HENRY LANSFORD

In late September and early October, hardly anybody in Colorado is apathetic about aspen. The U.S. Forest Service maintains a telephone hotline in Denver with information on where the fall color is at its peak throughout the central Rockies, local television weathermen include aspen reports along with their predictions of temperature and precipitation, and on the weekends all the highways and byways in the nearby Front Range are clogged with carloads of aspen fans.

Although aspens grow in every part of the United States except the Southeast and the Great Plains, many people think of the aspen as a symbol of the Rocky Mountain West. That region does have a lot of aspens. They comprise about 20 percent of the inventoried forest land in the U.S. Forest Service Rocky Mountain Region--about 4 million acres in Colorado alone.

Aspens don't stand out in eastern forests, where they mingle with oaks, maples and other hardwoods. But in the Rockies, where they are virtually the only hardwood in the subalpine zone, they are highly visible in summer as bright verdant patches scattered across great somber expanses of spruce and fir. In autumn, the contrast is even more striking. Because aspen leaves are thin and translucent, they seem to glow with an inner luminosity--even the air seems golden in an aspen grove on a sunny fall afternoon.

Aspen groves are hospitable as well as beautiful. A mature spruce-fir forest is a dim and silent place--the heavy shade discourages other plant species. Beneath the translucent leaves of an aspen canopy, however, bright wildflowers flourish in a rich understory that provides habitat and forage for many small animals. Aspen groves attract many more bird species than conifer forests do. Elk and deer bear their young beneath the aspens and often survive hard winters by browsing on young aspens that protrude from deep snow.

The species of aspen that grows in the western United States is Populus tremuloides--quaking aspen. Its leaves are almost constantly in motion. Early French trappers in the Rockies maintained that the trees trembled from shame because the cross on which Christ died was made of aspen wood, but the scientific explanation is more prosaic. The aspen petiole--the leaf stem--has an ovoid cross-section, with the thin dimension perpendicular to the leaf blade. The flattened petiole acts as a pivot, allowing the leaf to flutter frantically from side to side in even a gentle breeze.

In the fall, every tree in each stand of aspen turns the same color because they are genetically identical, springing from a single root system that silviculturists refer to as a clone. Although most aspens turn various shades of gold, a few groves display brilliant red leaves. The color is determined by the ratio of green, yellow and red pigments in the leaves. During the growing season, the green pigmentation of chlorophyll dominates, but the other two colors appear as the chlorophyll level drops in the fall. The ratio of yellow to red is genetically related, but it also is affected by growing-season temperatures, so the color of a genetically red aspen grove can vary from fire engine one year to orange the next.

Succession and Stability

Like everything else in this world, the autumn beauty of aspen groves is transitory. In many locations, even the grove itself represents a transitional stage in the process of ecological succession. Aspens grow in the subalpine zone at elevations from 7,000 to 11,000 feet, a region that they share with conifers such as spruce, fir and pine. Extensive networks of aspen roots usually underlie mixed stands of aspen and conifers. When fire wipes out the stand, new aspen stems quickly spring up. Recent scientific studies of aspen regeneration have revealed that, as long as aspen trees are growing, they produce a substance that inhibits the growth of buds on the roots, and that full sunlight on the soil surface is needed to stimulate suckering, or sprouting, from aspen roots. Thus fire triggers aspen regeneration in two ways: by eliminating the budding inhibitor and by letting in the sunlight. Growing as much as six feet in the first year, the suckers will become a mature aspen grove in 60 to 80 years. But if there are mature conifers nearby, spruce and fir seedlings, which have an aversion to strong sunlight, soon start to pop up in the shade of the aspens. If nothing happens to interrupt the process, the conifers eventually crowd the aspens out, and a climax spruce-fir forest replaces the aspen grove. If fire returns, new aspen suckers sprout through the ashes and the cycle begins again.

In addition to these seral (from sere, a series of successional ecological communities) aspen, there also are many stable stands. One scenario for the establishment of stable aspen hinges on the fact that conifers can drop their seed no farther than 100 to 200 feet from standing trees. A catastrophic fire may destroy thousands of acres of mixed aspen and conifers, leaving no conifer seed sources. Without conifers to succeed them, the aspen suckers that sprout from a clone after a fire eventually will become a stable stand of mature aspen. Partial regeneration may begin when something disturbs the stability of the clone--perhaps a defoliating disease kills some of the trees, opening up the stand enough to trigger the suckering response. Young suckers grow up, and a second generation of aspens develops. As more old trees die, they are succeeded by the young ones. Unless conifers are introduced back into the area somehow, this mechanism can maintain a stable aspen stand indefinitely.

A different mechanism probably produced the isolated stable aspen groves that are common on grassy or sagebrushy slopes in the region's mountain parks--the local term for high, open valleys. Aspen seed will germinate and grow only in a bare mineral soil seedbed with a constant supply of moisture during the first growing season, and summer moisture often is scarce during the current dry climatic regime in the Rockies. But envision a scenario in which one of the billions of seeds produced every year by Rocky Mountain aspen falls in a favorable location in a mountain park, and a seedling grows. Before it gets very big, perhaps a hungry elk eats it down to the ground. Its root system suckers, producing a second generation of half-a-dozen stems. As this cycle is repeated over time, more stems join the group. There are virtually no conifers present in many mountain park areas, so such isolated stable aspen clones can continue to regenerate and spread across the sagebrush or grassland. In some locations, aspen clones actually have migrated toward more favorable environments by regenerating on one side and dying off on the other. An isolated aspen stand that you see today may have been hundreds of yards away several generations ago.

Managing Endangered Aspens

The traditional U.S. Forest Service policy of suppressing fire on national forest lands is based on the conventional wisdom that preventing fires saves forests, but this may be an ecological fallacy that is threatening the long-term survival of some aspen groves. Aspen forests are among many western forest ecosystems that are fire-dependent, and human efforts to suppress fire since European settlement occurred have created unexpected problems. The efficient work of government firefighters has greatly reduced the extent of burned-over land where aspen clones can regenerate, and old aspens are not being replaced by young ones in many areas.

Although the Forest Service considered aspen a non-commercial species for many years, several thousand acres of aspen currently are being harvested annually on national forests in Colorado, mainly to produce a board made of compressed wood chips that is an inexpensive substitute for plywood. Aspen is not a high-value species--it usually goes for the lowest price at which the government is allowed to sell timber. But profit really isn't the goal, according to Forest Service officials who see commercial harvesting as a practical way to stimulate the growth of young aspen to replace mature ones as they die of old age. They say that harvesting has produced a profusion of young aspen stems in areas where, ten years ago, there weren't any because of fire suppression. Not everyone agrees that harvesting is a good thing, however, mainly because the immediate effect of harvesting mature aspen by clear-cutting is instant uglification--a brushy thicket appears where stately trees once stood.

Even more rigorous management is sometimes needed. Aspen is very resilient and can take care of itself in most natural circumstances. But silviculturists are working to learn more about how to manage endangered clones in seriously disturbed situations. For example, there are areas in the mountains of northern Arizona and New Mexico where browsing by large elk and deer populations and heavy grazing by livestock is interfering with aspen regeneration. In some locations, the only way that forest managers have found to allow such endangered aspen clones to regenerate is to build high fences around them.

The situation is not that desperate along Colorado's Front Range, but many aspen stands there are small and seral. If you drive south from Estes Park, you can see a lot of coniferous forest with aspen sprouting up wherever an area has been cleared to build a cabin or for other reasons. Aspen has grown there, but conifers have taken over. In the past, fire would have taken out conifers periodically, but the Forest Service can't use fire as a management tool--it's too dangerous. Harvesting aspen is sometimes controversial, and building fences around aspen clones is expensive, but Forest Service scientists and managers believe that, in some situations, such measures are needed to save the aspens.

To many of us who live in the West, the quintessential Rocky Mountain experience is hiking through a deep grove of old-growth aspen on a brilliant fall day. The air is suffused with a hazy golden glow, the pale-barked boles soar like columns in a cathedral, the forest floor is a carpet of soft duff and reddening kinnikinnick leaves and the only sounds are the gentle soughing of the wind in the leaves and the rasping cry of a Steller's jay. Whatever has to be done to manage aspens and ensure their survival, some places like this must remain. We need them now, and our children and grandchildren will need them even more.

NOTE: The author is grateful to Dr. Wayne Shepperd, silviculturist with the Rocky Mountain Forest and Range Experiment Station of the U.S. Forest Service, for providing a great deal of valuable information and advice for this article. Dr. Shepperd has worked with western aspen since 1978, studying stand characteristics and growth habits, regeneration and responses to harvesting and other management techniques.