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Why Do Trees Lose Their Leaves
in the Fall?
CLASS NOTES from Science Prof Online
Article Summary: Unlike herbaceous plants, deciduous trees stand tall all winter, minus some important accessories -- leaves. What trigger causes leaves to fall in the autumn?
Why Do Some Trees Drop Their Leaves in Autumn?
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.
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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.
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Snow covered branches of a deciduous tree during Michigan 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.
Shutting Down Operations for Winter
As winter approaches, the days get shorter and cooler. This change in day length and temperature triggers some trees to go dormant, and hibernate for the winter.
