Friends of Northern Arizona Forests

Aspen Ecology

Groves of quaking aspen are the most colorful forest communities in the Southwest. Their delicate spring leaves darken into a dappling summer canopy followed by sweeps of yellow, orange, and rose that brighten the slopes in autumn. In winter, the bare white trunks of aspen form exquisite patterns on the monochrome landscape.

Aspen groves are biologically diverse, providing habitat for myriad plants and a marvelous array of creatures. Their crowns admit enough filtered light to foster layers of life from small trees to shrubs, wildflowers, and grasses. Countless insects thrive in the cool, moist environment of the groves, pollinating flowers and shrubs and sustaining a wide variety of birds. Hawks, goshawks, and owls swoop among them while woodpeckers, flickers, and sapsuckers tap on their trunks. Snags shelter nests of mountain bluebirds, violet-green swallows, and house wrens. Mammals undefined from rabbits to porcupines to bear undefined feed on their buds, bark, and leaves.

Aspens in autumn have been called “trees of golden butterflies,” a description that is surprisingly close to the truth. Aspens are the sole host plants for a number of caterpillars including those of green comma and dreamy duskywing butterflies. Without aspens, these butterflies might very well cease to exist.

Bands of aspen follow mountain ranges throughout North America, making them the most widely distributed tree on the continent. In the Southwest, stands of aspens usually occur between 7,500 and about 10,000 feet. Yet some thrive well outside this elevational range, on cool north-facing slopes down to 5,600 feet or sunny south-facing slopes up to 11,500 feet. They are dioecious trees, with males more common at harsher, higher elevations and females generally in more favorable conditions down slope. New trees may emerge from tiny seeds but usually, shoots called ramets sprout from a widespread mass of roots. These roots are capable of producing half a million or more shoots per acre and can extend for considerable distances. They create forests of genetically identical clones and enable them to exchange moisture and nutrients with one another to support expansion into less favorable areas. Some masses of roots may be thousands of years old.

Aspens favor snowy winters but crave sunlight. Even their bark is composed of living cells that photosynthesize nourishment in the form of carbohydrates using the sun’s energy to process carbon dioxide and water. Aspens are fairly short-lived trees, partly because pine, spruce, and fir sprout at their feet and eventually grow tall enough to starve them of light. After the aspens die back, their roots lie dormant until a disturbance undefined avalanche, rockslide, or fire undefined allows sunlight to stimulate the sprouting of ramets once again.

Despite their remarkable ability to regenerate, for a number of reasons aspens are rapidly dwindling in the Southwest. One major cause has been decades of fire suppression that reduced sunny openings in the forest, which are necessary for ramets to sprout. Where natural or agency- managed disturbance has created openings, the young sprouts have been vulnerable to browsing by elk, deer, and livestock. Mature trees have been stressed by years of drought, making them more vulnerable to insects and disease.

In June of 1999, a catastrophic frost defoliated the aspens on the San Francisco Peaks near Flagstaff, accelerating their decline. Concerns about these dire conditions as well as the health of the ponderosa forest and Bebb willow community prompted USDA forest scientists to propose the Hart Prairie Fuels Reduction and Forest Health Restoration Project in 2009. Covering almost 13,000 acres in total, the Hart Prairie project includes restoration of aspens on about 3,200 acres by removing encroaching conifers, conducting prescribed fires, severing roots in the interiors of aspen clones to stimulate new growth, clearfell-coppicing (removing all aspens to create a sunny opening that will foster a fresh generation of aspens), and planting.

Much of the effort to restore the aspens could be wasted, however, unless fences are built and maintained to exclude the elk and deer that browse the young shoots. To help with this labor- intensive but necessary task, Friends of Northern Arizona Forests obtained a grant from the National Forest Foundation for start-up funds to initiate a long-term commitment to erecting, improving, protecting, and maintaining exclosure fences in the greater Hart Prairie area. National Forest Foundation funding was matched with a grant from the memorial fund of Kenneth Lawrence, who loved the forests around Flagstaff. The combined funding paid for tools and equipment such as ladders and fence stretchers, for metal signs explaining the project to persuade members of the public not to cut through the fences, and for ten-foot metal t-posts to make the fence high enough to prevent elk from jumping over it.

Text by Susan Lamb Bean

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