How climate change disrupts fall foliage 

Throughout the eastern U.S., fall colors have been arriving behind schedule, which scientists say is a sign of things to come.

Fall is a special time of year in the forests along the eastern seaboard, as trees explode in brilliant shades of red and gold. The air is crisp, the days are still warm, and the turning foliage offers an irresistible riot of color—one that, by one rough estimate, generates as much as $30 billion a year in tourism revenue from Maine to the Carolinas.

“The mountains here in general are gorgeous,” says Larry Deane, a professional photographer who lives in the foothills of the Blue Ridge Mountains outside Asheville, North Carolina. “You combine that with vivid fall colors and it’s just sort of magical.” 

But fall is getting warmer as a result of climate change.

From 2014 to 2021, the Northern Hemisphere experienced its eight warmest Octobers on record. And the Northeast, which is most famous for fall foliages, is warming faster than the rest of North America.

From Vermont to North Carolina, fall foliage has been appearing behind schedule—continuing a long-term trend that, according to one recent study of maples by researchers at George Mason University, has pushed the appearance of fall colors back more than a month since the 19th century. Temperature is not the only driver; precipitation or the lack of it, extreme weather, and insect infestations also play a role. As climate change affects all those factors, it’s making the timing of peak foliage harder to predict.

What’s more, climate-related delays in leaf coloration are disrupting annual cycles of growth and rest that trees undergo. What that means for forests—how well trees grow, where they can live, and whether they can keep storing carbon at the same rate—is still unclear.

“We ought to be concerned potentially, not just about timing of change for autumn, but whether or not it portends some forest collapse,” says George Mason University ecologist Rebecca Forkner. “While nobody wants to be the ‘sky is falling’ kind of person, we do understand these changes are the plants telling us something is not right.”

Why trees need rest

Like people, trees must prepare for the cold, dark winter. All spring and summer, their leaves produce a green pigment called chlorophyll that captures sunlight, providing trees the energy to create the sugars and carbohydrates they need to grow and survive.

As temperatures drop and days grow shorter, signaling an end to the growing season, trees respond by ending chlorophyll production and absorbing any remaining nutrients—in effect laying down stores for winter. 

But lurking under that chlorophyll all spring and summer are orange and yellow-hued chemicals, which are unveiled as trees begin to go dormant. In addition, shorter, colder days prompt some tree species, including red and sugar maples, to produce red anthocyanins. Scientists think these red compounds are like a leaf’s winter coat, helping stave off the cold and allowing trees to absorb any last nutrients from the leaves before a cold snap kills the leaves.

The process leading up to shedding is known as leaf senescence. Climate change is disrupting it in some species more than others, with unknown consequences. 

“Plants have this amazing capacity to cope,” says Forkner, but scientists don’t yet know the limits of trees’ ability to adapt. “If they’re not getting their nutrients back, we could see some problems in our forest, but it’s too early to know. We don’t have enough people researching it.” 

In Maine’s Acadia National Park, scientists have noted a possible link between warm nights in September and delayed fall colors.

“This is the biggest signal we have seen,” says Stephanie Spera, an environmental scientist at the University of Richmond who is studying leaf changes in the park. “The whole season is being jeopardized. I think it’s getting a lot shorter. Spring is starting earlier. Everything is shifting."

In the past century, the park has warmed by 3.4°F. Trees and other plant life are feeling this change. One out of every five plant species documented in the park a century ago can no longer be found today.

Sorting out the changes in trees

To gain historical perspective on changes in fall foliage, Forkner and Alexis Garretson, now a doctoral student at the Jackson Laboratory in Maine, turned to herbaria—the plant collections maintained by universities and other institutions. Sifting through digitized records of maple tree leaves collected from the central and eastern U.S. and dating back to the 19th century, they found that the first appearance of fall leaf color had shifted an average of about six hours a year since 1880. Over the course of more than a century, those delays added up to more than a month. 

“Both of us expected to see a change, and a pretty large one, but both of us were surprised that the signal was so strong,” says Garretson.  

The pair also examined the leaves for damage by pathogens or plant-eating animals. They found that the extent of damage has increased over time. It seems to be associated with an increase in summer drought—and also with leaves changing color later but falling off trees earlier: Damaged leaves tended to fall as much as three weeks earlier than undamaged ones.  

With temperatures staying warmer longer, and with spring arriving earlier, trees are experiencing longer growing periods with shorter transition times between warm days and cold frosts. In effect fall is getting shorter. One concern is that trees may not have a chance to finish absorbing the sugars and carbohydrates that remain in their green-tinted leaves before the first winter frost. 

“The harm could come if you get a sudden freeze. The trees may not be acclimated, and the leaves may fall off before they’ve withdrawn all their nutrients,” warns Howard Neufeld, a biologist at Appalachian State University.

That could impact how well a tree grows in the following spring. For a tree, the color change process is “how it goes to the grocery store and gets all the food it needs for next year,” says Forkner. “If it doesn’t do that, there are consequences for the lifespan.” 

How climate change affects trees, in turn, has a feedback effect on climate. Forests absorb an estimated 30 percent of all carbon dioxide emissions released every year, and if their health wanes, so too might their climate benefits. 

“These ecosystems are buffering us from the worst impacts of climate change,” says Mukund Palat Rao, a climate scientist at Columbia University’s Lamont-Doherty Earth Observatory who is monitoring the timing of fall leaves to better understand how changes might impact the carbon cycle. “If the ability of forests to do that decreases, then what happens to the carbon dioxide in the atmosphere?”

Rao says it’s unclear just how much carbon trees can absorb; a longer growing period may not equate to a bigger climate benefit. One recent study of trees in Europe found, as Forkner and Garretson did, that climate change is causing some trees to drop their leaves earlier than they used to. That could reduce the amount of carbon that forests remove from the atmosphere.

“Very severe droughts or stress events, makes the tree shut down more quickly,” says Paul Schaberg, a Vermont-based plant physiologist with the USDA Forest Service. “If it is just very, very dry, they’ll drop their leaves in August.”

A glimpse of changes to come 

Some trees may be more successful at adapting to climate change than others, and as the less adaptable trees die and recede from the landscape, fall colors may also change. Trees that produce red pigments tend to have more northern ranges, and some scientists speculate that the fall color palette could become more golden in southern latitudes. 

“Because of warming in New England, sugar maples will shift north to find cooler temperatures. Instead of Vermont maple syrup, we might have to think of Toronto maple syrup,” says Neufeld.

For now, falls will certainly keep getting warmer—just like springs, summers, and winters. 

“It’s part of a continuing trend we’ve seen over the past several decades. And that’s what climate change is about—individual events that are part of a longer-term trend,” says Benjamin Cook, a climate scientist at Columbia University. “[2021] fall was pretty warm in New York. In 30 years, it might seem like a cold memory.”

The annual ritual of admiring fall foliage is a good chance for people to reflect on that trend, Neufeld says. “Fall has the best weather. It's cool, it's crisp out. Fall color is big time tourism,” he says. “If more people get out and see nature and appreciate its fragility, they might be more conducive to doing something to protect nature.”

This article originally published on November 24, 2021.

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