Why do leaves change color in the fall?

Leaves look green in summer because chlorophyll — the pigment that captures sunlight for photosynthesis — is constantly being produced and broken down, masking the other pigments already in the leaf. As days shorten and temperatures drop in autumn, trees slow chlorophyll production and prepare to drop their leaves for winter. Without fresh chlorophyll, the green fades and the yellows and oranges that were hidden underneath become visible.

The reds and purples are a different story. Those come from anthocyanins, which the tree actually produces in autumn — they are not just unmasked. The leading theory is that they help protect the leaf’s remaining nutrients as it shuts down, or make the leaf less appealing to insects preparing to overwinter in fallen leaves. A dry, sunny late summer followed by cool fall nights tends to produce the most vivid reds.

Once the useful pigments and nutrients are pulled back into the branches, the leaf detaches at its base and falls. What remains on the ground is mostly the parts the tree no longer needs — which is why autumn leaves decompose faster than spring growth.