Autumn Leaves

New Jersey is one of the Northeast’s great wonderlands of autumn color. We owe our brilliant fall leaves to geography.

“When it’s warm in the daytime, trees produce a lot of sugars,” says Don Donnelly, the Hunterdon County horticulturist. “But we also have cool nights, which trigger the plants to begin cutting off the flow of nutrients from the roots. That’s when you get the most brilliant colors.” New Jersey can even claim the poet laureate of trees, Joyce Kilmer, who was born in New Brunswick and is now honored, as only the Garden State can honor its great citizens, with a rest stop on the New Jersey Turnpike.

In spring and fall, leaves absorb carbon dioxide from the air and get water through the tree’s roots. Leaves contain a green-pigmented chemical called chlorophyll, which absorbs energy from the sun and uses it to convert the carbon dioxide and water to glucose, the sugar that supports the tree’s life. In this process, called photosynthesis, chlorophyll breaks down and is constantly replaced.

People assume that cold air triggers the change of colors, and it does play a part, as does the amount of moisture in the leaves. But more important than either are the shorter days as fall heads toward winter. Without adequate light, photosynthesis slows and finally stops.

When daily darkness reaches a certain threshold, cells at the juncture of stem and branch, called the abscission layer, begin to dry and turn corky, cutting (3a) off the flow of moisture in and glucose out of the leaf. Without sufficient water or light, chlorophyll production slows and the green fades away, exposing the carotenoid (orange pigmented) and xanthophyll (yellow pigmented) cells that were present all along but in summer had been dominated by chlorophyll’s green (3b).

Glucose trapped in the leaves is converted into a group of red and purple chemicals called anthocyanins. The formula for brilliant colors is to have warm, sunny days, so that leaves continue to produce glucose, followed by chilly nights that promote production of anthocyanins. (Freezing temperatures, however, prevent anthocyanin formation).

As fall progresses, the cells in the tree’s abscission layer become drier and more brittle. Eventually they no longer can support the weight of the leaf, which drops to the ground. Trees in winter are like hibernating animals: They slow down their life processes and live on glucose they stored during the warm months. In spring, when the days lengthen and the sun strengthens, buds open and photosynthesis resumes.

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