Leaf-peeping at Aspen trees turning gold in the Colorado Rockies is such a tradition that the local news channels provide reports on the status and the best places to spot the aspen leaves each fall!

Heading up Highway 285 southwest from Denver toward Bailey provides many opportunities to see the leaves turning. Some of my favorites include Staunton State Park, Kenosha Pass, over Guanella Pass and many hiking trails in-between.

Leaf-peeping season is known locally along Highway 285 as the Gold Rush. Restaurants in the mountains plan their staffing around the many people that will show up hungry after a beautiful fall day in the mountains.

Although the Aspen leaves are stunning when they are at their peak, there are even more things you can notice about the trees and the landscape around them to make it a fascinating trip!

Here are some things to look for to take leaf-peeping up to a new level:

#1 – Fun Fact about why they are known as “Quaking Aspen”

Aspen leaves flutter with the slightest breeze, giving the tree the appearance of trembling, or quaking. The leaves flutter so easily because their stems (or “petioles”) are flattened, and attached perpendicular to the leaf which allow the leaves to easily pivot on their stems. Also the petioles are longer than the leaves, so they are more flexible.

This quaking benefits the tree by making it harder for insects to land on the leaves and also keeps the leaves cool on hot summer days.

#2 – Where you are often determines what other plants and animals you can see.

Aspen trees are native to Colorado and can be found in relatively moist environments from the foothills at around 6,000 feet all the way up to sub-alpine at about 11,700 feet, which is just below timberline! They are most common from 8,000 feet to 10,000 feet.

* Aspens Near Timberline

At the highest elevations, they are primarily found on warmer south-facing slopes.

* Aspens in Open Meadow Ponderosa Pine forests near 8,000 feet

At around 8,000 feet, in open meadows that face south, you can see a lot of Ponderosa Pine trees. Ponderosas are conifers with long needles and large cones. The bark is deeply furrowed and looks like jigsaw puzzle pieces put together. When the bark of a Ponderosa Pine is in the warm sun, you can smell vanilla or butterscotch when you get up close and take a whiff!

Aspen trees in these meadows are in groves that are filling in the open spaces.

Look for Abert squirrels, with long ear tassels, which depend solely on Ponderosa Pine trees for their food, habitat and protection. Other animals around Ponderosa Pines that you might see include Steller’s Jays, Pygmy nuthatches, Ground squirrels and Least Chipmunks. If you stop for lunch, you may see many of these birds and small mammals. Deer and elk live in this ecosystem but generally will not be easily seen on a day hike.

* Moist, dense Douglas Fir forests

Moist, north-facing slopes have denser forests with Douglas fir trees. Aspen trees in these forests tend to be smaller patches along the edges and individuals within the forest.

Because the Douglas fir trees are close together, there is not much plant understory in those slopes. Animals seen there include pine squirrels or chickarees and weasel-like pine martens. Birds can include Clark’s nutcrackers and ruby-crowned kinglets.

#3 – There is a difference between the Aspen stands on the West slope of the Rockies and the East slope.

The western slope of the Rockies generally gets more precipitation, which allows for large stands of aspen.

The eastern slope of the Rockies generally has small groves of aspen trees. These are along streams or lakes, the edge of forests or roads and as scattered individuals. Pay attention to the size of the Aspen tree stand and whether you are on the West or East slope of the Rockies.

On the East slope, see if you can spot a stream bed or wet valley by the aspen trees following its course. Frequently you will also see Colorado Blue Spruce there as well.

#4 – Notice how the trees are alike – individual Aspen trees are part of a larger organism!

Although Aspen trees can grow from seeds, that is extremely rare and is primarily on sites where the ground has been exposed by wildfire. More common is cloning, where new trees sprout up from the roots of an existing tree. This results in stands of Aspen trees that are all genetically the same.

Trees that are clones will have similar bark color and texture, will all produce leaves at the same time in the spring and turn the same color in the fall.

The largest identified clone of aspen has been named Pando (Latin for “I spread”) and is in Fishlake National Forest in South-Central Utah. It contains 47,000 trees and covers 107 acres. The root system of this clone has been estimated to be several thousand years old!

It is possible to have several stands of different clones of Aspen trees next to each other. One way to spot different clones is to notice that the leaves are turning different colors in the fall – ranging from yellow to gold to deep orange.

#5 – Look for Animal marks on Aspen Trees.

Aspen bark is covered with a white powdery substance which helps protect the tree from sun scald by reflecting the light. This bark characteristic also retains the evidence of how animals have marked the trees.

Scars of claw marks on the white bark of larger trees can show where bears climbed the trees to eat twigs or leaves or to look for eggs in nests.

Elk and mule deer eat the aspen bark, which contains salicylic acid, the main component in aspirin. Sometimes in the deep of winter, aspen bark is their main diet!

Their long, thin tooth marks can be seen on the bark. They also eat the aspen twigs and foliage during the spring and summer and evidence of this can be seen by stunted trees. Wider marks higher up on the trees are from bull elk and deer bucks scraping their antlers to remove the itchy antler velvet.

Beavers consume aspen trees and also use them in their lodge and dams. That evidence can be seen by aspen stumps with cone-shaped tops from the beavers gnawing the trees.

Many birds such as northern flickers and hairy woodpeckers hollow out nests in the bark, which are later used by other birds including tree swallows, white-breasted nuthatches, house wrens, mountain bluebirds and occasionally northern pygmy owls.

#6 – Notice the plants growing under Aspen Trees.

Aspen trees lose their leaves each fall, contrasted with conifers that don’t lose their leaves. These leaves improve the quality of the soil by breaking down quickly and allowing the nutrients to return to the soil.

Under a mature aspen stand, there is more organic content and better moisture-retaining qualities, which support plants in the understory. Depending on the location, there may be shrubs, grasses and/or flowers such as Colorado columbine, common lupine, golden banner and strawberry.

#7 – See if there are conifers growing up under the protection of the Aspen Trees.

Aspen trees are frequently part of a successional forest, especially in connection with Lodgepole Pine trees. They move in, growing quickly after a fire or logging, and then provide shelter for young conifers that will form the climax or long-lasting ecosystem.

Pay attention to any trees growing at the base of the Aspen trees.

Watching for these extra things in nature surrounding the Aspen trees will help increase your appreciation of these beautiful trees and make for a fun leaf-peeping trip!

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References and Further Reading

• The Naturalist’s Guide to the Southern Rockies, by Audrey DeLella Benedict, 2008
• Explore Colorado, a Naturalist’s Notebook, by Frances Alley Kruger and Carron A. Meaney, published by Denver Museum of Natural History and Westcliffe Publishers, 1995
• Botany in a Day: The Patterns Method of Plant Identification, by Thomas J. Elpel, 1967
• Flora of Colorado, by Jennifer Ackerfield, 2015
• Colorado Native Plant Database Data Portal, Colorado State University
• Wild at Heart: A Field Guide to Plants, Birds & Mammals, Snowmass, Aspen & the Colorado Rocky Mountains, by Janis Lindsey Huggins, 2008
• From Grassland to Glacier: The Natural History of Colorado and the Surrounding Region, by Cornelia Fleischer Mutel and John C. Emerick, 1992