The old leaves, buds, and inner bark all have a pigment that reads the seasons of the year. As the days shorten in fall, this pigment called phytochrome, tells the tree to close down for the winter. Getting ready for winter in a organized way is called senescence. Within a tree, a message is sent from the tissues with phytochrome, which signals senescence. Senescence in trees is an ordered shutting-down of summer growth and the conservation of valuable resources.
Senescence brings both the fall colors and leads to renewed spring growth.
Tree waste materials are left behind. The last bit of tree food is stockpiled in the living cells of the outer annual growth rings. Twigs, branches, and roots become the collection sites and warehouses of materials needed for another season to come. Within the tree, biological doors and windows are being closed and locked. From the moment last Spring's green leave expanded and began to make food, winter dormancy has been designed into the tree system.