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A Moment of Science

Falling Leaves

Every autumn, citizens of the North American Midwest and Northeast enjoy a spectacularly colorful display as leaves turn various shades of red, orange, yellow, purple, and brown. But autumn also entails the backbreaking work of raking all those beautiful leaves once they fall. Why can’t leaves change colors, stay on their trees, and change back to green again when spring comes?

Unfortunately, the chemical processes responsible for changing leaf color also make leaves fall. Trees prepare for their dormant winter period by undergoing senescence. Senescence involves recycling nutrients from leaves into the plant body for use in the Spring. As trees enter dormancy, they cut back on chlorophyll production. Because chlorophyll is the chemical responsible for making leaves green, its absence allows chemicals such as carotinoids, xanthophylls, and others to give leaves their orange, yellow, and various other colors.

As trees stop producing chlorophyll, they start producing sugars and amino acids that act as a sort of antifreeze to protect plants from the winter cold. Meanwhile, nutrients drain from the leaves, through the stems, and eventually into the roots, where they remain for the winter. When the nutrients have left the leaves, all that remain are cell walls and nutrient-depleted protoplasm.

Since all that now holds leaves to a tree are brittle, dry, and easily broken veins, a gust of wind is enough to tear leaves from the tree and send them fluttering to the ground. When dead leaves break away, they leave behind waxy cells that form a small bud called a bundle scar. When warm weather returns, each scar will give rise to a newly born leaf.

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