Why Do Leaves Turn Brown?

The longer night as summer turns into fall is responsible for the changing color of leaves in the autumn. As days grow shorter, the longer and cooler nights trigger biochemical changes in the leaves that turn them different colors, according to the U.S. Forest Service. Chlorophyll, the chemical that makes leaves green, decreases in response to shorter daylight hours, as other pigment-making chemicals appear more prominently.

The other pigment-generating chemicals include carotenoids and anthocyanins. Carotenoids, or yellow, orange and brown pigments, are present all summer, but the vibrancy of chlorophyll’s green color blocks them from showing. These carotenoids are the same chemicals that make daffodils and bananas yellow and carrots orange. As chlorophyll decreases, the yellow, orange and brown hues become visible. Anthocyanins develop in the leaves as they shut down in preparation for falling off. Anthocyanins contain the red and blue colors in strawberries, apples, plums and blueberries. Deciduous trees also prepare to survive winter by cutting off the food supply to their leaves. Once the leaves are walled off, they can fall.

Each tree species turns a different color, and each species turns the same color every year. The timing of leaf color and leaf fall is also dependent on the tree's species, because whether the tree lives in the northern or southern U.S., each species turns colors and drops its leaves at the same time.