The science behind fall colors

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All seasons have their pleasures, but fall is the family favorite, and not only because of Halloween. These rich autumn palettes are reminiscent of life itself — its cyclical patterns, its breadth and variety firing a blast of rich color before the big sleep.

We’ll soon see the pinnacle of fall colors in North Idaho. While the leaves of some deciduous trees are still green, others soon blanket the ground. We are treated to its splendor — a cornucopia of red, orange, yellow, purple, browns and — my favorite — trees sporting them all simultaneously. Often beginning at the top, with the changes working their way down over time.

But are they really changing, or just removing their summer coats?

The prettiest leaves have multiple colors, green and red, splashes of yellow and orange with a smattering of brown, interspersed or lining the edge. How can this be?

Light, falling night temperatures, and exposure.

Leaves are green because they contain chlorophyll; think of it as fuel. This extraordinary chemical absorbs energy from sunlight, transforming carbon dioxide and water to carbohydrates. Green leaves are essentially food factories.

Chlorophyll’s dominant green effect also acts as a curtain, hiding the leaves’ other pigments (e.g., carotenes and xanthophyll). So when autumn’s reduced sunlight and colder temperatures (timing generally depends on nighttime lows) break down the chlorophyll, the green fades and previously masked yellows and oranges gradually emerge. Anthocyanin pigments make those deep reds and purplish colors in some trees.

Each species is chemically different, as are amounts of chlorophyll residues even in individual trees and leaves, so the range and shades of visible pigments vary. That colorful diversity is what makes fall so beautiful.

Why do leaves fall as autumn fades to winter? It’s rather like a haircut. At the point where the leaf stem meets the tree, a layer of abscission (think “scissor”) cells develops which severs the tissues supporting the leaf. Before it falls, the tree has already sealed the exit point, leaving a little scar and preventing fluids from flowing past it.

Why shed leaves at all? To conserve precious resources to survive winter. Think of it as letting go of the kitchen staff in lean times, once the larder is full.

Temperature, light, and water supply have an influence on the degree and the duration of fall color. Low temperatures (above freezing) often mean more reds, but early frost weakens that. Rainy and/or overcast days tend to increase the intensity of fall colors, so if gray autumn days depress you, remember they offer in compensation this stellar artistry to enjoy.

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Sholeh Patrick is a columnist for the Hagadone News Network whose favorite month is October. Contact her if you agree at Sholeh@cdapress.com.

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