Ask A Naturalist: Why Do Leaves Change In Fall?

If you take a walk through Pittsburgh's parks in autumn, you’ll see an amazing array of colors as fall leaves change and drop.

Yellows, reds, and even purple leaves mix together as they make their way to the ground, making a lively mix of fall beauty. Trees that drop their leaves are called deciduous, and they are a great way to learn about the relationship of trees and their surrounding environment.

Leaves start out as green because they have a pigment called chlorophyll, which acts as a natural solar panel to get energy from the sun. Plants use this solar energy to combine carbon dioxide and water, and turn it into sugar and oxygen – a process called photosynthesis.

Leaves drop in the fall because the days get shorter, colder, and drier as winter approaches, and there is not enough light or water for leaves to support photosynthesis. As this process slows and eventually stops, the leaf closes down and eventually drops. Water in the leaves freezes and damages the leaves, adding to their eventual demise. While this is happening, the leaves also change color.

Some of the vibrant colors we see - yellows, oranges, and browns - are always there, just hidden by the green chlorophyll.  If you are out in our parks in the spring just as leaves are coming out, you’ll see that some of them have a yellow or orange hue. These pigments - called carotenoids - are still there after the chlorophyll dissolves, and the yellow, orange, or brown colors are revealed.

Bright reds, purples, and blues are caused by different pigments called anthocyanins which take the place of chlorophyll in some leaves. They combine with the carotenoids to create the most vibrant colors of fall leaves.

Take a walk through any of Pittsburgh's parks to see this amazing color change in autumn. Two trails that Pittsburgh Parks Conservancy staff love are Falls Ravine Trail in Frick Park, which has lots of yellow hickories, orange sugar maples, and bright red oaks and the trails like Upper Panther Hollow and Steve Faloon in Schenley Park, which has views of the ravines in the park, as well as fiery orange maples.