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.
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.
“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.”
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.
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 100 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 on to their carbon longer each year—perhaps an extra two or three weeks, as previous studies had suggested.