By Kathy Baugh
It’s sweater weather again — or, as the SNL skit famously put it, “Sweata Weatha.”
For many of us, those first cool nights of September bring mixed emotions. There is the excitement of pulling sweaters from the closet and watching the landscape slowly transform into a spectacular palette of yellow, orange, red, and purple. But there is also the knowledge that winter is waiting in the wings, bringing snow, ice, and those long days when we spend more time indoors.
Before we get too far ahead of ourselves, though, let’s pause to appreciate one of New England’s greatest seasonal spectacles: fall foliage. Each year visitors from near and far come to the Berkshires to witness the landscape’s remarkable transformation. But what, exactly, causes the leaves to put on such a dazzling show?
The answer lies in the chemistry of the leaf.
In autumn the normally green leaves of many deciduous trees and shrubs turn various shades of yellow, orange, red, purple, and brown. Two processes are largely responsible: the breakdown of chlorophyll, the pigment that gives leaves their green color, and, in some species, the production of anthocyanins, which produce shades of red and purple.
The yellow and orange colors are not necessarily new. They come from pigments called carotenoids, which are present in leaves throughout the growing season but are masked by chlorophyll’s dominant green. As long as chlorophyll is abundant, the green color prevails. When chlorophyll breaks down in the fall, the underlying yellow and orange pigments are revealed.
The reds, purples, pinks, and the many shades in between are produced by anthocyanins. Unlike carotenoids, anthocyanins are not generally present in leaves throughout the growing season. Instead, they are produced in many species as summer gives way to autumn.
The timing of these changes is influenced by several environmental factors. As the days grow shorter and temperatures cool, trees begin preparing for winter. Chlorophyll production declines, and the existing chlorophyll breaks down. Eventually, a process known as abscission separates the leaf from the tree, allowing it to fall.
Different species — and even individual trees among the same species — time these processes differently. That variation helps explain why the autumn display unfolds gradually rather than all at once, giving us the roughly two-month light show we have come to cherish here in Becket.
Weather also plays an important role in determining just how brilliant that show will be. Bright, sunny days and cool nights can encourage the production of anthocyanins and lead to particularly vivid reds and purples. In general the most spectacular displays occur when autumn days are bright and cool and nights are chilly but remain above freezing.
So, as we pull on our sweaters and watch the first hints of color appear, there is more going on than meets the eye. Every brilliant red maple, golden birch, and orange oak is part of a carefully timed biological process — a seasonal transition that has made New England a destination for leaf peepers from around the world.
Bottom line: our fall foliage is second to none, and it is an event well worth slowing down to relish.
