To understand why leaves fall off, it helps to first understand what the job of a leaf actually is.
Leaves Are Plant Food Factories
Plants and other photosynthesizing organisms have a very special talent. They can turn sunlight into food. It is a pretty neat trick that only photoautotrophs can do (photo=sun; auto=self; troph=feeder).
In order for plants to make food energy, they need water, carbon dioxide (CO2) and sunlight. From this special combination, a plant is able to make its own food, in the form of glucose, a type of sugar. Plants then use the glucose as food energy to live and grow. In order to harvest sunlight energy, plants have a green pigment called chlorophyll. This pigment is what makes a plant’s leaves appear green.
Shutting Down Operations for the Winter
As winter approaches, the days get shorter and cooler. This change in day length and temperature triggers some trees to go dormant, essentially hibernating for the winter. A tree’s woody roots, branches and twigs can endure freezing temperatures, but most leaves are not so tough.
It is also very energetically expensive for a tree to run its leafy food factories in the winter, when there is often little sunlight and freezing temperatures make water transport (from the ground into the tree’s trunk and leaves) a problem. So it’s more energy efficient for a leafy tree to close down operations in the winter and go dormant.
How Leaves are ‘Told’ to Drop
A tree is full of vascular cells that transport water and sap throughout, from root to leaf tip. As the amount of sunlight decreases in autumn, the veins that transport sap into and out of a leaf slowly close off. Then a layer of cells, called the separation or abscission layer, develops at the base of the leaf’s stem. When this layer is completely formed, the leaf falls off.
This process happens in all deciduous trees (trees that annually shed their foliage), with oak leaves as a notable exception. In oaks, the separation layer doesn’t fully allow the oak leaves to detach. That’s why most dead oak leaves remain on the tree through winter and even into early spring (much to the perpetual leaf-raking consternation of home owners with oak trees on their property).
Why Don’t Evergreens Drop their Leaves?
Evergreen trees include pines, spruces, cedars and firs. These trees don’t lose their leaves, or needles, in winter, because they employ a different survival strategy than do deciduous trees.
The needles of evergreens are covered with a heavy wax coating to help prevent moisture loss, and the fluids inside the cells contain substances resistant to freezing, essentially evergreen antifreeze. Evergreen leaves can live for several years, through all four seasons, before they are dropped and replaced by new growth.
More Information on Plant Growth Strategies
To learn more about the different growth forms and survival strategies that trees use see the New World Encyclopedia and Backyard Nature.