Damage and Damage Identification

Mountain beaver, Aplodontia rufa

Mountain beavers have damaged an estimated 300,000 acres (120,000 ha) of commercial coniferous tree species in western Washington and Oregon. Much of the affected land has the potential to produce timber values of over $10,000 an acre. The damage period extends to about 20 years after planting. The major losses occur from cutting tree seedlings during the first year after planting (Fig. 6). Secondary damage occurs during the next 5 years to surviving tree seedlings, followed by stem girdling and root damage for the next 10 to 20 years. Increased need for weed and brush control and occasional replanting costs add to the economic losses caused by mountain beavers.

Figure 6. Mountain beaver in feeding position.

Damage to conifer seedlings is identified by angular rough cuts on stems 1/4 to 3/4 inches (0.6 to 1.9 cm) in diameter. Mountain beavers climb larger trees and cut stems near the tips. Limbs are often cut a few inches from the stem. Small trees are usually cut near ground level while others may be cut several feet up the stem. Seedling damage occurs primarily in winter and early spring, but often continues throughout the year.

Figure 7. Mountain beaver–girdled conifer tree.

Most stem-girdling damage is at the base of 3- to 6-inch (7- to 15-cm) diameter stems (Fig. 7). Girdling damage can be distinguished from that caused by bears or porcupines in that mountain beavers do not leave pieces of bark scattered on the ground and they cut the bark smoothly along the edges. Girdling damage to older stems is more difficult to distinguish, but it can be verified by examining burrows near tree trunks where fresh girdling can be seen on the roots.

Root girdling may occur at any age, but small roots are usually cut instead of girdled. Trees with stems over 6 inches (15 cm) in diameter may die due to extreme root girdling. Root girdling may allow tree root pathogens to become established in individual trees and spread to other trees. It occurs in winter and spring, and may occur in other seasons.

Mountain beaver damage in 10-to 15-year or older stands appears to be increasing and is of great concern be-cause the crop trees are often selected at this time for precommercial thinning. Stem and root girdling may affect over 50% of the trees in a stand. Managers cannot achieve proper spacing in these damaged stands, and damage may continue on trees left as crop trees.

Damage to coniferous species is considered detrimental to forest production and can have long-term effects on habitats. This damage to commercial crops and other vegetation, however, does provide diversity of cover for other wildlife. In one area on the Olympic Peninsula in Washington, the excessive damage to conifers by mountain beavers caused a manager to change the area designation from reforestation land to wildlife habitat.

Legal Status

Mountain beavers are generally considered unprotected nongame species. Individuals wanting to control mountain beavers should consult their state fish and game agency to determine current regulations. A subspecies in California is considered endangered. Information on registered pesticides is available from the state’s Department of Agriculture.

Economics of Damage and Control

Mountain beavers cause considerable economic damage to reforestation. Most of their habitat is in timberland where the potential crop value is high. Well-stocked stands of Douglas-fir are usually commercially thinned once or twice before final harvest, and often produce timber values of thousands of dollars per acre. When mountain beavers prevent reforestation or cause expenditures for protecting reforestation, the value of the crops is reduced or eliminated. A planned Douglas-fir crop rotation period of 40 years on good sites can be severely disrupted if at 15 years the crop is lost to damage by mountain beavers. Since mountain beaver damage occurs on about 300,000 acres (120,000 ha) of commercial forest land, a conservative annual loss estimate of $100 per acre ($250/ ha) results in an annual loss of $30 mil-lion. Losses to mountain beavers may be $10,000 per acre ($24,700/ha) when damage causes failure of the timber crop.

Economic losses are caused by both direct and indirect damage. Cutting of planted tree seedlings is the most common damage. If it has been several years since planting, the site may need brush control by machine, hand, or herbicide before replanting can be done. Damage to tree seedlings also keeps the trees within a size range that is susceptible to damage by hares, rabbits, deer, and elk. If damage is not controlled, large areas may not be adequately reforested. Trees that escape early damage may be damaged later by girdling and undermining by mountain beavers, causing a loss of many years’ growth of commercially valuable species.

The mountain beaver currently has no commercial value. The pelt has no fur value and there is no market for the meat. The animal is of significant zoo-logical and medical interest, however, because of its limited range and unique physiological characteristics. Despite its limited range, however, the overall populations of mountain beavers have probably increased since timber harvesting began in the Pacific Northwest.

The burrowing and vegetation cutting activities of mountain beavers may improve soils and reduce competition by brush species. Sometimes, however, the burrowing activity has caused damage to roads and trails. Forest workers are periodically injured by falling into mountain beaver burrows.

An economic study of Pacific North-west forest animal damage indicates that damage control expenditures of about $150 per acre ($375/ha) are reasonable on average-site Douglas-fir forest land. On higher quality land the expenditure for damage control can be higher, particularly where mountain beavers cause heavy mortality in reforestation areas.