Throughout spring and summer, the leaves of trees work as food factories. But there are deciduous trees that stop food production, experience color change, and lose leaves. Changes in these trees are the vibrant markers of fall.

Chlorophyll, a chemical pigment in leaves, absorbs sunlight energy to turn carbon dioxide and water into carbohydrates (FOOD!), mostly sugars and starches. Chlorophyll is the pigment that makes leaves green. Working with the chlorophyll are carotenoids and sometimes anthocyanins. These are additional fancy chemical pigments that help with photosynthesis.

Once fall arrives, less daylight and lower temperatures signal the trees to stop producing food. First, chlorophyll is broken down and reabsorbed for its valuable nutrients. This gives the other pigments the opportunity to be seen. Carotenoids are vibrant yellows, browns, and oranges and anthocyanins are a spectrum of reds. A variety of leaf colors are visible depending on the pigments left over.

The next step is for the leaves to fall off in a process called abscission. The trees lose leaves in an effort to conserve energy and water through the winter. Sensitive to the shortening days, trees slow the release of auxin and increase ethene. Both are regulating hormones sent to tree leaves. This causes cells at the base of the leaf to weaken while some are instructed to expand. The expanding and weakening of the base cells tear the leaf’s connection to the tree. The weakened connection is similar to perforated paper and helps the tree shed its leaves.

Fall is happening on campus and throughout the Arb right now! Take a stroll through the Arb, lookout for all the stunning colors, notice a few beautiful fallen leaves, and listen to the rustle and crunch of the leaves.