Why do leaves change color in the fall?

By Ben Olson
Reader Staff

With the cooler temperatures of late, evidence of fall is everywhere. This time of year, the leaves of deciduous trees begin their magical transformation—exploding in a phalanx of autumnal color that dazzles the eye before they fall to the earth.

Have you ever wondered why exactly a leaf changes its color? Have no fear, dear readers, we’ve got you covered.

To understand the changing colors, you first must understand photosynthesis—the process plants undergo that uses sunlight to synthesize glucose (and oxygen) from carbon dioxide and water. The plant uses glucose as a food and as a building block for growth. We breathe the oxygen that is created as a byproduct.

A chemical called chlorophyll—which gives plants their green color—helps make photosynthesis happen.

As the days grow shorter and the temperature drops, trees begin to “prepare” for winter. Since there is not enough light or water for photosynthesis to take place in winter, the trees will go into a state of hibernation, living off the food they stored during the summer. The food-making factories begin to shut down. Chlorophyll breaks down and the green color disappears from the leaves.

As the green color fades away, yellow and orange colors begin to emerge. Small amounts of these autumnal colors have been in the leaves all along, but we can’t see them in the summer because they are covered up by the green chlorophyll.

When leaves turn bright red and purple, it means glucose is trapped in the leaves after photosynthesis stops.

As fall colors appear, other changes are taking place. At the point where the stem of the leaf attaches to the tree, a special layer of cells develops and gradually severs the tissues that support the leaf. At the same time, the tree seals the cut so that when the leaf blows off in the wind, or falls from its own weight, it leaves behind only a leaf scar.

While most conifers like pine, spruce, fir, hemlock and cedar retain their needles and stay green year round (hence the name evergreen), some species such as tamarack and larch lose their needles after turning brilliant shades of yellow.

Many factors influence the brilliance of fall colors, including temperature, light and water supply. Low temperatures above freezing will produce brighter reds in trees that trap glucose in the leaves while early frost will weaken the brilliant colors. Rainy or overcast days are also said to increase the intensity of fall colors.

Curious where to see the best fall colors around North Idaho? The International Selkirk Loop travels through North Idaho into British Columbia, passing through great fall colors against the backdrop of the Selkirk, Cabinet and Purcell Mountain Ranges. Further south, the St. Joe River Scenic Byway is also a great 89-mile route through a kaleidoscope of hues.

No matter where you go, don’t forget to stop and appreciate the natural fireworks display that marks the end of our glorious summer season here in the panhandle.