Why do Leaves Change Color?

When the Leaves Turn: The Science Behind Vermont’s Colors

Mike Stock

Nature’s paintbrush is just about done sweeping across the mountains here at Merck Forest, and the hillsides are alive with fall colors. Standing among the sugar maples, oaks, and birches, it’s impossible not to marvel at the transformation - from deep greens to yellows, oranges, and reds.

Here’s how this magic happens. Most of the color is already there in the leaves, hidden beneath the green of chlorophyll, which dominates throughout the growing season. As days grow shorter and nights get cooler, trees begin preparing for winter. At the base of each leaf, an abscission layer forms, a special zone of cells that slowly shuts down the flow of water and nutrients. With the chlorophyll fading away, the other pigments are revealed: the yellows and oranges of carotenoids, and the rich reds and purples of anthocyanins, formed from sugars trapped in the leaves.

The most spectacular colors come from healthy, well-hydrated trees. Leaves that are deep green in summer often produce the brightest autumn spectacle. Stressed or sick trees may change color earlier, and drought can cause leaves to brown and shrivel, especially on trees growing in shallow soils. Wilt diseases can also curl leaf edges into brown tips, reminding us that fall colors are as much a story of resilience as they are of beauty.

Even some conifers join the party. The larch, or tamarack, is unique among needled trees; it turns a brilliant yellow before dropping its needles like a broadleaf tree. And here at Merck Forest, as you walk along almost any trail or peer out over the rolling ridges of the Taconic foothills, you’ll see these shifts happening across the landscape, from the sun-drenched slopes to the cool, shaded valleys.

Plenty of warm sunlight, adequate water, and cool—but not freezing—nights combine to create the richest display. It’s a delicate dance of light, temperature, and moisture, and when the conditions are right, Vermont’s forests put on a show like no other.

If you’re looking for a colorful hillside to enjoy, we hope you’ll explore Merck Forest. Our trails, ridges, and open fields are always ready to welcome you. Witnessing these changes in person is a reminder of the cycles of life and the resilience of the forests we steward every day.