Regenerating Quaking Aspen: Management recommendations
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Figure 1. A densely regenerating aspen stand in northern Minnesota. There are about 30,000 aspen suckers per acre in this photograph which was taken less than one year after the stand had been clearcut.
Quaking Aspen (Populus tremuloides Michx.) is the most widely distributed timber species in North America and is commercially important throughout much of its range. Aspen is intolerant of shade, but grows on a broad variety of sites. It grows best on deep, loamy soils that have good soil moisture and high lime content. Aspen is a fast-growing species that is capable of vigorously invading an area following disturbance.
Figure 2. A poorly regenerating aspen stand in northern Minnesota. The mature stand in the background resembles the stand that originally occupied this site. This photograph was taken about five years after the stand had been clearcut.
For many years aspen had little recognized commercial value. Today, aspen is a valuable raw material for paper, waferboard, and lumber. Aspen contributes to soil and water conservation and provides abundant wildlife habitat for many species including ruffed grouse, woodcock, white-tailed deer, and moose.
Regeneration, the re-establishment of trees on a site following a timber harvest, is an important objective in any forest management program. Aspen management is well suited to this objective because harvesting activities can provide ideal conditions for aspen regeneration. However, foresters have observed that aspen may not regenerate equally well in all situations (Figures 1 and 2).
This publication examines the aspen regeneration process and describes appropriate forest management practices. Though most of the research for this paper was conducted in the Lake States region, the information presented here may apply to other areas as well.
Figure 3. One-year-old root suckers emerging from a parent tree. (Photo courtesy of USDA Forest Service.)
Though aspen produces abundant seed, it regenerates primarily by developing new shoots along the root system of the parent tree. The new shoots are called root suckers (Figure 3) and this process is called "suckering." Suckering is controlled both by growth-regulating compounds, or hormones, and by soil conditions (temperature and aeration). Aspen will not sucker when root temperatures are maintained below about 55° F or when soils are saturated. Suckering is inhibited by auxin, a growth-regulating compound. Auxin is produced in leaves and shoots and is transported to the roots. If the above-ground portion of the tree is killed (as happens during timber harvesting), auxin will stop flowing to the roots. Without the inhibiting effect of auxin, suckering increases.
Root suckers emerge primarily from bud-like organs called primordia. These organs are located beneath the bark on roots that are often less than three inches deep and run parallel to the ground surface. Roots are capable of suckering when they are about two years old and one-quarter inch in diameter. Aspen stands can have more than ten miles of such roots per acre, and aspen root systems commonly produce tens of thousands of suckers per acre one or two years following harvest.
Stands with many aspen trees per acre will contain many roots and consequently can produce numerous root suckers. However, a high number of trees per acre is not required for successful regeneration. Under ideal conditions, healthy, mature stands (age 45 to 60) with as few as fifteen to twenty evenly spaced trees per acre can regenerate a site. In practice, though, at least 50 well spaced aspen trees per acre (about 20 feet apart) are usually needed.
Regenerating a site from root suckering forms groups of trees called "clones." In other words, many new trees grow from the roots of a single, parent tree. The trees in a clone are genetically identical and their roots are often interconnected. Individual clones can occupy an area from a few tenths of an acre to many acres in size.
Aspen trees are not long-lived. By age 60 or so, trees within a clone often begin to die creating gaps in the canopy. As clones deteriorate, they are subject to high levels of root mortality as well as high levels of disease and injury. These deteriorating clones may produce significantly fewer suckers following harvest than their healthy counterparts.
A persistent question facing forest managers is, "How many aspen suckers are required following harvest to successfully regenerate a stand?" Aspen stands commonly contain between 200 and 500 trees per acre at maturity or harvest age. Unfortunately, it is difficult to specify how many root suckers will be needed after harvest to ensure that 200 to 500 aspen trees per acre will reach harvest age on the regenerated site.
Sucker stands reach their maximum density within two years following harvest. The density then decreases as individual trees die through a process of natural thinning. Mortality is controlled largely by competition between individual trees for light, moisture, and nutrients.
Figure 4. Aspen suckers infected with shoot blight, a common disease in the Lake States region. (Photo courtesy of USDA Forest Service.)
Disease and injury are another cause of death in sucker stands (Figure 4). Diseases in the Lake States region include hypoxylon canker (Hypoxylon mammatum), shoot and leaf blight (Venturia macularis), and root rot (Armillaria mellea). The forest tent caterpillar (Malacosoma disstria) is a common source of damage. Galls and lesions caused by insects, stem girdling caused by small rodents, and concentrated grazing and browsing by livestock or wildlife are other sources of injury.
Disease and injury are sometimes so serious that even dense sucker stands will not develop to their full potential. Frequently, however, disease and injury have no adverse effect on stand development. The number of individual trees killed by disease and/or injury is often about the same number that would have been lost from natural thinning. It is difficult to determine whether or not disease and injury will reduce yield; the outcome is determined largely by the condition of the stand and the specific disease- and injury-causing agents. A professional forester can best evaluate the potential impact of injuries and disease on yield.
Generally, 4,000 to 5,000 well-spaced suckers per acre, two years following harvest, will be adequate for regenerating the stand. A greater number of suckers is preferable, however, because they provide insurance against unexpected losses from disease or injury.
The effect of harvesting on regeneration
Timber harvesting affects the physiological and environmental factors that control suckering. Completeness of tree removal, the season of harvest, and the extent of site disturbance are the harvesting factors with the greatest effect on regeneration.
Clearcutting, the complete removal of the stand in a single harvesting operation, is the recommended practice for regenerating aspen. Clearcutting maximizes regeneration by stimulating suckering on the greatest number of roots and minimizing shade on the site. Soil warmup, sucker initiation, and sucker growth rate after emergence are all accelerated by clearcutting. In some cases it may not be desirable to remove all trees from a site; some of the trees may be unmerchantable or the landowner may wish to leave some trees for wildlife or aesthetic purposes. Nevertheless, no more than 20 to 30 mature trees per acre should remain standing following harvest.
The extent to which aspen regeneration occurs is largely controlled by the amount of carbohydrates present in aspen roots, length of time until the first frost, and soil conditions at the time of harvest. Harvesting during the spring and early summer results in the least and slowest growing suckers. Suckering increases as harvesting occurs later in the growing season. Harvesting in the winter, when the trees are dormant, results in the most and fastest growing suckers.
Carbohydrates stored in aspen root tissue sustain emerging suckers until they develop leaves and begin producing their own food. Carbohydrate reserves are lowest in the late spring and early summer when they are utilized for bud-break and leaf expansion. Thus, suckers produced as a result of early summer harvesting may be less vigorous. These root reserves gradually increase during the summer and peak in the fall.
Summer harvesting may also cause aspen suckering to begin after other vegetation on the site has already leafed-out. Shade from any source can slow the growth of aspen suckers. In addition, suckers initiated in the summer may continue to grow too late into the fall and suffer frost damage.
Perhaps the most critical factors associated with summer harvesting are the soil conditions present when suckering begins and the amount of root and site disturbance caused by the harvesting operation. Dormant season harvesting when the ground is frozen causes virtually no site disturbance. However, summer harvesting, particularly when soils are wet, can diminish aspen regeneration.
Figure 5. Rutting and the associated site disturbances caused by the operation of harvesting equipment on wet soils. These disturbances may result in injury to the shallow aspen roots. They also may disrupt the natural drainage patterns on some sites increasing saturated soil conditions. Disturbances of this kind can lead to reduced regeneration.
Soil strength, or the ability of a soil to support harvesting equipment, decreases rapidly with increasing soil moisture. Rutting, which occurs when tires or tracks break through the soil surface, develops more easily on wet sites (Figure 5). Rutting can reduce regeneration by severely damaging the shallow aspen root system and decreasing its ability to sustain emerging suckers.
Decreased soil strength also causes soil pores to collapse under the weight of harvesting equipment and, along with rutting, can aggravate flooded or saturated soil conditions. Low oxygen levels in saturated soils also restrict suckering. Warmer temperatures will reduce the time that roots can survive without oxygen.
A final harvesting practice that affects regeneration is multiple-entry (moving the harvesting equipment onto the site several times) or prolonged operation over the same area within a stand. When root temperatures are maintained above 55° F, the suckering process begins immediately after trees are harvested. Suckers are often visible two to three weeks after felling. Prolonged operation in an area after the aspen have been harvested can seriously injure regenerating suckers.
Logging slash, the tree tops and limbs left on the site after harvest, generally does not reduce regeneration success. This is particularly true when only a small portion of the site is covered by heavy slash accumulations that shade the ground surface. Slash contains large amounts of nutrients and its even distribution across the site lessens the amount of nutrient displacement associated with harvesting.
Management implications and recommendations
When the management goal is to regenerate aspen, the best strategy is to maximize the number of vigorously growing suckers produced following harvest. To accomplish this goal: (1) minimize disturbance and injury to the parent root system and (2) minimize site disturbance that can lead to saturated soil conditions. Fortunately, these two objectives are compatible.
Identify sites that are potentially sensitive to harvesting operations when planning a timber sale. These sites will be poorly drained and characterized by very little slope and a high water table. The most susceptible sites are those dominated by clayey soils. These soils have little strength when wet and take a long time to drain once saturated.
Delay harvesting operations on sensitive sites until they have dried out sufficiently to support harvesting equipment without rutting. On sites that rarely dry out, harvesting when the ground is frozen may be the only option. When harvesting sensitive sites in the summer, concentrate equipment operation on as few trails as possible to minimize the area of site disturbance and root injury. Rutting or displacement of surface litter to expose mineral soil may be detrimental to regeneration.
Figure 6. A back-to-front harvesting pattern designed to maximize regeneration on sites potentially sensitive to regeneration problems. Primary skid trails are arranged to minimize the area of the site subjected to concentrated skidding traffic. Trees are felled first in areas furthest away from the landing to avoid subsequent and prolonged operation of equipment over areas where the trees have been harvested.
Another effective practice is to harvest from "back-to-front" (Figure 6). Begin the harvest from the furthest reaches of the cutting area rather than starting near the landing. Harvesting from back to front will require skidding trees (dragging logs behind a skidder or tractor-like machine) through the uncut stand or on an adjacent tract. This pattern allows the harvesting operation to be concentrated on drier sites. The living trees remaining between the landing and the area being cut maintain drier soil conditions by intercepting precipitation and by utilizing soil water. The back to front cutting pattern may also shorten the delay when halting a harvesting operation for wet conditions. The uncut area will dry faster allowing operations to resume sooner.
Plan timber harvests so that operators do not have to gain access to a stand by going through areas being regenerated. If this is not feasible, concentrate equipment operation on as few trails as possible in the regenerating stand. In a single stand containing several species, remove all species simultaneously or remove the aspen last. For long-range timber sales, plan ahead to provide clear access from one stand to the next. By providing clear access, skidding through sucker stands can be avoided.
Though these recommendations will raise harvesting costs on some sites, they are designed to increase successful regeneration even on sensitive sites, thereby improving future harvests.
For more information on aspen management, contact a professional forester. Most states employ private forest management professionals who assist woodland owners at little or no cost; the government section of your local telephone book should list these contacts, generally under headings such as Department of Natural Resources or Department of Conservation. Private consulting foresters are listed in the yellow pages of the telephone book, generally under headings such as Foresters-Consulting. Some large forest industries provide forest management and marketing assistance to private woodland owners. Listings of public, consulting, and industrial foresters also may be available through county extension agents or extension foresters. Telephone numbers for county extension offices are listed in the Government Section of the telephone book.
Interested individuals may also consult the following publications:
DeByle, Norbert V., and Robert P. Winokur, eds. 1985. Aspen: Ecology and management in the western United States. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO. General Technical Report RM-119. 283 p.
Perala, Donald A. 1977. Managers handbook for aspen in the north-central states. USDA Forest Service, North Central Forest Experiment Station, St. Paul, MN. General Technical Report NC-36. 30 p.
Editor: Rich Sherman
Graphic Designer: Michael Mechavich
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