Many agents adversely affect the growth, health, and quality of western hemlock trees and stands.
Because of its thin bark and shallow roots, western hemlock is highly susceptible to fire. Even light ground fires are damaging. Prescribed burning is an effective means of eliminating western hemlock advance regeneration from a site.
Because of its shallow roots, pole-size and larger stands of western hemlock are subject to severe windthrow. Thousands of hectares of young stands dominated by coastal western hemlock have originated after such blowdown.
Western hemlock suffers frost damage in the Rocky Mountains, especially along the eastern edge of its range where frost-killed tops are reported (20,26). Snowbreak occurs locally; it appears to be most common east of the Cascade and Coast Mountains, and especially in the Rocky Mountains. On droughty sites, top dieback is common; in some exceptionally dry years, entire stands of hemlock saplings die. Suddenly exposed saplings may suffer sunscald. Excessive amounts of soil moisture drastically reduce growth.
Western hemlock is one of the species most sensitive to damage by sulfur dioxide (16). Spring applications of the iso-octyl esters of 2,4-D and 2,4,5-T in diesel oil can kill leader growth of the last 3 years.
Severe fluting of western hemlock boles is common in southeast Alaska, much less common on Vancouver Island, and relatively uncommon in Washington and Oregon. There appears to be a clinal gradient from north to south; the causal factor is not known.
No foliage diseases are known to cause serious problems for western hemlock.
Dwarf mistletoe (Arceuthobium tsugense) is a serious parasite along the Pacific coast from California nearly to Glacier Bay, AK; its presence on western hemlock in the Rocky Mountain States is unconfirmed. It increases mortality, reduces growth, lowers fiber quality, and provides an entryway for decay fungi. Uninfected to lightly infected trees may have a greater growth in volume (40 percent) and height (84 percent) than severely infected trees; in mature stands, volume losses as high as 4.2 m³/ha (60 ft³/acre) per year have been reported (29). Dwarf mistletoe in western hemlock is easy to control; success is nearly 100 percent if methods of sanitation are good.
Armillaria mellea, Heterobasidion annosum, Phaeolus schweinitzii, Laetiporus sulphureus, Inonotus tomentosus, Poria subacida, and Phellinus weiri are the major root and butt pathogens of western hemlock. Armillaria mellea occurs widely, seldom kills trees directly, and is not a major source of cull.
Heterobasidion annosum, the most serious root pathogen of western hemlock, can limit the alternatives available for intensive management (3). The incidence of infected trees in unthinned western hemlock stands ranges from 0 to more than 50 percent. In some thinned stands, every tree is infected. Heterobasidion annosum spores colonize freshly cut stumps and wounds; the spreading mycelium infects roots and spreads to adjacent trees through root grafts. Treating stumps and wounds with chemicals can reduce the rate of infection.
Phellinus weiri is a common root pathogen where Douglas-fir is or was a major component of the stand. In the Rocky Mountains, a similar relationship may exist with western redcedar. Phellinus weiri rapidly extends up into the bole of western hemlock. The first log is frequently hollow; only the sapwood remains. The only practical controls for P. weiri are pulling out the stumps and roots or growing resistant species.
High risk bole pathogens include Echinodontium tinctorium, Heterobasidion annosum, and Phellinus weiri. Echinodontium tinctorium causes extensive decay in overmature stands in the Rocky Mountains. It is less destructive in immature stands, although it is found in trees 41 to 80 years old; 46 percent of the trees in this age group in stands studied were infected. Echinodontium tinctorium is of little consequence on the coast. Heterobasidion annosum spreads from the roots into the bole of otherwise vigorous trees. On Vancouver Island, an average of 24 percent (range 0.1 to 70 percent) of the volume of the first 5-m (16-ft) log can be lost to H. annosum (24).
Rhizina undulata, a root rot, is a serious pathogen on both natural and planted seedlings on sites that have been burned. It can kill mature trees that are within 8 m (25 ft) of the perimeter of a slash burn (3).
Sirococcus strobilinus, the sirococcus shoot blight, causes dieback of the tip and lateral branches and kills some trees in Alaska; the potential for damage is not known (27).
Of the important insects attacking western hemlock, only three do not attack the foliage. A seed chalcid (Megastigmus tsugae) attacks cones and seeds; the larva feeds inside the seed. This insect normally is not plentiful and is of little consequence to seed production (14). A weevil (Steremnius carinatus) causes severe damage in coastal British Columbia by girdling seedlings at the ground line. In the Rocky Mountains, the western larch borer (Tetropium velutinum) attacks trees that are weakened by drought, defoliated by insects, or scorched by fire; occasionally it kills trees (9).
Since 1917, there have been only 10 years in which an outbreak of the western blackheaded budworm (Acleris gloverana) did not cause visible defoliation somewhere in western hemlock forests (28). Extensive outbreaks occur regularly in southeast Alaska, on the coast of British Columbia, in Washington on the south coast of the Olympic Peninsula and in the Cascade Range, and in the Rocky Mountains. In 1972, nearly 166 000 ha (410,000 acres) were defoliated on Vancouver Island alone. Damage by the larvae is usually limited to loss of foliage and related growth reduction and top kill. Mortality is normally restricted to small stands with extremely high populations of budworms.
The western hemlock looper (Lambdina fiscellaria lugubrosa) has caused more mortality of western hemlock than have other insect pests. Outbreaks last 2 to 3 years on any one site and are less frequent than those of the budworm. The greatest number of outbreaks occurs on the south coast of British Columbia; the western hemlock looper is less prevalent farther north. Heavy attacks have been recorded for Washington and Oregon since 1889. The insect is less destructive in the interior forests. Although mortality is greatest in old growth, vigorous 80- to 100-year-old stands are severely damaged.
Two other loopers, the greenstriped forest looper (Melanolophia imitata) and the saddleback looper (Ectropis crepuscularia), cause top kill and some mortality. The phantom hemlock looper (Nepytia phantasmaria) in the coastal forest and the filament bearer (Nematocampa filamentaria) play minor roles, usually in association with the western hemlock looper (28).
The hemlock sawfly (Neodiprion tsugae) occurs over most of the range of western hemlock. Its outbreaks often occur in conjunction with outbreaks of the western blackheaded budworm. The larvae primarily feed on old needles; hence, they tend to reduce growth rather than cause mortality (9). The hemlock sawfly is considered the second most destructive insect in Alaska (13).
Black bear girdle pole-size trees and larger saplings or damage the bark at the base of the trees, especially on the Olympic Peninsula of Washington. Roosevelt elk and black-tailed deer browse western hemlock in coastal Oregon, Washington, and British Columbia. The snowshoe hare and the brush rabbit damage hemlock seedlings, principally by clipping off the main stem; clipping of laterals rarely affects survival of seedlings (5). Mountain beaver clip the stems and lateral branches of seedlings and girdle the base of saplings along the coast south of the Fraser River in British Columbia to northern California. Four years after thinning, evidence of girdling and removal of bark was present on 40 percent of the trees (5). Mortality results from both kinds of damage.