A pedestrian is framed in a canopy of changing leaves along North Fairfax Street on Thursday November 12, 2020, in Alexandria, Va. The Washington Post

Before social media and smartphones, restaurant owner Nancy Aldrich took meticulous notes about her business on a clipboard. Starting in the 1960s, she documented how many people ate at Polly’s Pancake Parlor down to the hour of the day and noted the weather. Then in 1975, she began recording another important staple for life in New Hampshire: when fall leaves changed colors.

“Mostly this was necessitated because people would call us and ask, ‘When is peak going to be?'” said Kathie Cote, Aldrich’s daughter and third-generation owner of the restaurant who continues recording fall observations today. “She could look back and say, well, this is the average color when we normally have it up here.”

Predicting the peak of fall color is complicated. The timing and quality of color changes depend on a combination of temperatures, precipitation and sunlight. The best fall color displays occur after sunshine-filled days and cooler nights, following healthy doses of rain in the summer.

But Cote and researchers have found estimating fall peak has become even trickier in recent decades. Warmer temperatures have delayed the onset of fall, pushing the peak of fall leaf season back as much as a week in some areas over the past seven decades. Extreme weather events have also abruptly curtailed recent seasons or caused the color of the leaves to become duller.

“There has been and will be more variability year-to-year and more uncertainty,” said Andy Finton, a forest ecologist with The Nature Conservancy in Massachusetts. “With climate change, it just adds more complexity to an already complex phenomenon.”

This year, Finton expected a good fall season due to an especially wet summer, for instance. But a recent bout of warm weather in the eastern U.S. — one of many this summer — could keep the colorful leaves at bay a little longer and lessen their vibrancy.

Predictive maps based on historical and current weather data show peak season will occur in mid- to late October over much of the eastern U.S. this year, but climate change is expected to keep nudging these dates backward in the coming decades.

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Cote never expected her family’s rudimentary documentations to be used for any kind of analysis. She said they’re not exactly “scientific” and mainly based on eyeballing when most leaves were red, orange and yellow instead of green. The observations were on display at the parlor for customers to read while they were waiting.

But environmental scientist Stephanie Spera was thrilled when she saw the observations, which are now posted on the restaurant’s website. She said this is the “longest record of fall foliage” that she has ever found.

“Fall is sort of this understudied season because it’s so much harder to be like, ‘Is this peak?'” said Spera, an assistant professor at the University of Richmond. “It’s a lot harder to get data on when peak foliage has occurred anywhere.”

Suitable satellite data only documented fall foliage back to 2000, which was not long enough to discern any trends. Instead, Spera and her student had to crowdsource information and photographs before the 2000s from the public, national park reports and newspaper clips.

Through the Second Century Stewardship Foundation, Spera is analyzing fall foliage changes at Acadia National Park on Maine’s Mount Desert Island.

“We actually were able to piece together, from the 1950s to now, peak foliage is occurring a full week later. It’s actually delaying a day a decade,” said Spera. Now, peak season doesn’t occur until around the second weekend of October.

The delay is partly linked to warmer temperatures, particularly at night, said Spera. During the day, leaves use the sunlight to produce sugars. Cooler nights help trap the sugars in the leaf. The sugars lead to the production of pigments, such as anthocyanin that produce the brilliant red seen in maple leaves.

“Warmer days and overcast days reduce photosynthesis, which means less sugar in the leaf, and it’s those sugars that are required to produce the red colors,” said Finton.

As summer temperatures last longer into the fall due to climate change, the warmer temperatures can degrade anthocyanin pigments as well as postpone the peak.

Spera analyzed Polly’s Pancake Parlor observations and found a similar delay in northern New Hampshire. Peak fall season is also extending to the mid-month holiday weekend in October, which Cote said is “great for business.”

In 2019, Cote said the pancake parlor served more than 900 people on the Sunday over the October long weekend, which was their busiest in more than 80 years of business. In 2020, they served around 650 people on the same Sunday, which she said was amazing considering they were at reduced capacity due to the coronavirus pandemic. They expect another busy season this year.

Finton said the delayed onset is notable in New England due to the vibrant tree species and fall foliage tourism. Leaf peeping season generates billions of dollars for the local economy.

But he said these leaf peeping patterns could also be seen in deciduous trees around the world. The amount of delay differs depending on the region, with some studies showing a more pronounced trend in areas over others.

“Most studies show that it’s happening later and later, but the verdict is still out as to how far and how fast that will go,” said Finton. “The general trend is, yes, fall color change will continue to progress later and later in the year.”

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Rising temperatures could also shift the color palette of fall in several ways — from changing the chemical processes to the relocating tree species.

With less red produced in some species, the trees show off other pigments already in their leaves. Aside from green (chlorophyll) pigments, yellow (xanthophyll) and orange (carotenoids) pigments also organically reside in the leaves. Consistently warmer fall seasons could paint dominantly orange and yellow scenes where red has traditionally reigned.

Warmer temperatures could also change the distribution of colorful tree species, said Finton. New England could see less of the brilliant red Sugar and Red Maple trees, as southern species that flourish in the warmer weather could expand northward. Some of the southern species, like the oak or pine trees, have more muted hues.

Finton has already seen more yellow poplar trees, also known as the tulip tree, in southern Massachusetts. Projections from the U.S. Forest Service showed the yellow poplar trees will significantly expand northward within a century under the current, high emissions greenhouse gas scenario.

Extreme weather events fueled by climate change, such as drought and heat waves, can also diminish fall color. In 2020, parts of New England experienced a subpar season due to extreme weather. A severe drought during the summertime caused many leaves to shrivel before they had a chance to turn red. Then a strong storm with high winds at the end of September removed many leaves, cutting the leaf peeping season short.

For Finton, fall color is an important indicator of a forest’s health both in the long term and short term. When trees show vibrant fall colors, it is a sign that the trees are experiencing the appropriate amount of precipitation and livable temperatures, for instance.

“Anecdotally, when I when I see the colors hit the ball out of the park, I know that things are working well in that forest,” said Finton. “The ecological processes are functioning in a way that allows that forest to do all the other things we depend on it for — clean air, clean water, wildlife habitat, pulling carbon out of the atmosphere.”