Why fall leaves change color


Autumn is here, which means trees are about to put on a brilliant show. Find out why leaves change colors every fall, according to the U.S. Forest Service and the United States National Arboretum.

Trees need to protect themselves against the cold, harsh winter months. So they've either developed tough leaves that will survive the cold, like evergreens have, or they go through the process of shedding those leaves.

The bright green that leaves display in the summer is due to chlorophyll in the leaf. Chlorophyll lets trees turn sunlight into sugars through a chemical reaction called photosynthesis.

Trees are very sensitive to the amount of daylight they receive. As nights get longer, cells at the base of leaves start to divide, which starts to block the veins carrying carbohydrates and minerals between the leaf and the rest of the tree.

As the veins become blocked, chlorophyll production slows and then stops. The green hue fades from the leaf, allowing yellow xanthophyll and orange carotenoid pigments in the leaf to shine through.

Yellow xanthophylls and orange carotenoids are both always present in the leaf and help chlorophyll capture light energy, but their colors are hidden by the green chlorophyll for most of the growing season.

Blocked veins also trap sugars in the leaf. This lets red anthocyanin pigments develop.

Eventually, xanthophylls, carotenoids and anthocyanin pigments fade, too, leaving the brown tanins.

The cells blocking veins at the base of the leaf continue to sever the connection between the lead and the tree, eventually drying and weakening the connection enough that the leaf will fall.

Fallen leaves eventually decompose, helping restore nutrients to the soil and turning into food for organisms in the soil.

Temperature and rainfall can impact how brilliant colors are and the length of time leaves stay on trees.

Warm fall days and nights that aren't too cold let sugar develop in the leaf and stay there for a longer time, which will result in more vibrant reds. If fall is too warm, though, colors might be muted.

For more information, visit the U.S. Forest Service website at www.fs.fed.us or the U.S. National Arboretum at www.usna.usda.gov.