How bad is the damage?

Defoliation (removal of foliage by feeding herbivores or premature loss caused by disease) is the most visible damage to trees. However, the extent to which defoliation injures a tree depends on the abundance of the herbivore, the frequency and duration of the damage, and the innate ability of trees to tolerate loss of some foliage and remain vigorous enough to replace foliage following defoliation. For example, conifers retain several years of needles at any one time. Therefore, defoliators such as budworms, which feed mostly on current-year needles and seldom on older needles, are less injurious than hemlock loopers and tussock moths, which feed on all age classes of needles and so can kill a tree quickly when these insects are sufficiently abundant. By comparison, broadleaf trees produce and lose their leaves each growing season. If, as is commonly the case, defoliation occurs in the spring, most broadleaf tree species produce a second replacement cohort of foliage. If not, the root reserves of broadleaf trees are often sufficient to produce at least some foliage the next year.

Damage by wood-boring or sap-sucking insects is less apparent in the short-term because these herbivores are either very small (adelgids) or well hidden beneath the bark (bark beetles). Moreover, their damage is systemic such that external symptoms only appear after considerable damage has occurred inside the tree either via damage to the vascular tissues by tunnelling insects (e.g., mountain pine beetle) or by the pathogens some insects transmit (e.g., native elm bark beetle and Dutch elm disease). Gall-forming insects are rarely plentiful enough to affect the viability of a tree, although they can cause undesirable deformities in nursery and ornamental plantings.

Mammals such as deer and voles can cause extensive feeding damage in newly reforested areas, requiring costly replanting to achieve acceptable stocking levels. Damaged trees can result in malformed trunks, and the wounds serve as entry courts for pathogens.

Damage caused by forest pathogens can range from minor and aesthetic (seasonal leaf spots such as maple tar spot) to devastating, reducing the incidence of plant host on an environmental scale (e.g., Dutch elm disease and Chestnut blight). Commercial losses due to cull of timber from heart rot can be high, especially in cases such as aspen trunk rot where levels of decay in the stand cannot be detected until the trees are harvested. Perennial root diseases such as Armillaria root disease can cause growth loss and mortality in affected stands. Repeated defoliation of conifers from diseases such as Dothistroma needle blight can cause stunting or mortality, and render the surviving trees more susceptible to other insects and pathogens. Economic losses due to forest pests can also be caused by phytosanitary restrictions such as quarantines on Canadian logs or other wood products imposed by importing countries.