- [Narrator] Grandfather Mountain, it's mid-October and a bit cloudy, but it's near the peak of what people call fall color.

Right now, trees like maples, beech and birch are turning their traditional oranges and reds.

But in a warming world, the question is whether climate change will affect when and how the changing leaves will paint the mountains into a rainbow of colors every fall.

One day later, the sun has opened up at the nearby Blue Ridge Parkway and Linn Cove Viaduct.

[Hikers chatter indistinctly] It's here where hikers make their way up the Rough Ridge trail to take in some of these postcard-like scenes of Appalachia.

- You know, it's steep and there's rocks, and they've even cut into the rocks to make sort of little steps 'cause otherwise it'd be too difficult to get up.

- Hiking here is not for the faint and heart.

- Well this trail is, you have to know, you have to be in good shape for this trail.

[Hikers chatter indistinctly] - [Narrator] Every year these hikers and tourists flock to the area, in part because of the words of this man, Howie Neufeld.

Neufeld is known as the fall color guy, and he's a bit of a celebrity among those who love coming up here as much as he does.

- [Howie] Where are y'all from?

- [Hikers] Knoxville.

- [Howie] Knoxville?

- We're followers.

- Oh yeah, we are.

I always look every time, what are the leaves doing?

- Yeah, they're pretty good.

Well, I always get anxiety when people ask me when they should come up here, because I can't predict the weather more than about five or seven days in advance.

And so since the colors are really dependent on the weather, not so much the climate, but the weather, then, you know, if they ask me in May, I really can't get very accurate.

- [Narrator] Throughout the year, daily weather brings changes in sunlight, temperature, and precipitation, all things that affect how leaves live.

But the climate is what's happening in the long term, the big picture.

While Neufeld won't explicitly say whether climate change is affecting the seasonal weather in the mountains, and that includes when peak color arrives, he does say things are getting weird.

- I've been doing this for 17 years now, and so I went back and looked over all my writings and picked out the week that I said was peak color, and then I plotted it.

And from 2008 to 2016, all but one year were within the 10th to the 20th of October, so pretty consistent.

But starting in 2017, it was all over the place, it could be right on time, it could be a week late, it could be up to two weeks late, 2018 was two weeks later than normal.

And the variability in these last eight years is twice as high as it was in those first eight years, so it's getting harder to predict when it will be.

[water runs] - [Narrator] With so many questions about what's going on at this point, we may want to take a moment to look at the typical life of a leaf.

Plants need three things to stay alive, sunlight, water and carbon dioxide.

Water comes in through the roots, carbon dioxide through the leaves and other parts of the plant, and sunlight is absorbed through a chemical in the leaves called chlorophyll.

In the spring and summer, leaves use light to convert water and carbon dioxide into sugars and generate oxygen.

We call that photosynthesis.

The sunlight reacts with the water and carbon dioxide to produce those sugars, which is essentially plant food.

However, chlorophyll breaks down with the exposure to sunlight.

That's no problem during the summer, leaves just make new chlorophyll.

But when the sunlight fades in the fall, chlorophyll production drops.

- So for trees that turn sort of orange like that one right there, those pigments are there all summer, but they're hidden by the green chlorophyll.

When the chlorophyll degrades and goes away, these orange and yellows come out.

So they're pretty much constant from year to year.

But it's the red colors like on that bush there, and the reds you see on the hillside, those are not present in the summer, but they're made in the fall.

- [Narrator] So, we'll take red for example.

- So the red pigment acts as a sunscreen, it allows the leaves to stay uninjured when you get high light and cold, which can be very damaging to a leaf, and then it can withdraw those nutrients back into the twig and use those nutrients next spring to give the leaves a headstart.

- [Narrator] Neufeld says that climate change has been relatively slow to affect peak color of the mountains, but climate change can result in extreme weather events like flooding and heat.

He says they need more data, and says his big question is how hot weather would affect the timing of color in areas down in the central part of the state.

- For the Piedmont, I think you have a bigger chance of getting really extreme temperatures.

You know, an acute heat wave, like a short period where you have, say, several days in a row at 95 or a hundred, what that does to the fall color, 'cause it's unprecedented.

- We're definitely seeing the effects of climate change down here in the Piedmont.

This is a black gum tree, and it... - [Narrator] So three weeks later we went to the Piedmont and met with Nikki Hughes, who's known here as a premier leaf scientist.

Like Neufeld, she stops short of making a direct climate leaf connection, at this point.

- Well, what I would say is, climate change is extending the length of the season where we get to see red coloration, because some species are still changing early like this tupelo, and dogwoods are still changing on time, but others, like oaks, are waiting longer and longer.

- [Narrator] Hughes says to better understand how leaves change in the fall, you need to look at how different species do it.

When chlorophyll leaves a leaf and goes back into the stem, the pigments become the protections.

And there are a lot of pigments, xanthophylls, carotenoids and beta-carotenes give some of the yellows and oranges, anthocyanins give the reds, mixtures in some leaves of several pigments give a purple.

- Some species synthesize anthocyanins on top of that yellow.

When it's a small amount of anthocyanins, you get an orange leaf.

And when it's larger amounts, you get deeper shades of red, like we see in this Tupelo.

And so, some spots you can see the yellow xanthophylls, and carotenoids, but any oranges or reds you see are coming from anthocyanins.

We don't know as much as you think we know, scientists who study red leaves, because nobody's really researched it.

- [Narrator] Hughes says, even with all this science, they simply need more data to make that connection.

- We're starting to figure out trends with how soon it will be, like colder years, color change happens sooner, drier years, it happens sooner, but then you have warmer temperatures, making it happen later, so we have a lot of opposing forces at play.

- [Narrator] So as mountain leaves might be reacting to a warming climate, one thing is for sure, the leaves will change in the fall.

But we need more data if we're gonna pick the perfect time to schedule our mountain vacation.

- [Howie] I think the trend over the years will be for the peak fall color to come later, inevitably we'll get some warming up here.