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Advice for Leaf-Peeping Season: Don't Blink

With climate change, scientists say the prime window for breathtaking colors may be closing more quickly

By Craig Miller

As summer faded into September, Stephanie Spera loaded her backpack and headed for Acadia National Park in Maine. The park will be her office throughout the leaf-peeping season — and she'll be part of the show.

A rural road surrounded by trees with yellow and orange leaves. Next Avenue, leaf peeping, peak fall colors
Sunny days and cool nights are keys to brilliant fall colors  |  Credit: Craig Miller

Donning her bright orange T-shirt emblazoned with the challenge, "Ask me about my research," the 33-year-old biologist surveys hikers along the trail about the backdrop of fall foliage. Since 2019, the University of Richmond researcher has been weaving together a timeline of fall colors in the park, to see how the warming climate and other factors are changing their timing and intensity.

"Temperature and precipitation have been correlated with the length and vibrancy of fall color."

"That's why I'm here for the fall, is to understand how visitors are thinking about climate change and fall foliage and whether or not they're considering it when they plan their trips," explains Spera.

Understanding the changes is important to, among other things, fall foliage tourism, which is a multibillion-dollar industry. According to Spera, fall in Acadia has been "trending later," with foliage peaking in mid-October, about a week later than it did in the 1950s. And it appears to be moving back by about one day every decade.

The Effects on Fall Foliage

"Temperature and precipitation have been correlated with the length and vibrancy of fall color," she writes on the project website. "Some studies show that a hot summer, rainy fall and even nitrogen pollution are related to a shorter, duller foliage season, while a later color change is associated with warmer, earlier spring and fewer fall storms."

Spera says the key drivers that bring out fall colors are the amount of daylight and temperature. When days shorten and temperatures drop, the photosynthesis machinery in the leaves begins to shut down, drawing back summer's green curtain of chlorophyll to reveal bright pigments of red, orange and yellow that have been there all along (the process is slightly different for reds).

Spera says the optimal conditions for those brilliant reds, in particular, are sunny days paired with cool nights. According to federal climate forecasters, the entire continental U.S. is likely to see above-average October temperatures this year. And a key hallmark of climate collapse has been warmer nights, in particular, which could bode for less vibrant colors. Recent studies also show the northeastern U.S. warming more quickly than the country as a whole.

"What I've been doing is trying to actually tease apart the climate change piece," says Spera. "Like, what is driving that timing being later? Is it temperature? Is it precipitation? It's a few different things, but the thing that is consistent across the board is September temperatures."

"The timing and the speed are affected by ongoing climate change and drastically."

So far, says Spera, the data are telling her that every one-degree Celsius increase in the average September temperature creates about a two-and-a-half-day delay in fall foliage at Acadia.

But scientists aren't unanimous on the fall climate connection.

"The timing and the speed are affected by ongoing climate change and drastically," says Susanne Renner, 66, a biologist at Washington University in St. Louis. But Renner says that by fall, the clock is already set. "What our work shows is that the most important parameter to predict, to influence the senescence, is what happens in the spring," she says.

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Her latest research, currently in peer review, will show that the main climate impact occurs in the spring, when the leaves first emerge. "There are tons of data showing that leaves on the typical European and North American trees are now coming out two to four weeks earlier, depending on species, than they did a hundred years ago," says Renner, who claims the new findings will cause a "paradigm shift" in the way scientists think about leaf senescence.

When Autumn Leaves Begin to Fall

There's also a climate feedback effect. Data gathered by Renner and her colleague, Constantin Zohner of Munich, suggests that as the planet warms, deciduous trees in temperate European forests are dropping their leaves sooner, a process scientists call abscission. The results were more than a little counterintuitive, since simple logic would suggest that a warming planet would mean a longer growing season, with trees holding onto their carbon longer each year – perhaps an extra two or three weeks, as previous studies had suggested.

A white cabin surrounded by a forest with fall colored leaves. Next Avenue, leaf peeping, peak fall colors
Fall colors along Ten Mile Creek in Upstate New York’s Helderberg Hills  |  Credit: Craig Miller

It's important because trees are a critical player in the Earth's carbon cycle, sequestering almost a third of carbon emissions, helping to rein in runaway warming. That stops when leaves die and drop off of deciduous trees. Zohner and Renner's study of European forests suggests that climate change could actually shorten the photosynthesis season by several days. Though the research was done in Europe, the eastern U.S. has comparable ecosystems. So Renner says the implications of an earlier leaf drop are "humongous," amounting to gigatons of carbon dioxide every year that could remain in the atmosphere.

Renner does go along with at least one piece of homespun wisdom: that an early frost will really bring out the vibrant reds of autumn, just as your grandmother claimed. Delayed frosts from climate warming lead some scientists to predict that fall colors will be generally less vibrant than in past years.

Drought Enters the Fray

As in the past, however, each year will bring its own surprises. A "very severe" drought will cause trees to shut down their processes sooner, U.S. Forest Service plant physiologist Paul Schaberg told National Geographic last year. In parts of New York State's Catskill region, some leaves began drifting down in August, after an unusually dry summer this year.

A woman walking down a forested trail. Next Avenue, leaf peeping, peak fall colors
Biologist Stephanie Spera patrols the trails at Acadia National Park in Maine, gathering data on the timing of fall colors  |  Credit: Christopher Allen

"If we're entering a world with a lot more extreme drought, that's gonna be a huge game changer," agrees Spera. "If it's a big drought, like we've been seeing, you generally see those trees get stressed, have a big show of color pretty early, and then it's over." Too much rain and wind can be a showstopper, too. "The other thing is if you get that beautiful fall foliage showing, even if it's early, if you get one of these extreme rainfall events, there go all the leaves."

The divergent opinions are no surprise to Zach Fowler, 43, a forest ecologist who heads the Core Arboretum at West Virginia University. "We don't understand the whole process," says Fowler. "Our understanding of this is based on how we've been able to model it, but there could be things that we haven't even figured out yet that are important to the process and that we haven't incorporated into our model."

"The predictions are poor," adds Renner. "If the [tourism] industry really could predict when the leaves are turning in Vermont, it would be really great [but] it's not working well at all. The models that we have are very, very poor."

The bottom line on climate effects, says Spera: "I think it's just becoming a lot more unpredictable."

Spera invites visitors to Acadia National Park, past or present, to contribute photos that help document the park's changing fall colors over the years. Especially useful are photos from the "pre-cellphone," era, and before high-resolution satellite images were available, prior to 2000. You can submit photos at the project website, via Instagram or by email.


Photograph of Craig Miller
Craig Miller is a veteran journalist based in the northern Catskills of New York. His reporting is focused on climate science and policy, energy and the environment. In 2008 Miller launched and edited the award-winning Climate Watch multimedia initiative for KQED in San Francisco, where he remained a science editor until August of 2019. He’s also a proud member of his local volunteer fire department. Follow him on Twitter @VoxTerra.
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