Trees stop the production of green pigment chlorophyll, revealing carotenoid pigments that give way to yellow. Anthocyanin pigments are responsible for purple and maroon colors of leaves in fall. Fallen leaves can be raked and used as mulch in landscape beds and gardens or composted.
—LSU AgCenter/Heather Kirk-Ballard
Get It Growing: The science of fall foliage
Shorter, cooler days bring brilliant foliage color changes for many trees and shrubs in fall.