Why Do Fall Leaves Change Color?

Brian Handwerk
for National Geographic News
October 8, 2004

Fall foliage delights leaf-peeping tourists. But how does the change in color benefit trees? As scientists explain, there is a reason for the season.

John Shane, chair of the University of Vermont's forestry program, notes that the increasing darkness in the Northern Hemisphere this time of year "indicates to the plant that fall is coming on. So it starts recouping materials from the leaves before they drop off."

Evergreens protect their needle-like foliage from freezing with waxy coatings and natural "antifreezes." But broadleaf plants, like sugar maples, birches, and sumacs, have no such protections. As a result, they shed their leaves.

But before they do, the plants first try to salvage important nutrients such as nitrogen and phosphorus.

The process treats leaf-peepers to riotous displays of autumn color, as green leaves turn into brilliant or muted shades of gold, orange, yellow, and red.

Inside the Leaf

Chlorophyll gives leaves their green color throughout the growing season. The compound is essential for photosynthesis, a chemical reaction that converts sunlight into carbohydrates.

Leaves also contain carotenoids. These natural pigments, which produce yellow, orange, and brown hues in plants, from buttercups to carrots, are always present.

The colors of carotenoids are easily masked by green chlorophyll, at least until shrinking daylight and a nip in the air signal fall's arrival. At that time broadleaf plants slow and eventually stop their chlorophyll production, thus revealing the distinctive golden, orange, and yellow hues of carotenoid pigments.

What is Mother Nature's recipe for an especially spectacular season?

Since daylight wanes at a constant rate each fall, other factors, like soil moisture and weather, ensure that no two autumns are alike.

Continued on Next Page >>


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