PBS News Hour

JOHN YANG: Next Saturday is the eternal equinox the beginning of astronomical fall in the northern hemisphere.

For some though, fall doesn't really begin until the leaves change color.

But scientists say climate change is affecting both the timing and intensity of fall foliage.

Earlier I spoke with Bill Keeton, a professor of Forest Ecology and Forestry at the University of Vermont.

I asked him to explain why leaves change color in the fall.

BILL KEETON, University of Vermont: It's really a delicate dance between many factors that are driving both the onset of the fall foliage, as well as the intensity of the color that we see.

The basically what happens is that the trees sense the onset of winter as the contrast between daytime temperatures and nighttime temperatures increases, and as the length of the day shortens.

And as they sense that onset they begin shutting down, and they try to reabsorb as much of the energy that's stored in leaves as possible, these sugars and carbohydrates that the trees have been producing all summer long.

And as this is happening, the chlorophyll in those leaves, chlorophylls catalysts for photosynthesis and important for all of life on Earth, it makes vegetation appear green to our eyes.

The chlorophyll is beginning to degrade.

And as this happens, it reveals other chemicals that have been in the leaves all summer.

So these are the same chemicals that you find in bananas that made them appear yellow and in oranges that make them appear orange.

At the same time this is happening, the trees are trying to buffer those leaves and hang on to them as long as possible, so that they can absorb as much of the energy stored in those leaves as possible.

And to do that they produce another group of chemicals called anthocyanins.

These anthocyanins give us the reds and the purples that we find in some species like maples and oaks and add a lot of diversity to the color that we see on the landscape.

JOHN YANG: And that process you just described, how is that affected by climate change?

BILL KEETON: First of all, there's pretty good evidence that climate change, especially are warming summers, is delaying the onset of fall foliage.

There's even some research that's suggested that, for the onset of fall foliage may have been delayed by as much as a month over the last century.

The other thing is the climate change can dampen the intensity of the fall foliage.

And this happens, particularly after extreme droughts, or after summers, like we've just had with really extreme rainfall and wet, saturated soils.

So these types of climate extremes create stresses in trees, which can interfere with the production of some of these chemicals that give us color, and can interfere with the overall productivity and health of trees.

JOHN YANG: One of the things we've seen this summer has been record wildfires, putting a lot of particulate matter into the air into the atmosphere, does that affect the trees and the foliage in any way?

BILL KEETON: We don't have conclusive evidence on this yet.

But there's some evidence that yes, in some parts of the Eastern U.S., ash and particulate matter on leaves, can interfere a little bit with photosynthesis and reduce the overall productivity of trees, which could have a ripple effect, down the road on foliage on color.

But with the magnifier situation, increasing year by year in the western U.S. and now across boreal northern Canada, this may change into the future.

There are major differences around the country in terms of how climate change seems to be affecting for us.

So for example, in the southwest and part of the intermountain west, we've just come out of one of the worst droughts in something like 1,200 years, a 10, year plus drought.

And that certainly impacts tree health tree growth.

Whereas in the Northeast where I live, our climate seems to be getting rainier and wetter, we think that our rainfall has increased by something like 21 percent in the last couple of decades, and our winters especially are getting a lot warmer and rainier.

So these changes affect tree hill (ph), tree productivity, and their ability to produce some of these compounds that give us fall foliage, JOHN YANG: We know that forest and trees actually take greenhouse gases out of the atmosphere.

So does this become in any way a vicious cycle that as climate change, hurts the trees and forests, that they take fewer greenhouse gases out of the atmosphere making climate change worse.

BILL KEETON: We think that over the near term of maybe the next 50 years or so, the combination of longer growing seasons, because we have warmer summers, and higher concentrations of carbon dioxide in the atmosphere might actually increase forest productivity or again over the next several decades.

But over the very long term of maybe the next century, we might see those positive effects of climate changes overwhelmed by extreme climate conditions like drought and excessive rainfall.

JOHN YANG: Is there anything can be done now to mitigate the long term loss?

BILL KEETON: So for example, we can conserve forests by conserving healthy, complex forests, particularly older forests I might add, that are able to buffer their own climate beneath their canopies.

This might help for us resist some of these climatic changes.

There are also a lot of things that foresters can do to make those forests more resilient to climate change.

For example, reducing the stocking or the density of trees to make them less prone to drought of course in the West, all of the various techniques that we use to support and restore natural fire regimes.

So things like reducing fuel loading and the use of prescribed burning.

All of these things can make our forests more resilient to climate change.

JOHN YANG: Bill Keeton from the University of Vermont, thank you very much.

BILL KEETON: My pleasure.

Thank you for having me.