Big Tooth (Large Tooth) Aspen (Populus grandidentata)

DISTRIBUTION AND HABITAT: Aspen was historically considered a "weed" species until a number of industries found a niche for the fiber. Currently, aspen is an important resource used mainly as pulpwood for paper mills. Both species have become a concern to some land managers. Although aspens are native to Wisconsin, they are sometimes invasive because their prolific clonal growth pattern allows them to shade out herbaceous species in prairies and oak savannas.

Both species are normally found in woods and woodland edge habitats in Wisconsin, especially on cut-over or burned land. Bigtooth aspen grows from Nova Scotia to Minnesota, southward to North Carolina, and westward to Missouri. Quaking aspen inhabits a wider range: from Labrador to Alaska in the northern part of the continent, from New Jersey westward to Nebraska, and along the Rocky Mountains southward into Mexico.

While they are a natural part of early successional woods, aspens become a problem in prairies that have not been managed with fire for some time. Both species thrive on a wide variety of sites. Typically, quaking aspen is found in moist woods and along streams, while bigtooth aspen grows in comparatively drier soil. Both are found in young woods after disturbance and at the edges of mature woods.

LIFE HISTORY AND EFFECTS OF INVASION: Bigtooth aspen is a gap-phase tree of importance in the dry to mesic forests of Wisconsin. It requires soil disturbance for establishment, and is usually found in forest gaps created by fires or harvests. The ashes found on burned soil surfaces offer optimal conditions for germination. Quaking aspen is a pioneer invader following forest fires, logging, or other episodes of disturbance.

Both species produce an abundance of wind-dispersed seeds. Aspens flower in March and April; fruit ripens 4-6 weeks after flowering, generally from May to June. The seeds are small, very widely dispersed, and must germinate within a few days of their dispersal. Seedlings grow extremely fast, often at a rate exceeding three feet per year for the first decade. Clones expand radially by sprouting 3-6 feet of shallow, horizontal roots per year, depending on the site. By the time aspen individuals are 20 to 30 years old, their canopies expand and shade out other clones in the stand, thus encouraging fungal diseases to attack the shade-intolerant trees.

CONTROLLING ASPEN
Mechanical Control: Although it is labor intensive, girdling is most successful in clonal stands where most individuals are larger than 1" in diameter. This method is not effective on young clones that have resprouted. All stems in the clone with a diameter greater than 1" should be girdled. The girdle should be at least 2" wide around the tree to prevent the bark from bridging across the girdle. Girdle aspens in the spring up until leaves reach full size in May or June. It is easiest to separate the bark from the tree at this time. The technique of girdling requires making a cut just through the bark to the outer layer of sapwood. These cuts can be made with a bark spud (made from a sharpened car spring) or a crowbar. On smaller stems, a beveled butter knife may be used. Avoid using saws because they may cut too deeply, thus stimulating resprouting. After making the cut, insert a sharpened bark spud into the natural dividing region between the bark and the sapwood. Rotate the girdling tool around the trunk to remove the bark. Leaving the sapwood intact allows trees to continue pulling water, nutrients, and carbohydrates up from the root system. Removal of the bark prevents the shoots from sending carbohydrates to feed the roots. Roots starve slowly, and the trees usually live for 1 year after girdling. If removing trees, wait until they are completely dead.

Fire or ill-timed cutting of live aspen can make established clones very difficult to remove, and therefore is not recommended. Aspen responds to stem removal by generating an imbalance of hormones in the roots to promote the formation of root sprouts or suckers. Once the clones have been put into the hormonal "suckering mode," there are no known treatments to prevent their continued production of suckers. However, aspen may be controlled by using fire in August. Frost will kill reprouts.

Stem cutting is much less effective than girdling, but can be used as a control method. In order to avoid the formation of suckers, cutting must be timed to coincide exactly with maximum leaf-out in mid to late July, when most resources have been translocated to stems and leaves and root resources are at their lowest. This initial cut must be followed by repeated hand cutting of sprouts in the same growing season, or again at maximum leaf-out in subsequent growing seasons. Follow-up cuttings should be made by hand to allow the competing, shade-producing vegetation to remain standing. Ideally, the initial, well-timed cut will cause up to a 50% reduction in stem density. Cutting can be done with loppers, a chain saw, power brush cutters, or a brush hog.

Cutting can be effective if coordinated with some other mechanical control on sites other than natural areas. A large clone may have the overstory cut, followed by a leveling of the resprouts using heavy site scarification equipment. This has proven to be a cost-effective option in aspen control, but can be damaging to other vegetation in the area.

Scarifiers such as roller choppers, discs, and root rakes can be used to mechanically control aspen. If possible, a single pass in July should be followed by a second pass in August (after reprouting) for optimal control. Based on field experience, multiple passes during the growing season are more effective than a single pass.

Chemical Control: Basal injections or basal bark applications of triclopyr to uncut stems are the best means of controlling aspen chemically because application is easy and injury to other species is minimal. Every stem of the clone must be treated. Some damage to surrounding vegetation should be expected with these techniques. Young suckers or cut clones can be treated with a wick application of 25% glyphosate active ingredient (a.i.) on the stems, although this method has not proven completely effective. The herbicide 2-4D also works as a foliar application.