.
- [Alexandra] Some species really like cold temperatures.
And those are species that we project
may not do as well under a warming climate.
And that might mean that trees
that really like cold climates move north
and are not as prevalent in Vermont,
and we get new species that are coming in from the South.
And that might mean that we see differences in fall foliage.
- Before you get too worried, take a deep breath.
Whatever changes may come, they'll take decades to appear.
So there's still time to get climate change under control
so that we, and all the other leaf peepers,
can keep enjoying this legendary color show.
Hey guys, Joe here.
Thank you so much for watching
our latest episode of "Overview."
That was a good one to curl up
with a nice cup of hot chocolate or something, wasn't it?
Well, make sure that you subscribe here on PBS Terra
so you don't miss our next aerial adventure.
And let us know down in the comments
if there's any amazing stories that we should cover
where you live.
All right, see you next time.