Products and Uses
Quaking and bigtooth aspen (popple) are grown principally for pulpwood that is used in the manufacture of paper and particleboard, but they also are used for sawtimber and veneer. Paper (white) birch is in less demand than aspen, but can be used for pulpwood, lumber, fuelwood, dowels, and novelty items. Aspen stands are important for ruffed grouse, white-tailed deer, and moose.
Both aspen and birch commonly grow in pure stands, but also are found together or in mixtures with other hardwood and conifer species. This section focuses on aspen, which currently is in greater commercial demand than birch.
Aspen will grow on a great variety of soils, but the fastest-growing, best-quality trees often are found on moist soils that are rich in lime and have a high silt-plus-clay content. Trees on dry, sandy soils grow slowly because of low moisture and nutrient levels and are more prone to health problems than trees on better sites. Heavy clay soils do not promote the best growth because of poor soil aeration.
Site index commonly is used for evaluating productivity of different sites when aspen stands are at least 20 years old and have not been damaged by fire or overtopped by other species. Because of slow growth and health problems, poor quality sites may be better suited for other species, although some aspen can be grown there for nontimber purposes such as wildlife (particularly rough grouse) or aesthetics.
Aspen stands managed for pulpwood generally should be harvested at age 45 to 55. Stands grown primarily for sawtimber generally should be harvested at age 55 to 65. Stands may need to be harvested earlier if disease incidence is greater than 30 percent.
Aspen stands commonly are regenerated from root suckers (a sprout rising from stump or root system of tree, occurs on aspen stumps after harvest), but also may regenerate from seed if there is moist, bare mineral soil available during seed dispersal.
Because aspen is very intolerant of shade, optimum root sucker regeneration occurs when the stand is completely clearcut. Do not leave more than twenty mature trees per acre after harvesting the stand. If logging does not destroy undesirable trees and shrubs, remove them by felling, girdling, basal spraying, or controlled burn.
Clearcut when the soil is relatively dry or frozen to avoid damaging the parent roots. This is especially important in stands growing on clay soils with a high water table.
Root suckers grow fastest when trees are harvested during the dormant season and food reserves stored in their roots are at a maximum. Root suckering may not be satisfactory if trees are harvested in the spring or early summer shortly after the leaves and flowers have been produced.
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. Avoid damaging new sprouts by beginning the logging activity at the rear of the stand first and progressing toward the log landing.
Range of quaking aspen.
Old, decadent stands with low vigor and stands with fewer than fifty mature aspen trees per acre may be difficult to regenerate. In these stands you can encourage maximum suckering by harvesting during the dormant season and when the ground is frozen or relatively dry. If an old stand does not have a merchantable volume of wood, you still should fell the trees or shear them using a sharp blade on a bulldozer when the soil is frozen. This will stimulate suckering and help create a better-stocked, new stand.
Two years after clearcutting there should be at least 5,000 aspen root suckers per acre. Some stands may have up to 70,000 root suckers per acre. The more the better; you need many root suckers because aspen stands naturally thin themselves.
If root sucker density following the clearcut appears low, have a forester judge whether the stand is adequately stocked. If it is not, wait at least ten years, then clearcut the stand again. Following this second clearcut, sucker density should improve to a satisfactory level.
Aspen will not compete well in mixtures with other hardwood species such as maple, basswood, ash, and oak. Over time the aspen will die from disease and will be replaced by more shade-tolerant species. Mixed hardwood stands can be managed either for aspen or other hardwoods, but cannot be managed effectively for both. Clearcutting a mixed stand will favor aspen regeneration. Removing aspen during thinnings will favor regeneration of other hardwood species.
In the northern lake States it is common to find mature aspen with an understory of immature white spruce and balsam fir-two shade-tolerant conifers. These three species can be managed together. If you carefully harvest the aspen and leave the conifers undamaged, openings in the conifer canopy will be large enough to allow good aspen sucker development 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 spruce and fir.
Paper birch also is intolerant of shade and usually is regenerated by clearcutting, but recently success has been attained from the shelterwood method. Although small birch produce vigorous stump sprouts when cut, merchantable size trees do not sprout well and sprouts are normally of low quality. Natural seeding from narrow, progressive clearcut strips and small patch clearcuts is the most common regeneration method for birch. It is essential to knock down brush and expose bare mineral soil at the time of clearcutting.
If a clearcut through a birch stand has to be more than 300 feet wide, leave a sufficient number of seed trees scattered throughout the site to get adequate seed dispersal and provide for the survival of seed trees and protection of new seedlings. Remove seed trees within two years after acceptable regeneration.
On hot, dry sites a two-cut shelterwood system may be more successful than clearcutting for birch. The first cut should thin the canopy and provide more sunlight to the forest floor. A year after the first cut, disk the site to lightly bury the birch seeds, help control competing vegetation, and incorporate organic matter. Disking is especially helpful following a good seed fall. After the stand is sufficiently stocked with seedlings, it should be clearcut.
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 (see below). Dense stands also promote natural pruning. Artificially thinned stands may produce more sawtimber and veneer than unthinned stands, but thinning should be carried out 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 per acre may be appropriate. Take great care to avoid wounding residual aspen trees, since decay and discoloration can enter trees through those wounds.
The forest tent caterpillar (Malacosoma disstria) occasionally defoliates large areas. Gypsy moth (Lymantria dispar) may increase future defoliation in Minnesota. Severe defoliation reduces growth but rarely causes mortality unless coupled with other stress.
The major diseases of aspen are Hypoxylon canker (Hypoxylon mammatum), and white rot (Phellinus igniarius). Both can dramatically reduce merchantable volume. As stand age increases, volume losses due to insects and disease increase.