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Why Do Some Trees Drop Leaves in Autumn? - P2
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How Leaves are ‘Told’ to Drop
A tree is full of vascular plant 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.
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.
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.
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Above: Staghorn sumac (Rhus typhina) leaves changing color, before dropping, in Michigan autumn. Below: Time lapse of maple leaves falling.
That’s why most dead oak leaves remain on the tree through winter and even into early spring (much to the leaf-raking frustration of home owners with oak trees on their property).
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:
- Growth Form and Life History Strategies of Plants from Rutgers University
- Quick & Easy Habitat Education Activities: Evergreen and Deciduous Leaves by Starflower Foundation
- Deciduous vs. Coniferous Survival Strategy from the Roaming Naturalist.
