The Colors of Fall: An Expert Arborist Explains What Makes Local Fall Foliage Pop

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No need to go farther than Polly Hill Arboretum for breathtaking colors.

As the days get shorter and the nights colder, stands of muted-green roadside oaks morph into bright patterns of reds, oranges, and yellows.

Tim Boland holding tree.
Tim Boland describes how pigments in fall foliage determine color. -Photo by L. Thors.

It happens fast here on the Vineyard — trees and shrubs that one day are the same green they’ve been since June can shift into warm-toned abstract art overnight. Of course, leaf peepers revel in this time of year, but for Tim Boland, Executive Director of West Tisbury’s Polly Hill Arboretum (PHA), there’s more to the excitement than merely when local flora dons its fall foliage. “We’re still about ten days away from peak leaf season,” Boland explained as he greeted me at the arboretum’s visitors’ center on Oct. 23, which puts us pretty close to peak as you read this. 

Because PHA imports trees from all over the world, the full majesty of eye-catching foliage was on display in late October. Some trees were wearing their dark orange and brown leaves, while others still wore their warm-season foliage. “We have trees that are very hardy, and others that like their leaves to fall early,” Boland said.

 Boland noted that this year — as often happens on the Vineyard — the fall foliage season is late, due to the moderating effect of the moist ocean breezes, which lead to relatively warm temperatures and high humidity. “Plants are like ‘I don’t think it’s time to go to bed quite yet,’” Boland laughed. “That’s my plant impersonation.”

Although our maritime climate delays fall colors from popping, some trees make up for it with particularly brilliant leaves. Boland identified some standouts for me as we strolled around the PHA campus. “This is a stunning scarlet oak,” he indicated. “It’s one of the best in terms of fall color.” The hued boughs of the scarlet oak stood in stark contrast to a nearby black oak, which Boland said will later shift abruptly from green to a muted yellow.

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The reason some plants turn into Jackson Pollock paintings in the fall, while others skip the art show altogether, comes down to pigments. Plants track the seasons by using photoperiods (day length). Photoperiods dictate how much chlorophyll (the green pigment involved with photosynthesis) their leaves produce. During the summer, plants fill their leaves with chlorophyll, so as to absorb as much sunlight as possible.

As the days get shorter, plants begin to slow their production of chlorophyll, and the other pigments in the leaves of trees like the deciduous PHA kousa dogwood begin to shine through. Scientists theorize that such trees do this mostly to conserve energy during the months in which they receive less sunshine, according to Boland. As the season progresses, deciduous trees (trees that shed their leaves in the fall) begin a process called abscission.

Boland picked up a broad white oak leaf and pointed to the base of the stem. “This is called the abscission zone. This waxy substance called suberin develops between the leaf stem and the branch that cuts off nutrient transfer to the leaf,” Boland explained. “With a shorter photoperiod and cooler temperatures, there is no reason, from an evolutionary perspective, for the plant to photosynthesize.” As chlorophyll diminishes within the leaf, so does the summertime green. This is what paves the way for fall color. 

Stewartia tree with patterned bark and bright leaves.
The bark of the stewartia tree adds another layer of beauty to its fall foliage. -Photo by L. Thors.

But what pigments in a leaf make for such a brilliant fall color show? Boland explained that there are two different pigment groups. One, called carotenoids (think of the color of carrots), is present all year long in tree leaves, and is responsible for the oranges and yellows once chlorophyll production declines. As we walked through the winding paths of PHA, Boland pointed to some low lying shrubs that were showing off their carotenoids. American witch-hazel (often found in lotions and cosmetics) was blazing with them.

The other category of pigments seen in fall foliage are called anthocyanins. They give leaves the bright reds that are quintessential for this time of year. Unlike carotenoids, anthocyanins do not always exist within a leaf. In fact, anthocyanins remain a bit of a botanical mystery. According to Boland, these pigments are the result of sugars trapped in the leaf after abscission. Scientists have developed a few theories as to why anthocyanins might benefit a plant. One hypothesis is that the anthocyanins may act like a sunscreen to protect vulnerable leaves as they lose chlorophyll. Another proposes that plants produce anthocyanins in part to ward off insect attacks.

In addition to vibrant red huckleberry, brilliant anthocyanin specimens at the arboretum include scarlet Japanese maples and dreamy bronze tall stewartia. Native sassafras provided a good example of a tree with both carotenoids and anthocyanins. And because sassafras leaves are trimorphic (three different shapes), they turn into works of art. Of course, there are a few trees displayed at the arboretum that aren’t native to the Vineyard but still offer great leaf-peeping. We sauntered by a shimmering Japanese maple with lichen-covered bark. “You see, this maple is starting to senesce. It doesn’t happen with all trees, but for many, early fall color is a sign of stress. It makes you wonder what that tree is going through.”

The trees weren’t all that offered beauty at the arboretum. The field at PHA, filled with switchgrass and little bluestem, looked like a camouflage blanket with pinks and reds swaying in the breeze. “The little bluestem is that really iconic pink-looking grass that is very abundant here. It’s a great grass to replace your lawn with if you are looking to make a meadow,” Boland said. 

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Lucas Thors
Lucas Thors
Lucas Thors is an associate editor for Bluedot Living and program director for the Bluedot Institute. He lives on Martha's Vineyard with his English springer spaniel, Arlo, and enjoys writing about environmental initiatives in his community.
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