If you live in the Eastern or Midwestern part of the United States, the autumn season in which leaves change color seems like an immutable phenomenon. We've grown accustomed to that predictable explosion of red, purple, orange and yellow on tree branches as leaves near the end of their life cycle and lose chlorophyll, the sunlight-absorbing chemical that gives leaves their green tint. That, in turn, unmasks other pigments such as carotenes and xanthophyll, and creates a fall feast for our eyes.
But recent research suggests that autumn in years to come may be a bit different, as climate change alters the timing and duration of the colorful foliage season, and perhaps selection of the colors and their intensity as well. Exactly how those effects play out, however, may vary according to region and the type of trees in local forests, scientists say.
The fall foliage season is influenced by a variety of different factors, explained Howard Neufeld, a professor of physiological plant ecology at Appalachian State University in North Carolina, who writes a blog about the science of fall colors.
"One thing we're finding out is that trees don't always respond to environmental cues the same way," Neufeld said. "Some trees depend more on temperature, others on the day length. Rising levels of carbon dioxide can affect when a tree drops its leaves, but they react differently to it. And if climate change alters the weather, so that we get more or less rain, or more or less drought, that can make a difference too."
While we may not realize it, humans already have altered the autumn color palette, by changing the type of trees found in forests, Neufeld said. At the beginning of the 1900s, eastern forests had a lot more bright yellow, because the American chestnut tree was a dominant species. But then, importation of trees from Europe brought chestnut blight, a disease that killed off vast numbers of the large American chestnuts. Today, the species tends to grow only to 10 to 15 feet tall before it's attacked by the disease, Nuefeld said.
Climate change could alter the composition of forests even more, because tree species that are sensitive to temperature could gradually shift northward or to higher elevations, Neufeld said. "Some species that are currently in the Northeast, like the sugar maple, which gives that nice color, might move to Canada." The oaks and hickory trees that would replace the sugar maple would have less vivid colors, he said.
Additionally, some studies suggest that climate change could prolong the growing season and push back the start of autumn leaf changes. A 2011 study of satellite data by South Korean researchers found between 1982 and 2008, the start of autumn had shifted by more than nine days.
Other research suggests that trend could continue, A study published in PLOS ONE in 2013, which looked at leaves on eight different tree species in a New England forest from 1993 to 2010, projected autumn leaf displays could arrive as much as two weeks later. Those researchers also found that future forests might actually have more colored leaves than they do now -- though, as Neufeld pointed out in a blog post analyzing the results, the intensity of the colors of individual leaves wouldn't necessarily increase.
"Our results show that there is variation, year-to-year, in the timing and amount of autumn color, and at least some of this variation can be associated with temperature and precipitation during the preceding months," explained Andrew Richardson, one of the study's authors and an associate professor of organismic and evolutionary biology at Harvard University. "Also, our prediction that climate change will both delay and increase the amount of autumn color is I think interesting."
The timing of autumn leaf coloration also could be influenced more extreme weather caused by climate change, according to a study by University of Connecticut researchers, published in Proceedings of the National Academy of Sciences in 2014. Those researchers found that while northern New England could experience later leaf changes, the southern coastal part of the region might shift toward an earlier autumn.
Shifts that change how trees look could have a surprisingly big economic impact, since it would affect tourism to areas with vivid autumn colors.
"In the eastern states, you're talking about a $25-30 billion industry, once you factor in food, lodging, gas, and what people spend on apple cider," Neufeld explained. "It's the time of year when you can get out in nature -- it's the Goldilocks time of year when it's not too hot and not too cold, and you have the beautiful colors to look at. It's a pretty time of year, and I don't think people think what climate change might do to it."
From the State University of New York's College of Environmental Science and Forestry, here's a more detailed look at why leaves change color, and the changes that trees undergo in Autumn.