Nature News from Greater Yellowstone #13 September 14, 2020

Smoke is in the air and it is masking much of what we are accusomted to seeing with the onset of autumn.  The change of season, though is becoming quite real with longer, cold nights and the color changes in trees and shrubs.  Bull elk are starting to bugel and round up their harems of females, while osprey and other migratory birds are leaving on their annual migration to warmer climates.

Bull Elk and Aspens

Despite the fact most of the trees we find in the Yellowstone area are conifers, (80% in the park are lodgepole pine!), does not preclude the spectacular change of leaf colors on some of the other vegetation here. Aspen, Cottonwood, shrubby willow and other shrubs and forbes are influenced by changing daylight and temperatures. Many of the wildflowers, long gone to seed, have leaves that speckle the ground with color.

Aspen Leaves and Sticky Geranium

As days grow shorter, nights grow longer and cooler.  Biochemical processes in the leaves start the onset of changing colors. Changing daylight is the most significant trigger for these changes. No other environmental factor such as temperature or moisture is as constant as the longer darkness during autumn.  In the canyons, along riparian areas, the willow, chokecherry and dogwood will turn spectacular orange and reds.  Cottonwood leaves become a subtle gold before they drop and cover the ground.  One of the most brilliant displays in the Rockies will be the Quaking Aspen.

Quaking Aspen in Grand Teton NP, courtesy NPS

Later, I want to tell you more about Aspen, but first I will discuss the cycle of broadleaf plants and what happens in fall!

During the spring and summer the leaves develop and become the food makers for the trees and other plants. This is critical for their survival and growth. This food production takes place in the leaf in numerous cells containing chlorophyll, which gives the leaf its green color. Chlorophyll uses the energy from the sun to transform carbon dioxide and water to carbohydrates, (sugars) that are used as a food source for these life giving organisms.  Along with the chlorophyll are yellow to orange pigments, carotenes and xanthophyll pigments which, for example, give the orange color to a carrot. Most of the year these colors are masked by great amounts of the green chlorophyll. 

But in the fall, because of changes in the length of daylight (as well as changes in temperature), the chlorophyll breaks down and the green color disappears. The leaves stop their food-making process. At this time the yellow to orange colors become visible and the leaves change to their glorious fall appearance.  As the fall colors appear, other changes are taking place. Where the stem of the leaf is attached to the tree, a special layer of cells develop. In time that layer will cut off the connection tissue that supports the stem to the branch. When the leaf is blown off by the wind or falls from its own weight, it leaves behind a leaf scar.  This occurs because these leaf tissues could not survive the freezing temperatures as needles and branches do. The leaves that end up on the ground each year decompose and help add nutrients back to the soil. The nutrients feed soil organisms that are vital for the continuation of the entire forest. A spongy layer of duff covers the forest floor that will hold moisture. 

AND…think about how much fun it is to jump into a big pile of leaves that has piled up!

Nephew Robert many years ago

Ok, I did mention the significance of Quaking Aspen (Populus tremuloides) earlier. Not only do Aspen provide us with the glory of fall color change, Aspen forests provide critical wildlife habitat and are second only to riparian areas in terms of biodiversity richness. Aspens are considered a keystone species as they provide support for many species. Steve Cooper, vegetation ecologist with the Montana Natural Heritage Program, says aspen stands are important grizzly and black bear habitat. “The groves hold lots of forbs that bears eat in spring and fall” .  Beaver (another keystone species) thrive in the presence of aspen and ungulates like elk and moose which browse aspen.   These forests are particularly important to cavity-nesting woodpeckers and songbirds, such as bluebirds and wrens,

Mountain bluebird on Quacking Aspen

as well as warbling vireos, birds of prey and ruffed grouse.  Aspen grow in clones of genetically identical stems.  Healthy roots systems put out hundreds of shoots per acre. The sign of a healthy aspen stand is the presence of many young aspen surrounding the more mature trees. They can grow up to 10 feet in five years. Their bark is smooth, greenish-white, or gray to almost white in color. The green color is from chlorophyll in the bark. Their bark may become rough and fissured with age. Aspen leaves are pointed at the tip and rounded at the base, with many small rounded to sharply pointed teeth along their margins. The leave’s small stem (petiole) is flat. The flattened stems allow the leaves to quake or tremble in the slightest breeze; hence, their name, Quaking Aspen. 

It grows on many soil types, especially sandy and gravelly slopes, and is quick to pioneer disturbed sites (such as areas that burned recently). Fire reduces the overstory, stimulates shoots to sprout, and kills invading conifers growing in the aspen clone.

One more interesting fact:

 Acetylsalicylic acid is one of the most widely used drugs in the world. Its ancestry the salicylates, including salicin and salicylic acid, are found in the bark and leaves of the willow and poplar trees. (Quaking Aspen is a poplar)  The ancient Sumerians and Egyptians, as well as Hippocrates, Celsus, Pliny the Elder, Dioscorides and Galen used these natural products as remedies for pain, fever and inflammation. In the Middle Ages these remedies were used for fever and rheumatism by Hildegard of Bingen and Henrik Harpestrengne

( taken from

Wishing you all a good September. It’s one of my favorite times of year. You can contact me at and see this blog as well as past blogs at

Happy Trails to all of you, Leslie

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