Scientists say climate change affecting fall foliage
In the 19th century, Eastern forests looked very different. Huge American chestnut trees, their trunks up to 10 feet in diameter, dominated forests from Maine to Mississippi. Their bright yellow foliage gilded Appalachia every autumn.
Then, a shipment of imported trees arrived in New York in 1876 carrying a stowaway: Cryphonectria parasitica, a fungus native to Asia. Within a few decades, the fungal blight wiped out hundreds of millions of chestnuts. Oaks, hickories and red maples took over, turning yellow autumn forests more scarlet and bronze.
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 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.”