Fall foliage was at peak when this photo was taken the morning of Oct. 4, 2023 at Heart Lake in Lake Placid. The property — home of the Adirondak Loj — is owned by the Adirondack Mountain Club and is one of the most popular places to embark on backcountry hiking trips in the High Peaks Wilderness. (Enterprise photo — Andy Flynn)

SARANAC LAKE — With the leaves just starting to show the first signs of fall, many are wondering how spectacular of a fall foliage show the Adirondacks are in for this year. Understanding how the weather plays a part in the process can offer hints as to what is in store for this year.

One local expert says early indications point to a more vibrant fall foliage season than normal this year.

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It starts with the sun

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The one constant factor, regardless of weather and climate, that kicks off the process is decreasing daylight as the northern hemisphere receives less and less sunshine at this time each year when summer transitions into winter.

Decidious trees — or trees that shed leaves — sense this change, and, when it is no longer efficient for them to keep harnessing the sun’s energy, lose their leaves for the season, according to the U.S. Forest Service’s website.

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Fall weather makes a big difference

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Beyond the shorter days each season, things get more complicated.

Temperatures, cloud cover and a slew of other factors determine just how brilliant of a blaze the leaves will erupt in before falling. While the Adirondacks consistently produce some of the most vibrant foliage in the country, some years are exceedingly spectacular, while others are relatively muted.

Summer weather can have an impact on overall leaf health, although, the most determinative time for a phenomenal versus lackluster fall foliage season are the couple of weeks preceding leaf change, according to Professor Justin Waskiewicz, who teaches and researches forestry at Paul Smith’s College.

“Temperature, especially night-time temperature, and sunshine. The basic recipe for really bright colors is bright, sunny days, and clear, cool nights in the fall,” he said.

Waskiewicz emphasized that it takes several weeks for trees to “turn off” their leaves.

“Leaf drop is a process. The leaves don’t just die and then get colorful. The tree begins to extract resources that it has put into the leaf over the summer and spring so it can reuse them. Some elements are recyclable for the tree, essentially,” he said.

It is during these weeks when the weather really matters.

Waskiewicz said a crucial aspect of bright foliage is the tree continuing to photosynthesize and produce sugar as the tree stops drawing that sugar from its leaves. The more sunshine during daylight hours, the more photosynthesis takes place and the more sugar the tree has.

The colder it gets at night, the less of its sugars the tree uses. Instead, they accumulate and become trapped in the leaf. A combination of high sugar supply from the leaves and low demand by the tree makes for vibrant colors.

“The brighter and sunnier it is, the more photosynthesis that the remaining chlorophyll is going to do, the more of the sugars, and therefore the more anthocyanin is produced. But then the colder the night is, the less of that sugar is going to get respired, and the more of it accumulates,” Waskiewicz said.

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The science behind the palette

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Fall foliage colors are predominantly driven by three pigments, according to Waskiewicz: Chlorophyll, carotenoids and anthocyanin.

Chlorophyll is responsible for the leaves’ green color throughout much of the summer. It allows plants to produce energy from the sun through photosynthesis.

Carotenoids are another pigment in the leaves throughout the summer. They produce orange, yellow and brown colors, notably in things such as carrots or bananas. Like chlorophyll, trees rely on the pigment to help absorb light for energy production. However, in most deciduous tree leaves in the region, carotenoids are masked in their visible appearance by chlorophyll. They cannot be seen by the human eye as long as chlorophyll is present in the leaves.

Carotenoids take longer to break down than chlorophyll, as it is a more stable compound, according to the U.S. Forest Service. Waskiewicz explained how this comes into play in the fall.

“As the green chlorophyll gets broken down and its components get reabsorbed, you see a yellow. That’s carotenoids. Those are also going to get broken down but it takes longer (than the chlorophyll), so they sort of linger there and all else equal, you get this period of the leaf appearing yellow,” he said.

Yellow from carotenoids serves as the base color, in a sense. Anthocyanin is then added to complete the palette. Waskiewicz detailed that aspect of the autumn colors process.

“Anthocyanin are the reds. Mixed with the carotenoids, you get the orange color. Those are created while the lingering chlorophyll that hasn’t yet been broken down is still photosynthesizing and still producing sugars. The anthocyanins are sort of a byproduct of that. But because the tree is starting to cut the leaf off, the chlorophyll that is still waiting to be disassembled is still producing these sugars, but then those sugars are not being respired because it is cold, or they’re not being exported back into the tree because the tree is starting that process of cutting the leaf off,” he said.

The more anthocyanin is in a leaf, the more red, or even purple in high enough concentrations, it appears. The less of it, the more orange or yellow the leaf looks. While weather impacts this color spectrum each year to a degree, much of the palette is pre-determined by the tree species’ genetics.

It’s why some trees, such as red maples, regularly turn brilliantly red, while other species tend to produce less anthocyanin and appear only predominantly yellow before falling.

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Summer’s impact

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Waskiewicz explained that while early fall weather matters most for fall colors, summer conditions can have an impact.

“One way weather in the summer will affect foliage is diseases. There’s a huge host of all your endophytes that are fungus live in the leaf. When you hold a leaf, you’ve got dozens and dozens of different kinds of organisms in your hand. Some of those are harmless, some can hurt the tree,” he said.

“It can also depend on the conditions the leaves and the organisms inside of them receive,” he added. “Something that might have been harmless could begin to eat the leaf if you get warmer and wetter conditions, for example. Those in turn will affect the color you get or don’t get if the leaf were to just turn spotty and brown before falling.”

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All eyes on 2024

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Waskiewicz was optimistic about the weather conditions during this year’s critical period for foliage. High pressure in place throughout much of this week will result in abundant sunshine and cool nighttime temperatures, according to forecasts from the National Weather Service.

“It’s still early but it’s looking like the next week or maybe even two would be the right recipe looking at the weather reports. If that continues, this year could be quite nice,” he said.