The pattern continues as human activities transform not just the health and composition of forests, but their colors, too. Introduced pests, pathogens and invasive species are causing immediate changes to the fall color palette. And scientists are beginning to see a framework for how climate change may shape the forest colors of the future.
“These species have been adapting for millions of years, and we’re putting them through a stress test in a very short period of time. It’s shocking their system,” said Tanisha M. Williams, the Burpee postdoctoral fellow in botany at Bucknell University. “But they are adapting.”
Autumn’s longer nights and cooler days kick-start the seasonal color change, known as leaf senescence. Trees respond to the difference in temperature, precipitation and light by slowing photosynthesis. As the chlorophyll — the energy-producing compound that makes leaves green — breaks down, new chemical compounds emerge. Carotenoids, the same pigments in carrots and buttercups, make leaves appear orange, yellow and amber. Some tree species also produce anthocyanins, compounds found in blueberries and grapes, giving leaves red, purple and burgundy tones.
But wildly multicolored forests are under threat. Foreign pests and pathogens, arriving unnoticed in imported lumber or even packing materials, can alter whole landscapes in a short time, said Howard Neufeld, a plant ecophysiologist at Appalachian State University.
“They can take out trees, and if other trees come in that are different colors, that can have a dramatic effect,” he said. Under the moniker “Fall Color Guy,” Neufeld issues foliage color reports on the university’s website and on Facebook.
For example, an insect called the hemlock woolly adelgid has been wiping out Eastern hemlocks, a dark-green conifer, since the 1980s. The emerald ash borer, a pest arriving in wooden packing crates less than 20 years ago, decimates ash trees that normally turn yellow and burgundy. Both can quickly convert trees from dazzling to drab.
Weather patterns also affect when the leaves grow, turn color and fall off, Williams said. Moderate heat and drought extend the growing season and delay leaf senescence. But extreme drought exacerbated by high heat — the exact conditions in New England this summer — can accelerate senescence, causing leaves to change earlier and faster.
In some years, “some trees will continue to turn color early; others will delay and turn color later,” Neufeld said. “So instead of having this explosion of color in a short period of time, you get different groups of trees turning color over an extended time.”
Stressed trees may also fail to produce anthocyanins in their leaves, and go from greenish to brown without passing red. It’s hard to predict because of different species’ drought tolerance, said Amanda Gallinat, a postdoctoral fellow and plant ecologist at Utah State University. “Maples are often more sensitive to drought than oaks. Since maples contribute brighter colors than oaks, these different responses could lead to a less colorful autumn landscape,” she said.
Scientists say the long-term effects of climate change on fall colors are already apparent.
Warming temperatures tend to help invasive species, Gallinat said. Because they tend to come from places with milder winters than the eastern United States, they readily adapt to the longer growing season and remain an active threat for more of the year.
Invasive shrubs are overwhelming forest understories and adding more red tones with their leaves and fruit, she said. Pests and pathogens may continue to ramp up their attacks on certain species and allow others to take over, shifting the fall palette.
And as the climate gets warmer and drier, forests in New England that appear mostly red and orange today might not stay that way.
“If the forest changes from having maples and sourwoods and black gums, which are the red trees, to one that’s oak and hickory, then you’re going to get less colorful,” Neufeld said.
Some species are already on the move toward more genial habitats. A 2008 study found hardwoods in Vermont, such as the iconic sugar maple, had migrated upslope to cooler territory occupied by red spruce and paper birch, both of which had declined.
Sugar maples have also tried to move northward to Canada as temperatures rise, Gallinat said. But like many plants, the trees are limited in their ability to survive in colder climates. “It’s most likely that the vibrant colors those trees give us — not to mention the syrup — will be in shorter supply in the future,” she said.
The emerging tree communities — and their color combinations — have no precedent.
“If we see the migration of these trees, the fall colors are going to look different, because we’re going to have a completely different species landscape,” Williams said.
There’s no need to panic about subpar pigments over the next few decades, Neufeld said, but leaf-peepers should be ready for change.
“I don’t think we have to worry, in the immediate future, that we won’t be able to see fall colors,” he said. “We’ll just see different fall colors. And maybe at a different time.”