Beavers often prefer to eat seaweed, clover and other herbaceous vegetation during warm weather, instead of the green bark (cambium) of trees. But, in areas with harsh winters, they normally need to prepare an underwater food cache of branches. They also use peeled sticks to build their lodges, and dams. Because one beaver family (colony) often builds several lodges, the number of lodges is not a reliable way to estimate the population. They have only one litter per year and rarely overpopulate since two-year-olds leave each spring to seek new homes, and reproductive rates decline as the area is filled. Removing beavers rarely gives a lasting solution since survivors have larger litters, and others will resettle good habitats.
Beavers prefer fast-growing trees, such as poplar, willow, cottonwood and alder, which normally have little commercial value. Although the felling of these trees appears destructive, such pruning often results in more, bushier growth in the spring. For example, each cut willow stem can lead to three to four new stems. If the beavers then use the branches for a dam that creates a wetland, great benefits can result, such as erosion abatement, flood control, water cleansing and more biodiversity.
When it is desirable to protect trees from beaver felling, consider that most cutting occurs within five yards of shore, and that the likelihood of damage decreases as the distance from shore increases. Also, while beavers prefer certain tree species, they do not necessarily take them in order of preference. Leave the trees that are already down, while protecting others, and in urban areas tree trimming companies may drop off branches to provide alternative food. When planting trees along shorelines, consider less palatable varieties, such as spruces in the eastern U.S. and cascara in the West.
Cylindrical cages are the best way to protect valuable trees. Make them of hardware cloth or sturdy 2 x 4 inch welded wire fencing, about four feet high (three feet is adequate in areas without snow). Encircle the trunk, leaving a space of about six inches between the tree and the fence. Cut every other horizontal wire and bend into hooks to connect with the other side. Cages can be anchored to the ground with stakes.
Turkey or chicken wire is less reliable, but it is sometimes used to protect many small trees as, for example, with mitigation plantings. If this type of wire is used, stake close to the trunk to prevent crushing by beavers.
A newer method to prevent beaver gnawing involves coating tree trunks with a sand and paint mixture. Use 8 ounces of fine sand (30 mil, 70 mil or mason sand) to one quart of oil or latex paint. Stir often and paint trunks about four feet high. The paint can be clear or color-coded to match the trees. Avoid painting young trees less than about six feet tall as this may be harmful.
Low fences can be used to protect groups of trees, and normally need not surround the entire stand, since beavers dislike being separated from the water. Have the fence fit tightly to the ground and trail each end toward the water. Monitor often in the beginning for burrowing. If digging occurs, two concrete blocks tied together can be used to block the tunnel.
An electrified wire strung 4 - 6 inches off the ground serves the same purpose as a low fence.
Repellents can protect saplings and foliage plants from beaver for a few months, and work best when there's other food available. Research indicates that repellents containing sulfur compounds, such as Deer Off and Big Game Repellent Powder, are effective as temporary deterrents, but they are not yet registered for this use with U.S. EPA. In a recent trial where Chew-Not was painted on trunks, this product has been effective for a year.
Success has also been reported in California using volunteers to lure beavers from a problem area to a new location. Methods used include leaving scat from otters at the site, spraying almond extract to attract beavers to willow stands that need thinning and placing favorite food (such as fresh willow or poplar branches) where it is wished to have the beaver relocate.
A year-long trial to protect hundreds of restoration trees with a combination of "4 the Birds," or "Birds Away," and sand was successful, according to biologist and beaver consultant Mary Tappel. The sticky substances were applied with a brush that was also dipped in sand, and a strip was painted about four feet up the trunks of the saplings. This method is not suitable for older trees, where trunk climbing birds might be harmed.