Every other Friday, the Outside/In team at NHPR answers listener questions about the natural world. Today's question comes from Jarrett from San Francisco, California.
"I just learned about this back east past-time called 'leaf-peeping.' It just seems so cool that leaves change to mark that fall. Out here, I just wait for pumpkin spice lattes and Uggs to be back in style. I’d love to know more about what makes leaves change color, and how they know to drop leaves to prepare for the coming winter."
Outside/In producer Justine Paradis donned her wool sweater and called up some experts to find out.
Chlorophyll.
As many of us learn in school, that’s the pigment that powers photosynthesis and helps make sugar inside plant leaves.
It also happens to be green.
But then, as summer tilts into fall, trees are picking up on certain cues, especially shorter days and falling temperatures. When they get the message that it's that chlorophyll production breaks down, revealing other pigments that were hiding in the leaves all along.
These pigments are called carotenoids, and they are the oranges and yellows of autumn. The season when people like Stephanie Spera come into their power.
"Fall’s my favorite season. I love wearing sweaters. I love changing the colors. I love crisp air. I love feeling cold. I miss feeling cold," said Spera, a geographer studying autumn foliage and climate change.
Spera is a professor at the University of Richmond in Virginia, but she grew up in New England and maintains a strong bias towards east coast foliage. But there is actually something that gives New England’s leafscape an edge.
It’s another pigment, called anthocyanin. It shows up in lots of red and purplish fruits and veggies, like strawberries, cherries, and red onions. As some trees are trying to squeeze the last bit of sugar out before winter, they actually produce anthocyanin in their leaves as those leaves are dying.
Trees do this all over the place. But there’s a particularly high abundance of trees in New England which produce anthocyanin in the fall.
"Those are your reds. So, if you have cool nights with sunny days... in the fall, you're going to get brilliant reds because that enhances sugar production and anthocyanin production," said Spera.
'When trees are stressed, they just don't have the energy to allocate into like leaf production and like maintaining and, honestly, just being their best selves.'Stephanie Spera, a geographer studying foliage and climate change
But even within the same year, the vibrancy of the foliage can vary across the region. Last fall, we got another question about just that from Eben, who called from Lincoln, Massachusetts, just outside of Boston.
"I've been walking around Lincoln recently noticing that the leaves as they're turning colors have been much more vibrant here than they were farther north over the past few weeks," Eben noted. "And I'm kind of curious as to why, because usually it's the other way around."
That happens to be "a hilariously complicated question," according to Spera.
"Last year, I joked that it was like the year without a summer… it rained all the time, right? Particularly Vermont, New Hampshire," Spera said. "What happened was a lot of these trees actually got a fungus that really dampened the colors... outside of Boston was a little bit drier."
As a result, Boston enjoyed better — or, at least more colorful — foliage.
The dynamics last fall were a bit contrary to what we typically expect around foliage, but a big reason people associate New Hampshire, Vermont, and Maine with foliage is that there are just so many trees, and so the effect is spectacular.
But local weather conditions can make a difference in terms of color.
"When trees are stressed, they just don't have the energy to allocate into like leaf production and like maintaining and, honestly, just being their best selves," said Spera.
When we are blessed with a particularly spectacular foliage year, it’s because the region is experiencing what people often refer to it as “Goldilocks” conditions, when all or most of the trees turn at once.
"A warm but not super hot summer. You had adequate rainfall. You don't have pests like the emerald ash borer decimating the leaves," Spera said. "It is all of these things working together to create this season."
Watching the leaves change isn’t just pretty; it can also be science in action.
Spera uses satellite imagery to track how the timing of “peak” foliage is changing in Acadia National Park in Maine. Anyone can participate in this project by sharing your photos of fall foliage, anytime from 1950's to present day, with the Acadia National Park Fall Foliage Project.
This year, some are predicting a banner season. So, get out there and enjoy that beautiful anthocyanin.
If you’d like to submit a question to the Outside/In team, you can record it as a voice memo on your smartphone and send it to outsidein@nhpr.org. You can also leave a message on our hotline, 1-844-GO-OTTER.
