Why do leaves change color? The science behind fall foliage and best places to view around Summit County | SummitDaily.com

Why do leaves change color? The science behind fall foliage and best places to view around Summit County

Best places for leaf-peeping in the mountains

As the days begin to shorten and crisp mornings bring the anticipation of winter, Summit County takes one last breath of brilliance before succumbing to the snow. The explosion of reds, yellows and oranges from the fall foliage along the trails and across the peaks give the High Country breathtaking views almost anywhere traveled.


Although fall in the mountains is relatively short, the turning of the leaves can be counted on each year.

“There’s three factors that influence the leaf color,” said Adam D. Bianchi, deputy district ranger of the United States Forest Service — Dillon Ranger District.

Those factors are leaf pigments, the length of night and the weather, he said. “Typically we think that weather really affects the change, but realistically it’s more the length of the night. … Every calendar year we can predict when the colors are going to start to change. When the days get shorter and the nights get longer, a biochemical process starts to occur inside the leaves.”

The process affects three pigments produced in the leaves: chlorophyll, carotenoids and anthocyanins. Chlorophyll gives leaves their basic green color and is produced in photosynthesis through sunlight. With shorter days and longer nights, the chemical reaction of photosynthesis slows.

“The sunlight that it was using to manufacture some of the sugars that cause photosynthesis begin to slow down, and then it pushes all of those sugars into the root system,” Bianchi said. “So when that happens, we tend to see more of this carotenoid pigment that’s in the leaf all year round, we just don’t see it because how much chlorophyll is there.”

Carotenoids are the yellow, orange and brown colors seen in the fall, which is often the pigment seen in carrots, rutabagas and corn, Bianchi said.

While sugars are being pulled down into the root system and chlorophyll is no longer being produced, anthocyanins comes into play because the tree is trying to produce as much sugar as possible to pull into the root system for dormant season. Anthocyanins gives the leaves that red, purple color — the same pigment seen in cranberries, red apples and blueberries, Bianchi said.

“Basically, during the summer growing season, chlorophyll is continually produced, broken down, and so the leaves are green,” he said. “As the night length increases in fall, the chlorophyll production slows down, stops and eventually the chlorophyll is then destroyed, and that’s why we see the carotenoids and anthocyanins already present.”


In Summit County, the vibrant colors are only seen a few weeks out of the year — here one minute and gone the next. How long the leaves stick around in those colors varies by tree species and when they start to turn is based off of latitude in the United States.

“It’s kind of genetically inherited when the colors come on and how long they stay,” Bianchi said. “In late September in New England states, they will start to change color and move southward across the United States. But at that same time — it’s basically based off of latitude in the United States — so at that same latitude here in Colorado and in the high mountain elevation, you’ll see stuff changing the same time you might see something change in New England.”

How long the leaves will stay in their autumn colors is affected by weather conditions. Warm, sunny days and cool, crisp nights brings more spectacular color displays, because that weather pattern produces more sugars inside the leaves, and cool nights and gradual closing of the veins within the leaves prevent the sugars from moving out quicker. With this type of weather, the colors come on later and last longer into the fall.

“A lot of times you’ll see that more with the reds and purples,” Bianchi said. “So on the flip side, more of the yellows, the aspens that we see, are more dependent on soil moisture. If we have a late spring, or severe summer drought, it can delay those colors.”

A warm period during the fall will also lower the intensity of the colors. So the most favorable conditions for vibrant yellow colors are a warm, wet spring, and warm, sunny fall days with cool nights. In the mountains, the leaves begin to change first at higher elevations, and move down to the valley bottoms.

“If you get a cold snap in, or a nice frost, it could really shut things down quickly,” he said.


Acorn Creek

North of Silverthorne

Acorn Creek trailhead can be accessed by driving north on Highway 9 from Silverthorne for approximately 10.6 miles. After you cross the Blue River you will immediately turn right onto CR 2400 (Ute Park Road). At the first junction, continue left following the trailhead sign. Then turn right onto FDR 2402 (Rodeo Drive) and travel approximately 0.6 miles to the trailhead/parking lot. (Directions from US Forest Service website.)

“I really like Acorn Creek. … That to me is a great place to view (leaves),” Bianchi said.

Beaver Creek Trail

Fairplay, Park County

In the summer, Beaver Creek Road is open to traffic, but there are also numerous hiking, biking and 4WD trails in the vicinity to get off the main path. It takes roughly an hour to get to the area from Frisco, but the views even from just the road are exploding with color right now.

Boreas Pass

Elevation: 11,488 feet

On the south end of Breckenridge is Boreas Pass. The road is open to vehicles during the summer, or park in the lot and hike or bike up. The road has a gradual ascent to the summit, making it a relatively easy hike. Boreas offers an expansive view of the Blue River Valley and the Ten Mile Range, and also boasts views of Breckenridge Ski Resort.

Cataract Lake area

North of Silverthorne

This trail is roughly 25 miles north of Silverthorne near Heeney. Getting to Upper Cataract Lake is a hike — it requires about six hours of hiking over 10.5 miles of trail, with an elevation gain of 2,000 vertical feet. Lower Cataract is easier, the trail is about 2 miles long with minimal incline.

Fremont Pass

Elevation: 11,319 feet

Fremont Pass forms the Continental Divide on the border between Lake County and Summit County. Take the Copper Mountain exit (195) and follow CO-91 south — it takes about 20 minutes from Frisco.

Guanella Pass

Elevation: 11,670 feet

Guanella Pass Scenic and Historic Byway is a 23-mile route through Pike and Arapaho national forest land that links Georgetown and Grant. The road is rugged, which means less traffic. Guanella Pass takes about an hour to get to from Frisco.

Hoosier Pass

Elevation: 11,542 feet

Hoosier Pass separates Summit and Park counties. There is a large parking lot at the top of the pass for picture taking, as well as hiking trails. Coming down Hoosier Pass into Park County also lends itself to spectacular views of the valley.

Kenosha Pass

Elevation: 9,997 feet

Getting to this pass takes a little over an hour from Frisco, but it is one of the most popular areas for leaf peeping. This also means beware the crowds — weekends especially — so watch for slowing traffic and pedestrians when getting close to the top.

Originally published in September 2016 and regularly vetted for accuracy.

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