Rocky Mountain Aspen
From their first greening in spring to the golden crescendo that symbolizes autumn in the Rockies, aspen forests occupy a special place in the hearts of most westerners. - Audrey DeLella Benedict
Aspen is the distinctive slender broadleaf tree of the Rocky Mountain mountain forests, renowned for its golden autumn foliage. It grows in locations from 6500 to 11000 feet in elevation, usually in groves, often in moist sites, and can reach tree line. It commonly grows to 20 to 60 feet in height and less than 20 inches in diameter, but a few Aspen attain 100 feet in height and 3 feet in diameter. Individual Aspen trees are short-lived, rarely exceeding 100 years, and are easily damaged by fire, wind, and a very large number of kinds of insects, fungi, and animals. When individual trees are fairly short-lived, groves of Aspen are persistent since young aspen trees are constantly appearing.
Aspen is one of the most widely distributed tree of North America. It is found right across the North American continent from western Alaska to St. John's Newfoundland, and south to Mexico. The North American Aspen is very closely related to the aspen of northern Europe and Asia, if not exactly the same tree. The Aspen of the Rockies is noted for its bright golden-yellow fall foliage, not typical of other Aspen, and at one time this Aspen was considered a different species or a variety on that basis. The bright yellow fall color, which appears sometime from mid-September to early October, is a distinctive hallmark of the Rockies.
Aspen sprouts will shoot up from roots quickly and vigorously in cut or burned areas, and Aspen is one of the important pioneer species in the Rocky Mountains. Aspen groves provide shelter where young seedlings of other kinds of trees can survive, protected from the hot drying sun. Engelmann spruce, Subalpine fir, and lodgepole pine all can get a start in Aspen groves. Due to its important role in forest regeneration, a great deal of attention has been paid to aspen by foresters. A book could be written about this tree and its role in the forest.
Aspen is the only broadleaf tree of the mountain forests. Other broadleaf trees grow in the mountains, but only along streams. For other trees the short growing seasons and harsh conditions of the high forests seem to provide too little chance for recovery of the loss of growth associated with the long winter and the fall of leaves in the autumn.
Aspen can actually stay active and growing in the winter. Aspen twigs appear to absorb some of the leaves' nutrients before leaf fall, and there is chlorophyll in Aspen bark, making this tree in a sense an evergreen! At midwinter an "evergreen" subalpine fir in shade is dormant and may be entirely frozen, while across the glade a bare Aspen in full sun may be doing a bit of phototsynthesis.
Aspen tends to grow in sunny areas with fertile soil, with comparatively moderate climates and long growing seasons -- for the Rocky Mountains! It builds up soil nutrition by shedding leaves. It can grow in many kinds of soil and in many degrees of moisture. This allows Aspen groves to persist in places where other kinds of trees can not do well.
Seeds are produced, with "cotton" much like cottonwoods, but the seeds hardly ever germinate and grow. Aspen reproduces by sprouts growing up from the roots. The new sprouts may appear up to 25 feet from the parent tree. Each "tree" is often one stem of a much larger organism. Entire groves of aspen actually may be the same single organism.
Some aspen groves appear to be the largest and oldest living things on earth, easily reaching several thousand years in age. Colorado University biologists Jeffry Mitton, Michael Grant, and Yan Linhart have described an Aspen grove in Utah with some 47,000 trunks from a single root system. It covers 106 acres and weighs over 6000 tons. If Aspens can shoot up new stems from old roots, and make new roots from the new stems, there is no obvious limit to their existence. Even a fire which destroys all aspen growth above ground will not kill the plant.
Aspen bark contains salicin, a chemical closely related to Aspirin, and the bark was used by Indians and pioneers to treat fevers. Beavers love to eat the bark, and use stems and trunks for lodges and dams. The seeds are eaten by grouse. The twigs, bark, and buds are eaten by pika, beaver, deer, elk, mountain sheep, moose, bear, squirrel, rabbits, and porcupine. Mountain men - early fur trappers and explorers who preceded the miners and settlers by decades - fed aspen leaves and bark to their horses when other forage was lacking. Bark damage by browsing animals, especially elk, makes an entry point for disease.
The scientific name is Populus tremuloides , meaning trembling poplar. This tree does not grow well where the average annual temperature is above 47 degrees Fahrenheit, so it does not grow naturally in the cities of the Rockies on the plains or lowermost foothills. Aspen does best in upper Montane and Subalpine forests.
The leaves are nearly round, a broad oval, or heart-shaped; 1 1/2 to 3 1/2 inches long and wide; sometimes larger especially on young shoots. Pale green to pale yellow green in color; bright yellow in autumn. 20 to 40 small teeth per side of the leaf blade. Leaf stalk is 1 to 3 inches long and flattened, which allows the leaves to flutter back and forth in the slightest breeze. The leaf color and fluttering motion are distinctive.
The bark is mostly smooth and very pale green to pale white with a dusty look, or cream, pale to dark yellow-green, or a whitish green. Near the base of the trunk or in the lower few feet old trees have gray to very dark gray slashed or fissured bark, caused in part by winter feeding of animals especially elk. Any bark damage will cause rough black scar tissue.
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 sagebrush 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.
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