Nature’s Secrets

There's a lot of science behind fall leaf coloration

October 6, 2013 

If one really loves nature, one can find beauty everywhere.

Vincent van Gogh

There are a few awe-inspiring events hosted by Mother Nature that every child deserves to see – snow falling, sunset on a beach, birds flying, lightning ... and fall foliage.

The miracle of fall foliage is a special punctuation mark in the seasonal story of trees. Throughout the temperate forests of North America, red and yellow hillsides appear almost magical each autumn. In North Carolina, the onset of fall foliage spawns economic, emotional and biological activities. Foliage forecasts are featured on some local telecasts because these botanical events create major tourist attractions for visitors to our state.

Life within the phylloplane (“leaf surface”) is more complex than anyone but a botanist could imagine. A leaf represents one of nature’s most extraordinary and complex factories. It houses the only machinery on Earth that manufactures sugars from sunlight. Called chloroplasts, these small units of green tissue create energy which forms the basis of all life on our planet. Green plants, especially trees with their thousands of leaves, accomplish this highly productive process both quietly and efficiently: no fanfare, no noise, no toxic wastes – just nature’s chemistry.

The green coloration of foliage is derived from a pigment called chlorophyll housed within the chloroplasts of each green leaf. Chlorophyll is the main substance that leaves employ to absorb light for photosynthesis, that complex process in which plants turn sunlight into sugars as the basis of all food chains. During spring and summer, chlorophyll pigments mask the presence of orange, yellow and red pigments that are also present within leaf tissue. These accessory pigments assist in photosynthesis by capturing additional energy from sunlight that chlorophyll pigments can not efficiently absorb.

When fall temperatures become cooler, leaves stop manufacturing chlorophyll, and no longer reflect green light. This exposes the underlying pigments of orange and yellow carotenoids and red anthocyanins for a brief period before the leaves fall.

This protective process that allows trees to drop their leaves during winter – it’s called abscission – is essentially a shutdown valve that facilitates energy conservation for the tree. The end result of this chlorophyll die-back is hillsides of gorgeous fall coloration. Mother Nature is a practical and efficient manager of her foliage. Trees lose their leaves when conditions become harsh, and then grow a new set when spring arrives.

No human-constructed factories can compete with the elegance and efficiency of a leaf. Its machinery is the basis of all life on Earth. Its annual coloration is an inspiration for literature and art, as well as for science.

Meg Lowman, Ph.D., a forest canopy expert, is senior scientist at the N.C. Museum of Natural Sciences and research professor at N.C. State University.

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