As temperatures cool and daylight hours grow shorter, trees across Pennsylvania begin their annual transformation, turning hillsides and forests into displays of yellow, orange and red.

While many people associate autumn with colorful foliage, the pigments responsible for those vibrant shades have been present in leaves all along. During the growing season, they are largely hidden by chlorophyll, the green pigment that helps plants convert sunlight into food.

As fall arrives, chlorophyll begins to break down, revealing other pigments and, in some cases, allowing new ones to form.

Green: chlorophyll

Chlorophyll gives leaves their green color throughout spring and summer and plays a vital role in photosynthesis, the process that allows trees and plants to convert sunlight into energy. As trees prepare for winter and food production slows, chlorophyll fades, making way for other colors to emerge.

Red: anthocyanin

Unlike other pigments, anthocyanins are produced in the fall as chlorophyll breaks down. They are responsible for the brilliant red colors seen in trees such as red maples and scarlet oaks, as well as red sumac. These pigments create some of the season’s most striking displays.

Orange: carotene

Carotene pigments create shades of orange in leaves. Sugar maples are among the best-known examples, often lighting up forests and roadways with brilliant orange foliage. Carotenes are also the pigments that give carrots their familiar orange color.

Yellow: xanthophyll

Xanthophyll pigments produce yellow hues found in trees such as beeches, ashes, birches and aspens. The pigment is also responsible for the yellow coloring seen in crops such as corn and some varieties of squash.

The most vivid fall foliage often occurs when late summer is dry and autumn brings sunny days paired with cool nights. Those conditions help create the brilliant colors that attract leaf-peepers each year and make autumn one of nature’s most colorful seasons.