“Season Creep” is Affecting Fall Folliage

Image: Craig Adderley/Pexels

If you’ve ever noticed that there’s something off about the timing and duration of fall foliage where you live–you’re not imagining things! As with many ecological processes, human activity is shifting the arrival of our seasons through what’s described as “season creep.

As the Washington Post wrote this past weekend:

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.”

Rising temperatures as a result of climate change aid the spread of invasive species and tree diseases while drought stresses trees which affects the vibrancy of their foliage.

Why This Matters: Fall foliage is a splendor of nature many of us enjoy. However, as climate change forces trees with the brightest foliage to migrate northward, some species like sugar maples may have nowhere to go as they’re limited by their tolerance for the cold. This isn’t to say forests won’t put on a fall show, but it may not be as impressive.

The Impacts: As Sara Peach wrote for Yale Climate Connections, for trees, the purpose of the leaf changing process is to retrieve nutrients from the leaves before they fall to the ground. For people, it’s big business, with foliage-related tourism worth billions of dollars in New England alone.

  • As the climate has warmed during the past few decades, the onset of fall colors across much of the Northern Hemisphere has been delayed.
  • In the eastern United States, fall foliage arrives an average of two weeks late compared to the 1980s and 1990s, said Yingying Xie, an ecologist at Northwestern University.

The Process: A warming planet alters the signals trees rely upon to begin their transition to cooler seasons. As Scientific American explained, as autumn rolls around, a variety of cues—including the shortening days and falling temperatures—tell trees that it’s time to begin shutting down for the winter and preserving their nutrients for the next growing season.

  • Trees stop producing the chlorophyll that makes their leaves green and is a key part of the photosynthesis that provides them with food.
  • Left behind in the leaves of some tree species are carotenoids, which color the leaves orange and yellow.
  • Red leaves, on the other hand, are colored by anthocyanins, produced only in the fall.

But it isn’t just the environmental cues during the autumn that impact the timing and intensity of leaf color; conditions during the spring and summer growing season also influence how vibrant fall foliage might be. Shorter springs and longer summers can bring drought and cause foliage to turn earlier and appear duller.

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