The dropping of leaves by deciduous trees is called abscission. It occurs on the cusp between autumn and winter, as part of an arc of growth, maturity, and renewal. In spring and summer, leave cells are full of chlorophyll, a bright, green substance that absorbs sunlight, fueling the process that converts carbon dioxide and water into the starch and sugar that allow the tree to grow. But at the end of the summer, as the days grow shorter and the temperature falls, deciduous trees stop making food. In the absence of sunlight, it becomes too costly to maintain the machinery of growth. The chlorophyll begins to break down, revealing other colours that were always present in the leaf, but which were masked by the abundance of green pigment: oranges and yellows, derived from carotene and xanthophyll. Other chemical changes take place to create red anthocyanin pigments. The exact mix is different for each tree, sometimes producing bright yellows, oranges, and browns, and sometimes displaying as reds or purpose.
But while this is happening, a layer of cells is weakening between the stem and the branch: this is called the abscission zone. Gradually it severs the leaf from access to water, and the leaf dries and browns and in most cases falls off, either under its own weight or encouraged by wintery rains and winds. Within a few hours, the tree will have released substance to heal the scar the leaf has left, protecting itself from the evaporation of water, infection, or the invasion of parasites.
Even as the leaves are falling the buds of next year’s crop are already in place, waiting to erupt again in spring.
From Katherine May’s Wintering: The Power of Rest and Retreat in Difficult Times.