Learn About Fall Colors

Aspen Trees Turning to Gold
A stand of aspen are turning to gold, fall colors are in full swing


Fall in Rocky Mountain National Park is marked by shorter days, cooler temperatures, bugling elk, and shifting colors.

At high elevations, the alpine tundra is cloaked in deep red and purple hues as plants go dormant for the season. Across the park, deciduous trees like aspen and cottonwood adopt their gold and orange palette before losing their leaves completely.

The landscape collectively shifts from vibrant greens to autumn’s golds, oranges, reds, and browns signaling the approach of winter in the Rocky Mountains.

NPS Photo_Fall Colors on the Timber Lake Trail
The leaves on aspen are changing to gold as chlorophyll production begins to slow, signaling the arrival of fall


Why Do Leaves Change Color in Autumn?

While all leaves are green during the summer, leaves turn to gold, orange, red, purple, and brown due to the different pigments found within leaves.

Chlorophyll produces shades of green, carotenoids produce yellows, oranges, and browns, while anthocyanins produce deep reds and purples.

During the spring and summer growing seasons, plants appear green as they produce chlorophyll that supports photosynthesis – the chemical process that allows plants to convert sunlight into sugars for food. Shorter days signal chlorophyll production to slow and ultimately cease, preparing the plants for winter. As the remaining chlorophyll pigments break down, the yellow and orange hues of underlying carotenoids begin to emerge.If sugar concentrations are high enough, anthocyanin pigments are produced resulting in dark red or purple leaves.

Sunlight, soil moisture, and temperature all have an influence on sugar concentrations, which in turn help determine the color of the leaves during the fall season.

Aspen Leaf_Green


View of an aspen leave that is green in color
Aspen Leaf_Green_Yellow_Red


This is an example of an aspen leaf that has turned to gold, with pigments of red along the edges.
Aspen Leaf_Red-Purple


This is an example of aspen leaves that have turned red/purple in color.

Last updated: September 28, 2022

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