It’s officially fall in the northern hemisphere, but depending where you are, it might not look like it.
From Maine and Vermont to New York, tourists often flock to the U.S. East Coast to see the maples and oak leaves transform into vibrant oranges and reds. Leaves can change their color from as early as mid-September all the way through early November.
[time-brightcove not-tgx=”true”]
But pay closer attention and you might notice that it doesn’t always follow a predictable pattern from year-to-year. Research shows that climate change is impacting when leaves fall along with the vibrancy of their colors—a tree that was previously barren by the end of October might still have leaves come November, while a drought stricken region might see dull, brown leaves that simply crumble off the tree.
Here’s what to know about why leaves change color in the fall—and how the process is being impacted by climate change.
Why do leaves change color in the fall?
When temperatures drop and the days get shorter, trees get less sunlight, causing the energy-absorbing chlorophyll, which gives them their green hues, to break down and reveal the other color pigments.
“They’re present the whole time,” says Stephanie Spera, a professor at the University of Richmond who is currently studying shifts in the fall foliage season. “They’re just masked by all the chlorophyll.”
However, not all trees turn red in the fall. Some, like Aspens, the most widespread tree in North America, turn yellow. “Aspen trees, and also some other trees, get extra nitrogen from fungi in their roots, so they don’t need to make such a big effort to recuperate the last nitrogen from their leaf,” says Susanne Renner, honorary professor of biology at Washington University in St. Louis.
Since maple and oaks can’t do the same, they have to produce an energy-consuming “sunscreen” that allows the leaves to stay on the branches—and absorb more nutrients—a little longer.
Some don’t even change color at all. Evergreens, for example, have protections that prevent them from losing their leaves altogether. “Their needles are really thin and covered in wax,” says Spera. “And because their needles are so thin, it’s not like the big energy-intensive situation where you have a giant leaf, taking all this light.”
How is climate change impacting fall foliage?
There’s no one-size-fits-all answer for how climate change is impacting the natural phenomenon. Impacts vary depending on the type of tree and the region it lives in.
“It is not possible to predict across the board how climate warming will impact leaf coloration,” says Renner.
One thing’s for certain, which is that the color change that we see in the autumn is caused by a chemical process—and that isn’t likely to be impacted by climate change.
“The maple trees will continue to be red no matter the temperature, because of the underlying chemical ability of the plant to produce this or that color,” says Renner.
However, trees need cold temperatures to produce the vibrant hues we’re used to seeing. If the leaves in your area are a disappointing brown, it could be that the temperatures didn’t drop low enough to produce bright colors.
“Fall colors in some regions and some years are pretty dull because the nights weren’t cold enough,” says Renner.
Higher temperatures in the summer and fall can also impact when the leaves begin to fall. Without a stronger decline in temperature, “the trees don’t have that cue to start shutting down chlorophyll production and shutting down photosynthesis,” says Spera. In the region of Maine that Spera has been studying, fall foliage has been delayed by almost a day each decade, mostly due to warmer summer temperatures.
Rain and drought can also impact the leaves, with too much leading to fungus and too little leading to “drought scorch.” “The leaves literally crisp off the trees,” says Spera. After a summer of drought and erratic rainfall, experts predict that New England’s famous fall foliage, which brings an estimated $8 billion to local economies, will be “bright, brief and early” this year.