Why Do Leaves Fall in Autumn?

Why Do Leaves Fall in Autumn? 

In temperate regions of the world, autumn is marked by the brightly colored foliage that slowly drops from trees and shrubs to carpet the ground. But why do some plants shed their leaves before winter? It turns out autumnal leaf drop is a form of self-protection. While evergreen plants in cold climates have thick waxes and resins to protect their leaves from freezing and fracturing, deciduous species generally have thin leaves that are susceptible to cold temperatures. Because water expands when frozen, soft leaf cells burst during the winter, rendering them unable to photosynthesize. Without shedding these leaves, such a tree would be left with thousands of non-reproductive appendages and no means of making food! As if that weren't enough, the surface of all those leaves will also pose a threat to the physical integrity of the plant. The winter months are often windier than other seasons, and wind blowing against the cold, brittle leaves of trees can cause significant breakage. The same goes for the weight of snow accumulating on all those leaves. Finally, at the end of summer, many leaves are eaten by insects, diseased or damaged. Letting them fall gives the plant a fresh start in spring, and nutrients from the decaying leaves are recycled to help grow the next generation of leaves. 

Interestingly, autumn leaves are not simply blown off the tree by the wind, but are separated from the tree in a tightly controlled process. As day lengths shorten and temperatures cool, plant hormones are activated to begin the abscission process. Chlorophyll production stops and pigments begin to break down, often revealing vivid reds and yellows obscured by greens. The vessels that carry water to the leaves and sugar to the rest of the plant are closed and a layer of cells, called the abscission layer, begins to grow between the leaf stem and branches to maintain it. These cells are responsible for cutting leaves slowly without leaving open wounds. When the leaves fall, the tree will hibernate, saving energy for large buds to bloom in the spring.