By Lowell and Kaye Christie, F47246
Quaking aspen trees satisfy Westerners’ needs for bright fall foliage.
One one of our favorite walks, the slightest breeze brings the sounds of a flowing mountain spring. But thirsty as we may be, we don’t bother looking for moving water. It’s actually the sound of quaking aspen leaves fluttering in the breeze.
Depending upon the speed of the air, the sound can range from the almost inaudible hiss of soft rainfall (some call it the music from thousands of butterfly wings) to the rush of a mighty mountain stream. If the air is moving at all, there will be some sound, accompanied by a shimmering light show as the leaves dance.
Unlike the leaves of other trees, the quaking aspen leaf is designed for movement. The stem of the leaf is flat, and the leaf itself is attached to the stem at a 90-degree angle. No matter the wind’s direction, the slightest puff of air can start the leaves trembling. Even the tree’s scientific name, Populus tremuloides, points out this attribute.
We love aspen groves in any season, but in the fall these trees set the Western mountains aglow. Depending on the altitude, starting in late September and continuing through October, aspen leaves change into their fall attire. Usually it’s a brilliant yellow, but sometimes a grove of aspens takes on a reddish glow. What surprises many is that an entire grove will usually make the same color changes at precisely the same time. To understand why, you need to look below the surface.
An aspen grove isn’t just a collection of individual trees; it’s actually a large underground root system connecting all the trees in a colony. Some say that the biggest aspen grove is actually the largest organism (by mass) in the world. The most famous example is found in Utah and has the nickname Pando (Latin for “I spread”).
It’s a telltale name, because these trees do spread in two ways. Like most other plants, aspens can reproduce through seeds. A single aspen tree can create as many as a million seeds “” soft, fluffy, wind-transported masses that sometimes collect in what look like snowdrifts. The tree also uses vegetative reproduction, sending up shoots from an expanding root area.
Even with the millions of seeds produced, most aspens begin as sprouts from the massive root base. The aspen seed has a relatively short life span, and unless it finds precisely the correct environment, it will not survive, particularly in the arid West. It’s amazing that the aspen is still the most widely distributed tree in North America.
Because they sprout from the same root source, each aspen tree in a grove has identical genetic characteristics; the same angle of branching from the main stem, the same color of bark (usually white, but in some groves, it ranges from orange to green), the same shade of green leaves in the spring, and the same shade of foliage in autumn.
When looking down from mountain roads, you can identify different aspen groups by their color. Even in early spring, the difference is visible. One grove of aspens will appear green, having sprouted young leaves, while another grove nearby may show only white bark, since the leaves have yet to appear. Come back a week later and both groves may be green, but perhaps different shades. In the fall, the colors will probably turn at a different time for each grove, and the colors themselves will vary. Even the leaves from each grove fall on their own schedule.
The Pando grove in Utah is one of the largest in the world. It covers an area of 106 acres and has an estimated mass of more than 6,000 tons. By actual count there are more than 47,000 aspen stems growing from Pando, with an average age of 130 years (based on tree ring dating). The real question is the age of the entire organism. Most experts agree that it’s at least 10,000 years old, but some date its age to almost a million years.
Aspen groves often mark the location of a previous disaster. The trees require ample sunlight, so many aspens are found where the ground has been laid bare by an avalanche, a mudslide, or a fire. In fact, over a very long period of time the absence of fire can lead to the end of an aspen colony.
Beneath an aspen canopy there exists an ideal environment for shade-loving plants. If trees that grow taller than the aspen take root, the colony will be replaced, leaving only its underground root system. Fortunately, that root system can lie dormant for centuries, waiting for just the right conditions to send new sprouts upward. A massive fire provides just those conditions.
After the fire, while other seed-bearing trees try to get their start, the aspen root sends up shoots that grow as fast as three feet each season. The speed allows them to race ahead of trees developing from seeds, and a “new” aspen grove is born. Even without a pre-existing rootstock, a burned area provides the ideal habitat for aspen seeds blown in with the wind.
But it is really the root system that controls what happens in an aspen grove. If one area of the grove exists in drier soil, the roots can transfer moisture to it from another part of the grove. The same thing can happen if one section of the colony needs nutrients that exist only in another area. With so much coordination of effort, maybe it’s no surprise that aspens thrive in so many places.
This coordination actually occurs in both directions. As an aspen grove develops, the individual trees send hormones back into the root system to suppress the production of new stems so there will be less competition within the colony. But if part of the colony dies, from fire or from any other disaster, the hormone flow stops and the root system immediately begins sending more aspen shoots to the surface.
Since we spend much of our time in the West, we depend on the aspen for the brightest of our fall colors. The timing of the peak in aspen colors, just like the changes in other fall foliage, varies with the temperature and moisture conditions of a particular year. To catch the best of the aspen fall colors, do as we do. Check with the U.S. Forest Service’s annual fall foliage hotline at (800) 354-4595. The number is activated each September and works through mid-November.