“Fall’s changing colors have been delayed up to four days per decade in North American forests since the early 1980s,” says Andy Finton, a forest ecologist who is the landscape conservation director with The Nature Conservancy in Boston. This year has been particularly pronounced due to a long hot summer in much of the country, followed by an unusually warm fall. The disruption in fall color is happening in several ways: Summer droughts are making leaves turn brown and fall off before they can take on those desirable shades of red, orange, and yellow. Hurricanes and their rainy, windy remnants strip the leaves off the trees entirely. And unseasonably warm weather keeps the trees from knowing when it’s time to trade their green leaves for fall colors. “This is a climate-driven phenomenon,” Finton says. “We’re seeing trees track with the changes that we’re seeing in the weather.” Finton says as the average temperature in the United States has risen over the last two generations (it’s more than 3 degrees higher than it was in 1970), fall color has been appearing progressively later. As a result, peak leaf season now arrives in some areas more than two weeks later than it did in the 1980s. “It’s never been easy to predict peak leaf season, but climate change is making it harder,” Finton says. Peak leaf season usually occurs in mid- to late-October for much of the Eastern United States, and in mid- to late-September in the Rocky Mountain West. This year it was later in most places in the United States. Finton says to expect the trend for later falls to continue in the future. “Most studies show that fall color change will continue to move later and later.”
What makes leaves change color later?
To understand why warmer weather is bad for fall color, you must understand how nature paints a forest in autumnal hues. Shorter days and cooler air temperatures cue trees to change their leaves from green to red or gold. They’re shutting down their food-making operation (photosynthesis) for the season and preparing to go dormant for the winter. If the air stays warmer longer, the color change will be delayed. Heather Alexander, associate professor of forest ecology at Auburn University, agrees that late fall leaves are a sign of climate change. “Changing temperature is something that we’re seeing, obviously, and it’s getting warmer on average across the globe,” Alexander says. “The trees are sensing this.” It doesn’t take much to throw the leaf change cycle off-kilter, Alexander says, because trees are super sensitive to air temperature. She says there’s a three-day delay in leaf color change for each 1.8-degree increase in air temperature. That’s why the warm fall weather that’s affecting much of the nation right now has trees staying green and leaf peepers feeling disappointed. It’s not just abnormal fall weather disrupting the trees’ leaf cycle. Summers of extreme heat and drought are also stressing trees, which delays and diminishes fall color. In Washington, a summer of record-breaking heat and a 51-day drought made many leaves fall off the trees early, before they could change colors. The leaves that remain have a duller color due to environmental stress. Too much rain can disrupt fall color, too. The New York State Department of Environmental Conservation blamed a summer of unrelenting rain (thank you, Hurricane Ida) for the late fall foliage there. The storm pulled down many leaves too early, so the remaining leaves on the trees had to work overtime doing photosynthesis to feed the tree. That means they stayed green longer, and that there won’t be as many leaves to peep at when they do turn color.
Colorful trees equal a healthy forest.
Fall leaves aren’t just pretty to look at. They’re also an important indicator of a forest’s health, Finton says. Those vivid fall colors are a sign that trees are getting the rainfall and temperature they need to thrive. Finton sees later, paler fall leaves as an omen. “It’s a challenge for forests to keep their resilience and vigor in the context of climate change,” Finton says. “But there’s a hopeful message. Forests have a lot of inherent resilience. They can adapt. If we’re successful at protecting and conserving large, intact forests, we can help them cope with climate change.”
