from May 1999
Avoiding Wildlife Damage to Northwest Conifers (Part one of a three-part series)

Dan L. Campbell

Warning Signs: Most wildlife damage to trees is not spectacular, and therefore may go unnoticed until too late to provide an economical remedy. Damage from wildlife, however, may be as costly as damage from insects, disease, or fire. Damage to root systems by voles, pocket gophers, and mountain beavers may go unnoticed until tree seedlings are yellow or dead, or until larger trees with clipped roots fall during wind or wet snow. Many poorly stocked conifer stands west of the Cascades are the result of wildlife damage. Many under-stocked acres in conifer stands, often occupied by low quality alder or other species, are still occupied by mountain beavers. Browsing damage by deer and elk may appear insignificant until brush species overtake and shade out the browsed conifers. Deer browsing on over 15 percent of the tree seedlings in a plantation, during the first spring after planting, usually means there will be much heavier browsing during the next two or three years. Deer and elk browsing on Christmas trees can cause several years’ loss of growth, and badly deformed trees. Bears may start “sampling” the bases of trees as stand canopies close. These small bites often occur a year or two before bears begin more extensive tree girdling.

Snowshoe hare clipping of planted tree seedlings diminished from being the number one reforestation problem in the 1950s and 1960s, to currently being insignificant. Research on snowshoe hare behavior was conducted by biologists, and forest managers were then able to avoid most problems. It was shown that snowshoe hares required winter hiding cover and that Douglas-fir seedlings of larger size and stem diameter were resistant to clipping by hares. Replanting with large stock soon after timber harvest has avoided most hare damage problems. Under-stocked sites with cover may require other tree protection. If the factors leading to damage by wildlife can be recognized, many serious problems can be avoided. Extensive wildlife damage to plantations is often a result of several species injuring the trees. Early recognition of potential wildlife damage problems are necessary to avoid damage.

Habitat Conditions, Wildlife Behavior, and Populations: The density of wildlife populations are dependent on many factors. Burrowing rodents such as mountain beavers and pocket gophers are territorial and solitary within their own defended burrow system. Each animal has an individual nest, a nearby fecal chamber, and several underground food storage caches. When an individual mountain beaver or pocket gopher occupies a burrow system in good habitat, the removal of that individual does not assure a vacancy. In fact, the vacant burrow system and nest is often rapidly reoccupied by an adjacent animal or a young animal looking for a territory that is not defended. Some individual nests, which may be six feet deep, are often maintained for many years, even into new crop rotations. If a nest becomes uninhabitable, or can be made uninhabitable, that burrow system may be vacated. Individual mountain beavers usually occupy burrow systems of about one-fourth acre, but they seldom occupy cleared sites used for Christmas trees which do not have suitable above or below ground vegetation to support the animals. Scarification may help reduce populations as long as slash and soil is not piled to provide new homesites. In sapling size stands they may begin girdling conifers as understory vegetation is shaded out. Mountain beavers, and even pocket gophers will persist almost indefinitely, but usually at lower densities, in older stands having adequate understory vegetation.

Pocket gopher behavior is very similar to that of mountain beavers, except that pocket gophers are very tolerant of open areas where the roots of palatable plants are available. Individual burrow systems of pocket gophers may require only about one-sixteenth acre, but they are very dependent on their individual nests. Vole populations are usually higher in plantations where grasses have become dense. Fecal droppings will often be seen on small bare patches of ground, and runways can be seen in the grass. Damage from girdling and clipping can be expected when these indicators of vole activity are seen. The amount of deer and elk damage to conifer plantations is usually related to animal densities and travel patterns, and in seasonal changes in availability of forage plants. The palatability of the conifer species being grown is usually a major factor in the amount of damage that can be expected. Western hemlock, for example, is usually lower in preference than Douglas-fir or western redcedar. True firs such as noble fir have been considered less palatable, but recent deer damage to noble fir Christmas tree plantations is causing concern and need for protection. At the other end of the palatability scale, western yew is very palatable and new seedlings seldom become established because of its high preference by a variety of wildlife.

Most deer browsing on conifers is a cumulative process, and even individual deer may cause significant browsing damage in certain areas. An individual deer often uses similar daily travel routes within a period of time, such as in May and June, browsing just a few conifers each day, but causing extensive damage over the time period. If several deer or an elk herd have similar behavior and travel patterns in an area, there may be significant damage to the conifers. In springtime, during the first period of conifer budburst, it often appears that there is an abundance of spring forage for deer. However, during the period of most growing season damage, from April to June, there is often a shortage of preferred forage, particularly flowering forbs. As the season progresses, damage to conifers usually decreases as flowering of preferred forage increases in abundance. During the second or third bud-burst of Douglas-fir there is usually not much conifer damage by deer, because other forage is usually adequate by mid-summer.

Winter damage by deer to conifers in western Oregon or Washington is usually caused by gradual depletion of key forage species such as trailing blackberry from November into February. January and February may have severe shortages of forage. Elk damage to conifers can be more intensive because of their normal activity in groups. This group behavior may cause trampling as well as browsing. Elk will often pull more newly planted seedlings than deer. Editor’s Note: This is the first part of a three-part series by Dan Campbell. He is a retired USDA Wildlife Research Biologist and heads the private consulting firm of Wildlife Services, Inc.