Geology professor Stephanie Spera

3 Questions: How climate change is delaying fall colors

September 18, 2020

Research & Innovation

Geography professor Stephanie Spera uses photos of Acadia National Park, taken over time, to explore how climate change is changing the onset of peak fall colors — as well as how much time park visitors have to see them. She explained how she tracks the changes, based on recent and historic photos from visitors to the park.

How does climate change affect when leaves change color?

There are a few different ways climate change can impact the timing, and even duration, of fall foliage. Most deciduous trees use two big environmental cues that let them know the seasons are changing, and that they should stop using their energy to make chlorophyll, photosynthesize, and create green leaves. Those two cues are photoperiod (or daylength), the amount of time the sun is above the horizon — and temperature. Climate change will not affect daylength — that’s determined by the latitude on Earth where you’re located and the Earth’s tilt — but it is affecting temperature. So as temperatures are getting warmer, even as daylength is getting shorter, these trees aren’t getting all of the cues they need to tell them to shut down their chlorophyll production.

The fun reds, yellows, and oranges that we see in the fall are present in the leaves even during the spring and summer, but leaves are producing so much chlorophyll that it really masks out any of those other colors, so only when a tree stops producing chlorophyll do we get those beautiful fall colors. But if it’s not receiving the cues it needs to shut down that production, particularly if summer temperatures are stretching further and further into the fall season, we can expect that the timing of when the leaves start changing color will be delayed. Depending on which tree relies on which environmental cue more (temperature or daylight), we might expect some of these brilliant displays of color to become out of sync. And another interesting result of changes in climate, particularly in New England, is an increase in extreme precipitation events. So if the leaves start changing, say, on Oct. 12, but you have a big storm come through on Oct. 15, you only have three days where the leaves are on the trees.

What’s the latest with your research in Acadia National Park?

We’re at an exciting time because we’re pulling together some of the most important pieces now. Based on our historical and photo analysis, since 1935 peak fall foliage has been delayed by a little over a day a decade. So in the 1950s, in Acadia National Park, peak fall foliage would occur around Oct. 4. But now, 70 years later, peak fall foliage typically occurs around Oct. 12-13. And what’s great — as a scientist — is we see the same trend using just 20 years of satellite data. Now we’re working hard to disentangle which specific climatic drivers are responsible for this shift in timing of fall foliage.

How have you used historical photos to track the onset and peak of fall colors?

We’re using historical photos to track the timing of fall foliage in a few different ways. Photos we receive from the last 20 years help us validate that what we’re seeing in the satellite data is actually what’s happening on the ground. If our algorithm shows that on Oct. 5 in 2010, about 50% of the leaves had already changed color, and we get a photo from Oct. 7, 2010, and see that, yes, about half of the leaves have changed color, we know our algorithm is working. We have a lot of photos from the post-cell-phone-camera era. The other way we use photos — but we don’t have as many of these — is to fill in gaps in our historical analysis. So, we’ve gone through old newspapers, park reports, magazines, everything we can to piece together how fall foliage has changed before we had satellite data, and photos help us fill in the gaps for when we can’t find these data. And to me, these are the photos that are super fun to receive.

Fall colors in Acadia National Park
Fall colors appear in Acadia National Park, on the coast of Maine.