Fall's Finest: Appreciating Autumn Through the Eyes of an Arborist

The poet William Cullen Bryant called autumn "the year's last, loveliest smile" and there's no question that Central Park in the fall can bring a smile to any visitor's face.

But tree experts (known as arborists) at the Central Park Conservancy observe this yearly spectacle with a deep appreciation for nature's handiwork, taking note of how Central Park's unique conditions cause its trees to behave differently than other deciduous trees during this dazzling season. (Fun science fact: The word "deciduous"—what we call trees that shed their leaves annually—comes from the Latin "to fall off.")

We sat down with Alan Clark, a Conservancy arborist, to take a glimpse into the life of Central Park's changing leaves.

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Why do leaves change color?

As Alan says, the changing hues of a tree's leaves mark their "last hurrah" before going dormant for the winter.

He explains that around this time every year, a tree will begin shutting down its food-making process, called photosynthesis, to prepare for the seasons ahead. It will store excess energy in its trunk, since during the dry, short, winter days there won't be enough light or water for it to convert chlorophyll—a bright green chemical—to sugar.

Once the trunk has filled with the necessary moisture and sugar to survive the winter and reach spring with adequate nutrition, the tree will seal off its leaves (a process called "abscission") to prevent taking in any excess energy via photosynthesis. Then the leaves will begin to change. The colors you see in an autumn leaf are the result of certain always-present chemicals (carotenoids, flavonoids, and anthocyanins) becoming visible as the leaf's chlorophyll decreases and breaks down. Soon after, you'll find those leaves carpeting the ground.

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What makes Central Park's trees different?

It is a misconception that temperature and moisture are the only influences that determine when leaves begin to change color in autumn. They are important, but as Alan explains, the tree's exposure to light plays a significant role as well. As days become shorter and shorter, trees will detect the oncoming winter not necessarily by colder temperatures, but by the angle at which the rising and setting sun hits its leaves. Certain angles signify the beginning of winter, at which point the tree will begin its preparations.

This has a unique effect on Central Park's trees, which are often obscured by many shadows cast by surrounding buildings. We are incredibly lucky to have this green oasis in the center of our busy metropolis, but as a result of the light and temperature gradient, the trees often have a distorted sense of the seasons. As a result, Central Park will often experience a later foliage turn than other parts of New York.

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What if you miss peak foliage?

These varying factors that contribute to fall colors make it hard to predict a "best time" to witness peak foliage. But even if some trees have shed their leaves for the winter by the time you get around to seeing them, there's still a beauty to them.

Alan has some advice on how to appreciate a tree that's already gone dormant for the winter. To an arborist, abundant leaves often obscure parts of the tree that can be particularly arresting—namely the branches and shape of the canopy. (Alan recommends the "bow branches of a maple," "gothic canopy of a locust," and "conical architecture of an oak.") These characteristics differ for every tree species, providing a beautiful excuse to look up even after the leaves have fallen down.