  Privacy
| Legal |
Table of Contents Back | Next | Home | Cover Page |
 Technology & Development Center |
Timber
Damage by Black Bears:
Approaches to Control the Problem
When black bears (figure 1) emerge from their winter dens, foods are relatively scarce. Because trees are already producing sugars (carbohydrates) during the early spring, bears strip the bark and eat the newly formed wood underneath.
Figure 1—Black bears strip the bark from trees to eat the sapwood.
Bears have caused significant damage to some stands of timber, particularly in the Pacific Northwest. This report considers why bears cause such damage and what managers can do to reduce the damage. Sugar concentrations vary, depending on the tree species and the season. For instance, bears generally forage on hemlock before Douglas-fir, which probably reflects an earlier flush of nutrients in the hemlocks. Similarly, bears may have sampled a tree early in the spring, only to return a few weeks later and strip it, presumably when sugar concentrations are greater.
Bears use their claws to strip bark from a tree, then feed on the sapwood (newly formed outer wood) by scraping it from the heartwood (older central wood) with their teeth. Scattered remnants of bark strewn at the base of a tree and vertical tooth marks indicate bear activity. Occasionally a tree will be “frilled,” with bark strips loosened at the base and pulled up away from the tree. These strips may hang from more than 5 meters up on some trees, such as western red cedar.
Bears usually forage on the lower trunk of trees, girdling the bottom 1 to 1.5 meters. Some bears may climb the tree and sit on branches to feed higher up. Bears have been known to strip entire trees. Damage within a stand can be extensive. A single bear can strip bark from as many as 70 trees per day.
Stripping trees for food is different than marking trees to stake out territory. “Bear trees” are rubbed and scent-marked by both sexes, especially by adult males before and during the mating season. Marks are usually made by biting or clawing conifer or deciduous trees about 1.5 to 2 meters above the ground. Although marked trees are common in most areas where black bears are found, damage caused by marking is not severe.
Tree species stripped by bears vary depending on the location, probably reflecting the species available. In the Pacific Northwest, bears frequently girdle (strip bark all the way around the trunk) of Douglas firs, primarily immature smooth-barked trees ranging from 15 to 30 years old. Girdled trees die because they cannot transport nutrients from the branches to the roots. Trees of any age are vulnerable to bear damage. Although western hemlocks are sometimes stripped, they are stripped earlier in the year and are stripped less frequently once Douglas fir trees break dormancy. Bears appear to prefer redwoods in northern California, western red cedar in British Columbia, and western larch in interior forests. Other species reported to have been stripped by bears include silver fir, balsam fir, grand fir, subalpine fir, noble fir, bigleaf maple, red alder, western larch, Port Orford cedar, Engelmann spruce, white spruce, red spruce, Sitka spruce, whitebark pine, lodgepole pine, western white pine, aspen, black cottonwood, bitter cherry, willow, and northern white cedar.
 |
E-mail: