Damage from deer to a Scots pine. Courtesy of Jan Liska, Forestry and Game Management Research Institute, Bugwood.org (#2113036)
Birds tend not to be a big problem associated with Christmas tree production, but damage occasionally occurs to the tops, particularly of taller trees (Figure 1). This is especially common during spring, when the new growth is supple and not hardened off. The weight of the birds causes the terminal to bend and break. This damage is sometimes mistaken for white pine weevil damage; however, the birds will cause a sharp bend or break instead of the curled, wilted damage caused by the weevil. Taller trees are targeted because birds need a good view in order to defend their territory. Most growers having repeated damage will install poles or perches above tree height to alleviate the problem.
Figure 1. Bird perching on a tree, which can cause the leaders to bend or break. Courtesy of Rayanne D. Lehman, PDA
Later in the year, birds may cause another type of damage as they perch on the stiff, mature terminals. Needles may be pulled from the terminal by the birds’ feet. This damage is minimal and will be corrected with another year’s growth. Noise-producing devices have been used, but birds learn to tolerate them over time.
Deer will feed on all conifers, but spruce is the least preferred. Feeding damage may occur during winter when preferred foods are not available. The ends of deer-browsed shoots appear ragged because deer lack upper incisors and must bite down and rip food sources in order to feed (Figure 2).
Figure 2. Damage from deer browsing. Courtesy of John H. Ghent, USDA Forest Service, Bugwood.org (#0796079)
The most common damage from deer occurs in late summer and fall when bucks polish their antlers by rubbing them against the trunks of small trees (Figure 3). Damage may be more evident along the edge of blocks adjacent to woodlots as deer are an edge species. White-tailed deer cause more damage to Christmas trees than any other animal.
Figure 3. Conifer bark and branches damaged by bucks polishing their antlers. Courtesy of Sarah Pickel, PDA
Methods of limiting deer damage to Christmas trees include fencing and repellents. Fencing must be justifiable by being economical and of low maintenance. Studies on types of deer fence have shown three kinds to be effective. Woven wire fence is the best choice where deer damage is intolerable. Multistrand, electric, high-tensile fences are good in situations when total deer exclusion is not a necessity and the cost of fencing is not an issue. An electric poly-fence is good for seasonal protection. Each grower must look at all fencing options, farm operations, finances, and goals to decide the best choice for their farm.
A second possible option for limiting deer damage is the use of repellents. Feed treated with lime provided the best repellency in one study; charcoal was the second most effective. Bittering agents were shown not to be effective over extended periods in another study. Growers have hung bars of Ivory soap from the trees with mixed results.
Some other options for deterring deer are keeping a dog in the vicinity of the tree fields and allowing hunting on or near the property.
Mice and Voles
Damage to trees from these rodents is highest in the winter and early spring when a shortage of their preferred foods is present. During winter, both mice and voles will feed under the snow cover and consume bark cambium and phloem layers and the roots of trees (Figure 4). A decrease in tree growth results from the sublethal feeding injuries, but smaller trees may die if completely girdled (Figure 5).
The best way to control these rodents is by maintaining weed control in the rows and particularly around the base of the tree. The bare ground is not attractive to the rodents and exposes them to predators.
Figure 4. Rodent feeding on root tissue. Courtesy of PDA
Figure 5. Girdling at the base of the tree from rodent feeding. Courtesy of Rayanne D. Lehman, PDA
Rabbits will feed on young trees. Shoots cut off at a 45-degree angle or girdling at the base are symptoms of rabbit feeding. Damage frequently occurs in winter when rabbits are unable to reach their normal food sources because of snow cover. During these periods, they will feed higher on the tree and may remove significant amounts of bark. Good weed control in the rows and around the trees removes hiding places for rabbits and makes them more vulnerable to predation.
Groundhogs can do minimal chewing damage to tree bark, but the bigger problem is their burrows. These holes are hazardous to farm equipment and employees on foot. When groundhogs burrow under a tree, the root system is disturbed and the tree may be weakened. Maintaining good weed control, either chemically or by mowing, and eliminating brush piles will discourage groundhogs in the field. Some recommendations include live trapping to relocate, smoke bombs, coyote or fox urine around the burrow, or motion devices to frighten these timid but destructive animals.