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Forest Damage Inflicted by Select Species

Information for this section is quoted from NOLTE, D.L. AND M. DYKZEUL. 2002. Wildlife impacts on forest resources. Dale L. Nolte and Mike Dykzeul. 2002. Pages 163-168 in L. Clark, and J.K. Yoder, editors, Human conflicts with wildlife: economic considerations. Proceedings of the 3rd NWRC Special Symposium (August 1-3, Fort Collins, CO). National Wildlife Research Center, Fort Collins, CO.

The negative impacts of wildlife on forest resources can be extensive. Although damage is most often considered in terms of reduced productivity or delayed harvest cycles, attempts to replace trees after a delayed harvest cycles, attempts to replace trees after a harvest or a fire can also be complete failures because of foraging wildlife. The full impact of wildlife on forest resources is frequently difficulty to assess because of the complexity of the resource...This paper provides some insight into the economic and environmental consequences of wildlife damage to forest resources and a brief overview of the damage inflicted by select wildlife species.

The temporal and spatial scales of forests ensure varied habitats, and wildlife species change with the habitat. For example, high populations of pocket gophers may occur in young stands but are unlikely to be present in mature stands, and mountain beavers exist in high numbers in stands along the coast of Oregon and Washington, but are never found in inland forests. Vulnerability to damage also depends on stand age. For example, deer may inflict significant and repeated damage to young seedlings, but rubbing damage to older stems is rarely detrimental to the tree. Conversely, bear rarely damage trees less than 15 years of age.

Therefore, whether calculating potential damage losses or figuring costs to implement preventive measures to protect forest resources the estimates must be based on the current state of the forest, reflecting the species present which, in turn, affect the potential type and the extent of damage and future controls that might be needed as the nature of the resource changes and becomes vulnerable to a new suite of wildlife capable of inflicting damage.

A brief overview of the type and extent of damage inflicted by a few select wildlife species is provided below. The reader is referred to Black (1992, 1994), Hyngstrom et al. (1994), Nolte et al (1996) for more complete descriptions. We selected these species because of the severity of frequency which they inflict damage to forest resources.

Bear (Ursus americanus). Bears feed on the vascular tissue of trees by removing the bark with their claws and scraping the sapwood from the heartwood with their incisors. Bears generally feed on the lower bole of trees in stands between 15 and 30 years old. Any age tree, however, is vulnerable, and bears occasionally strip and entire tree. Damage within a stand can be extensive and a single foraging bear may peel bark from as many as 70 trees per day. Damage inflicted through this behavior can be extremely detrimental to the health and economic value of a timber stand. The severity of timber loss is compounded because bears tend to select for the most vigorous trees within the most productive stands or where stand improvements (e.g. thinning) have been implemented.

Beaver (Castor canadensis). Beaver activity can have severe negative impacts on agricultural resources and infrastructure developments. In the southeastern United States alone, economic losses attributed to beaver have been estimated to exceed US $40 billion over a 40-year period. Most of the damage is a result of flooding and the subsequent losses of timber, crops, roadways, and other resources. Less, but substantial damage occurs through bank burrowing, and tree cutting or girdling. Conical-shaped stumps and large wood chips at the base of stumps are prime indicators of beaver damage. Peeled sticks with uniform horizontal tooth marks also are generally found in the vicinity of beaver activity.

Deer (Odocoileus spp.) and Elk (Cervus spp.). Browsing by big game species, such as elk and deer, inflicts the most widespread form of damage to forest resources. The similarity of deer and elk damage ofter prevents specific assignment of cause of damage. However, the wider distribution of deer suggests they are probably the most prevalent cause. Although lateral branches are browsed, damage to the terminal leader causes the most problems. Repeated annual browsing terminal shoots distorts growth, suppressing tree height and converting seedlings into a bushy growth. Delayed growth lengthens the rotation period for timber stands. Extensive browsing can cause mortality. Unlike elk, deer rarely trample seedlings or pull them from the ground and most deer damage occurs below 6 feet. Elk, on the other hand, can pull seedlings without we established root systems out of the ground, and elk traveling in herds can severely trample new stands. Additionally, stems browsed by elk are often splinte red, and during the spring the bark below the break may be stripped from the stem.

Mountain beaver (Aplodontia rufa). -Reforestation efforts can be difficult, or impossible, on sites occupied by high numbers of mountain beaver. Mountain beavers clip seedlings up to an inch in diameter. Their diagonal cut is typical of rodents, but multiple bites may create a serrated edge. Although, mountain beavers are mot often associated with seedling damage, they also gird the base and undermine the roots of larger trees. Mountain beaver girdling can be readily distinguished from bear girdling because the damage is lower on the bole and mountain beavers leave horizontal tooth marks and irregular claw marks. This damage generally occurs as the canopy begins to close and shading reduces the availability of forage more preferred by mountain beaver. Over time, as these trees suffer mortality, substantial meandering openings may appear across a forest stand. Prime indicators of mountain beaver activity are numerous shallow burrows and burrow entrance: along with fresh digging, or fresh vegeta tion and debris piled near burrow entrances.

Pocket gopher (Thomomys spp.). Reforestation efforts are often severely hindered on sites that contain high populations of pocket gophers. Efforts to establish tree seedlings on sites infested with pocket gophers can be futile unless protective measures are implemented. Pocket gophers commonly prune roots of seedlings and girdle or clip seedling stems. Small seedlings, less than .25 inch in diameter, are the most vulnerable. The stems generally are clipped at or near ground level and pocket gophers may pull harvested seedlings into their bur- rows. Pocket gophers also prune the roots and girdle the stems of larger trees. Extensive above-ground girdling is fairly easy to detect. Damage to roots, however, may go unnoticed until seedlings tip over or become discolored. Non-lethal damage causes poor overall growth, shortened needles, reduced internodes, premature needle drop, and needle discoloration.

In all of the above examples, the economic impact caused by a species is a function of the current damage plus future losses plus the costs associated with replacement. For mature trees, the loss must account for time to reestablish the tree to a harvestable age. Thus, in time, the resource loss is the monetary value anticipated at time of harvest plus the monetary value for protection up to the point of damage, plus the time-integrated costs associated with reestablishment to future harvest.

Literature mentioned in the text above:

Black (1992, 1994), Hyngstrom et al. (1994), Nolte et al. (1996) for more complete descriptions.

BLACK, H.C. 1992. Silvicultural approaches to animal damage management in Pacific Northwest Forests. USDA Forest Service, Pacific Northwest Research Station, General Technical Report PNW-GTR-287, Portland, Oregon, USA.

BLACK, H.C. 1994. Animal damage management handbook. USDA Forest Service, Pacific Northwest Research Station, General Technical Report PNW-GTR-332, Portland, Oregon, USA.

HYGNSTROM, S.E., R.M. TIMM, AND G.E. LARSON. 1994. Prevention and control of wildlife damage. University of Nebraska Cooperative Extension, Lincoln, Nebraska. USA.

NOLTE, D.L. 1999. Behavioral approaches for limiting depredation by wild ungulates. Pages 60-69 in K.L. Launchbaugh, J.C. Mosley, and K.D. Sanders, editors. Grazing behavior of livestock and wildlife. Idaho Forest, Wildlife and Range Experiment Station, Moscow, USA.