The Science of Fall Foliage and Climate Change: UVM Expert

Across North America, climate change and drought are impacting leaf colors

With leaf peeping season here, University of Vermont forest scientist William Keeton explains the science behind why trees explode into colors each fall, and where in North America to find the most vibrant colors.

Besides dazzling the eyes, Keeton says fall foliage generates over a billion dollars for the U.S. economy—$800 million in Vermont alone. However, without action, climate change, drought and invasive species put this seasonal phenomenon at risk, he says.

Virtual leaf peepers: For drone photos and videos of fall foliage, follow UVM Spatial Analysis Lab director Jarlath O’Neil-Dunne on Twitter. Multimedia is available to news media for download upon request to: [email protected]. 

What U.S. climate trends will impact colors this year?

The Western U.S. is facing one of the most severe droughts in history, which impacts foliage. Droughts tend to make leaves less vibrant, more brown, and fall from trees early from physiological stress.

Southern Vermont and New England have seen droughts and outbreaks of the invasive spongy moth, which can dampen foliage colors—but these have not really hit northern New England as badly, where we are seeing very vibrant colors.

In mid and higher elevations out West and in the Rockies with less severe drought, I've been seeing evidence of beautiful foliage in the Rockies and the Sierras. In the coastal ranges, and intermountain West forests, the aspen forests could be a vibrant yellow this year, too.

What state typically has the best foliage?

Vermont is definitely the best in North America in terms of the vibrancy and diversity the colors—at least in my humble opinion. Fall in the Rockies is stunning, but they don't have as much of the reds and purples the way we do. The Appalachians have beautiful foliage there too, but it's probably more tilted towards reds. Other parts of North America have beautiful bright yellow foliage. But if you want that perfect mix of all the colors, you have to come to Vermont.

What’s the science behind fall colors?

Fall foliage a chemical reaction, influenced by external factors including climate, water, and length of day. Parts of the process are still actually a bit of a mystery. That makes is tricky to project how vibrant the colors will be.

The best conditions for fall foliage are cool evenings contrasted with warm daytime temperatures, paired with a shortening length of the day—known as the photoperiod. Together, these signal the tree to begin shutting down, and degrading chlorophyll and other key chemicals in the tree and leaf, which reveal and produce vibrant colors.

What causes the chemical reaction?

The chemicals in the leaves are mainly carotenoids and flavonoids, the secondary photosynthetic compounds that make carrots look orange and bananas look yellow. Reds and purples are produced by different chemicals called anthocyanins, but still not fully understood. They are produced during the fall and may play a role in protecting the leaves as they begin to shut down for the winter.

One of the really fascinating things about fall color is that there can be multi-year lagged effects. It's not as simple as what has happened just last summer. If the previous growing season was a good one, with better rainfall that allowed for more production of photosynthate and energy reserves, it can result in much better foliage production than you might have expected, even if there is a drought happening that particular year. But when we get into multiple years of drought, and tree reserves are tapped, that’s when problems start.

How important is fall foliage to the economy?

Fall foliage peeping brings something like $800 million, maybe even a billion dollars, in revenue to Vermont alone. Our tourism industry depends on this heavily. I think with climate change and winter sports declining, ski resorts and other destinations are becoming more like for season resorts. They may be emphasizing summer recreation and fall recreation more, and it takes on even more of an economic significance. In terms of the culture, identity, who we are, and the sense of place, fall foliage is inextricably connected to that.

There's a reason Vermont looks the way that it does. As a state we’ve invested in the landscape and keeping our forests together. The fall colors that make Vermont an international destination are one of the many benefits.

How will climate change impact this?

Drought is a major vehicle by which climate change will affect everything we've talked about. Climate change is tricky in Vermont and northern England. On one hand, we're seeing more summertime precipitation, so our summers are getting wetter. But on the other hand, we're seeing more extreme weather, including greater frequency of drought. In the future, that could have a dampening effect on foliage.

The other is the contrast between nighttime and daytime temperatures. If night temperatures are warmer later in the fall while the photoperiod remains the same, you don't have the same kind of degradation process for the chlorophyll. That also might mean that trees maintain their greenness longer with less of a reveal of colors. Or it happens a little later. Or it's for a shorter time period. Or maybe a little bit less vibrant.

What are other threats to fall foliage?

Besides climate change, drought—airborne pollution, invasive species, pathogens and acid deposition can impact tree productivity and foliage production. For example, Beech bark disease has decimated our large beech trees, which produce a beautiful, vibrant yellow color. In the future, Emerald ash borer may eliminate our ash. A major concern would be Asian Longhorn beetles, which attack Maple trees. Think of the future with no Maple trees, or much less. That would be devastating for both our economy and our culture.