Forest Types
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Aspen

| Description | Products and Uses | Site Conditions |
| Regeneration | Immediate Treatments | Pests and Diseases |

Description

Range of quaking aspenThe aspen forest type, dominated by quaking aspen (also called “poplar” or “popple”) and bigtooth aspen, covers more area than any other forest type in the Lake States (Figure 6-1). Paper birch and pin cherry are common associates. Balsam poplar (balm of gilead) is found on moist sites. Aspen is very intolerant of shade and relatively short-lived.

Stands can be invaded readily by more shade-tolerant species. On dry sites, aspen may be replaced by red pine, red maple, or oaks; on sites with intermediate moisture, by white pine; on fertile sites by northern hardwoods, white spruce, and balsam fir; and on the wettest sites by balsam fir, black spruce, black ash, and northern white-cedar.

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Products and Uses

Aspen: Aspen often grows in nearly pure standsAspen species are used principally for paper and particleboard, but also for lumber, studs, veneer, plywood, shingles, matches, novelty items, biomass fuel, and animal feed. Aspen stands are important habitat for ruffed grouse, woodcock, snowshoe hare, beaver, porcupine, white-tailed deer, moose, and black bear.

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Site Conditions

Aspen grows on a wide range of soils from shallow and rocky to deep loamy sands and heavy clay. Good soils are well-drained, loamy, and high in organic matter, calcium, magnesium, potassium, and nitrogen. The best sites have soils with silt-plus-clay content of 80 percent or more. Aspen prefers a water table from 2 to 8.2 feet deep. It grows poorly on sandy or droughty soils and on heavy clay.

Site index commonly is used to evaluate site productivity when aspen stands are at least 20 years old and have not been damaged by fire or overtopped by other species (Appendix B-1: Site index curves for quaking aspen). Manage aspen for timber only where the site index is 60 or better.

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Regeneration

Aspen seeds are dispersed long distances by wind, but require a water-saturated, mineral soil seedbed for germinationRegeneration from seed is possible, but unreliable. Good seed crops occur every four to five years on 50- to 70-year-old trees. Seeds ripen in May to June and are dispersed long distances by wind and water. They require a water-saturated seedbed for germination. A moist, bare mineral soil is best.

Aspen stands most commonly regenerate from root suckers that grow from lateral roots after a stand has been harvested or killed by fire or wind. Because aspen is very intolerant of shade, optimum root suckering occurs when the stand is completely clearcut. Do not leave more than 20 mature trees an acre after harvesting. If logging does not destroy undesirable trees and shrubs, remove them by felling, girdling, basal spraying, or controlled burning. Root suckering is most prolific when:

  • Clearcutting is done when the soil is relatively dry or frozen to avoid damaging the lateral roots within 4 inches of the soil surface that produce the suckers. This is especially important on clay soils with a high water table.
  • Harvesting occurs during the dormant season when food reserves stored in the roots are at a maximum, especially on fine-textured soils.
  • Soil temperature is 74° F. (High soil temperature inhibits suckering.)
  • Parent trees are healthy and have high carbohydrate reserves. Grazing, repeated cropping, killing of sucker stands, and insect defoliation will lower carbohydrate reserves.
  • There is no excess soil moisture (to impede aeration) or severe drought.

Two years after clearcutting there should be at least 5,000 aspen root suckers per acre.If a stand is harvested during the growing season, root suckers will begin to grow immediately after trees are felled. Do not drive heavy equipment across young sprouts or they will be killed. To protect new sprouts begin logging at the rear of the stand, then progress toward the log landing. Harvest stands for pulpwood at age 45 to 55 and for sawtimber at age 55 to 65. Harvest earlier if more than 30 percent of the trees are diseased (indicated by the presence of fungal conks or bark cankers).

Old, decadent stands with low vigor and stands with fewer than 50 mature aspen trees an acre may be difficult to regenerate. Encourage maximum suckering by harvesting these stands during the dormant season when the ground is frozen or relatively dry. If an old stand does not have a merchantable volume of wood, you may kill the old trees and stimulate suckering by felling the trees, by shearing them with a sharp blade on a bulldozer when the soil is frozen, or by setting a prescribed fire.

Two years after clearcutting there should be at least 5,000 aspen root suckers an acre. Some stands may have up to 30,000 root suckers an acre. The more the better, since aspen stands naturally thin themselves. If root sucker density following the clearcut appears low, ask a forester to judge whether the stand is adequately stocked. If stocking is not adequate, wait at least 10 years, then clearcut the stand again. Following this second clearcut, sucker density should improve to a satisfactory level. Aspen will not compete well with other hardwoods such as maple, basswood, ash, and oak. Over time the aspen will die from disease and be replaced by more shade-tolerant species. Clearcutting a mixed species stand favors aspen regeneration. Removing aspen during thinning will favor other hardwood species.

Where you find mature aspen with an understory of white spruce and balsam fir (two shade-tolerant conifers), clearcutting aspen while damaging the conifers will reproduce mainly aspen. In contrast, carefully harvesting the aspen while leaving the conifers undamaged will enable aspen root suckers to survive in scattered patches. In 40 to 50 years the conifers and aspen will mature. Clearcutting then will regenerate a stand of mainly aspen with a few scattered conifers.

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Intermediate Treatments

Once an aspen stand has regenerated, trees grow rapidly. A densely stocked stand thins naturally; artificial thinning is unnecessary to produce pulpwood and may increase losses from hypoxylon canker and rot. Dense stands also promote natural pruning. Artificially thinned stands may produce more sawtimber and veneer than unthinned stands, but thin to grow these products only when disease incidence is low and the site index is 70 or higher. One thinning at about age 30 leaving approximately 240 trees an acre may be appropriate. Row thinning of sapling stands has produced faster volume growth on residual trees, but results are inconsistent. Take great care to avoid wounding residual aspen trees, since decay and discoloration can enter trees through those wounds.

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Pests and Diseases

Aspen is highly susceptible to fire damage. Major insect pests are forest tent caterpillar, large aspen tortrix, gypsy moth that feed on leaves, and various wood borers that weaken and degrade the stem. Small conch on trembling aspenSeveral species of fungi cause stem cankers or white rot that reduce the volume of usable wood.

To reduce losses from these pests, do not grow aspen for timber where the site index is less than 60. Stands growing on poor sites are highly susceptible to pests. Try to regenerate 30,000 suckers an acre and maintain a high number of stems an acre to discourage poplar borer and hypoxylon canker. Do not thin aspen; wounds on residual trees will favor establishment of poplar borer and hypoxylon canker.

To minimize pest damage, harvest trees by age 40, unless the site index is at least 75 and veneer is the desired product. If fewer than 15 percent of the trees are infected with hypoxylon canker, stands may grow longer than 40 years. If 15 to 25 percent of the trees are infected with hypoxylon canker, harvest early and regenerate aspen. If more than 25 percent of the stand is infected, consider converting to an alternate forest type or species. Harvest early if white rot affects more than 30 percent of the basal area.

Repeated defoliation by forest tent caterpillar will weaken the trees, increasing their susceptibility to disease. Insecticides may be required to protect the stand during prolonged outbreaks.

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