The science behind High Country fall foliage

Thinking Outside the Classroom by Keystone Science School

Carrie Scheick
Keystone Science School program coordinator
Aspen leaves pictured Sept. 15, 2018, in Breckenridge. Warm, sunny days with cool (not freezing) nights lead to the most spectacular color displays.
Photo by Hugh Carey / Summit Daily archives

As the colors of the wildflowers give way to the changing colors of the leaves, we know fall has arrived in Summit County. Our High Country’s forested landscape may be dominated by evergreen trees, but it’s the deciduous trees that draw people here during this time of year. Deciduous means “fall off,” and these tree species annually lose their leaves on a seasonal basis. Here in the mountains, we are primarily known for our aspen trees.

So what happens at this time each season to create such beautiful fall foliage? Let’s start by going back to elementary school where we learned all about plants. Plants use sunlight to create their own food in a process called photosynthesis. Chlorophyll, a pigment found in plants and other photosynthesizing organisms, is essential in this process and is what gives plants their green color. 

As the length of daylight and temperatures slowly change, the leaves stop photosynthesizing and no longer put effort into this energy-intensive, food-making process. The chlorophyll in the leaves breaks down, and the green color disappears, giving way to the vibrant yellows and oranges we associate with fall. Other chemical changes within the leaves create the reds and purples that are seen in other parts of the country. 

The trees conserve their energy by having this color-changing process take place before the leaves fall off. They move the chlorophyll out of their leaves and reabsorb the molecules that create the chlorophyll so that next spring they don’t need to create it from scratch. Pretty smart, huh?

The deciduous tree species we have in Summit County are the thin-leaf alder, aspen, waxleaf and narrowleaf cottonwood, and balsam and white poplars. We often think only of the aspens creating our fall foliage, but these other native species also contribute to the beautiful yellow and orange colors we enjoy. 

The duration of the season and the vibrance of the colors are largely determined by weather conditions throughout the summer and beginning of fall as the chlorophyll begins to break down. Temperature, light and moisture all play a role in this process. Warm, sunny days with cool (not freezing) nights lead to the most spectacular color displays. The leaves produce sugars during the day that then stay in the leaves overnight. A lot of sugar and light lead to the creation of the pigments within the leaf’s cells, which produce the brilliant colors. Higher amounts of precipitation and soil moisture during the spring and summer prior also tend to intensify the colors. 

The majority of the colors you’ll find in the High Country will be shades of yellow and orange. Thin-leaf alder leaves will be yellow as well as the poplar leaves; balsam poplars turn a darker yellow while white poplars are a paler yellow. Both the narrowleaf and waxleaf cottonwoods turn yellow and orange. Of course, we can’t forget the iconic aspen trees that have bright yellow leaves with some orange and light red mixed in, as well. 

What are you waiting for? Grab a camera and a friend, and get outside to leaf peep! How many species can you find? 

Thinking Outside the Classroom is written by the staff and volunteers at the nonprofit Keystone Science School.

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