Autumn Foliage Feels the Heat

By Michael Caduto

It is inspiring to see how the allure of fall foliage draws millions of travelers to New England’s winding back roads, where even the most jaded traveler stops to appreciate the palette of colors that burnish the hills. “Leaf-peeping” in northern New England is an annual pilgrimage that generates well over $2 billion; income that many small towns, like Woodstock, rely upon. 

No one likes to rain on a party, so now that the peak of this year’s leaf season has passed, let’s look at how climate change is impacting fall foliage. In 2021 the displays of leaf color were rich and vibrant. But in some recent years the autumn coloration has been notably pastel, with many leaves turning from green to brown before dropping. With autumn color so variable, how can the long-term effects of climate change be measured?

Dr. Barrett Rock—professor emeritus of the Institute for the Study of Earth, Oceans and Space at the University of New Hampshire—has a PhD in the microscopy of plants and has devoted a lifetime of research to looking down at vast swaths of northern forest through eyes set literally in the sky: NASA’s Landsat satellites. As it turned out, the launch of the first Landsat satellite in the early 1970s corresponded with the time frame when the climb in average global temperature due to climate change was accelerating. While most of us get caught up in observing the natural world in short time frames—a temporal version of “not seeing the forest for the trees”—Rock has spent more than three decades viewing forests from a long-range planetary perspective. 

Said Rock, “It’s like looking through a microscope, and then stepping back 500 miles—moving from the microscope to the macroscope.” Landsat satellites map and measure visible color bands in the blue, green, and red spectra. “But the longer infrared bands measured by Landsat satellites, which are not visible to the naked eye, tell us more about changes in leaf color than the visible spectrum.” Satellite data is then compared with spectrometer readings taken in the laboratory to measure foliage using short- and long-wave infrared. 

“Serious change in our climate became really noticeable in the mid-1970s,” says Rock, “as if a switch had been thrown at that time, including increased temperatures and reduced air quality. Significant changes in forest health also began in the 1970s and all data indicates that it is now occurring at an increasingly rapid rate. This corresponds well with what the climate change models predicted.”

The first hard frosts, which bring out the most vibrant leaf colors, used to occur around the third week in September. But in recent years, these frosts have arrived later. As Rock has observed, “There was a time when Columbus Day was widely recognized for planning a visit to New England for spectacular colors. Now the foliar change may not have even begun by early October, and frost sometimes comes as late as early November.”

Parallel trends have been reported by researchers at the University of Vermont as well as at the Harvard Forest Research Station in Petersham, Massachusetts. Cornell plant biologists found that stress placed on trees due to climate change and other forms of air pollution, such as high ozone levels and acid rain, is causing many leaves to brown off and die without much color change. Leaves are also more prone to being invaded by fungi and bacteria seeking sugar to feed on.

Abby van den Berg—research assistant professor at the University of Vermont’s Proctor Maple Research Center—noted that shortening day length and decreasing temperatures initiate autumn leaf colors. Bright sunshine also triggers foliage coloration. “If you change the timing of the onset of cool temperatures, you alter when chlorophyll breakdown starts. Even though we have no good way to predict how climate change will affect the process that creates the colors of foliage season, it will change how the landscape will look over time.”

Drought, late-season warmth, and the cloudier days associated with climate change all have the effect of muting autumn leaf colors, which now often progress, as Rock has observed, “from green to pale yellow, followed by leaf fall. Stressed trees have difficulty making anti-fungal compounds, which makes it hard for them to create the beautiful bright colors.”

As seen from the outermost layer of the atmosphere to changes detected in individual leaves on trees growing nearby, the impacts of climate change on autumn foliage are growing ever closer to home. New Englanders, however, are accustomed to the vicissitudes of weather, and most take the changes in stride. Foliage season remains tightly woven into the fabric of the region’s identity as an iconic event by which we celebrate the seasons.

Photo credit: Michael J. Caduto

What You Can Do:

  • Learn what can be done to improve forest health and ameliorate climate change by visiting the International Union for Conservation of Nature (IUCN) website:
  • Young people around the world are active in fighting climate change and protecting forests through Jane Goodall’s “Roots & Shoots” project:
  • See the list of suggested actions in the author’s “What You Can Do About Climate Change” article on page 2D of the October 7, 2021 edition of the Vermont Standard.

Article adapted with permission from Through a Naturalist’s Eyes: Exploring the Nature of New England by Michael J. Caduto (University Press of New England).


Learn more about our Vermont Standard articles.