White fir saplings and poles are susceptible to fire damage or kill, but trees become more resistant to both with age and size. White fir is considered more fire resistant than its associated species at high elevations (37,41), but less resistant than its associates at low elevations (47). Fire scars, commonly found in old-growth stands, provide an entry court for a variety of disease and decay organisms.
White fir is sensitive to spring and fall frosts. Spring frosts can kill developing buds as well as foliage. Damage to established trees, other than Christmas trees, is not usually significant. On some sites, repeated damage to new fir growth can give a competitive advantage to more resistant species. Cold damage to mature trees takes the form of frost cracks and ring shake. Frost cracks are associated with some rot and decay loss (9).
Sudden rises in temperature during May and early June can cause damage nearly identical to that of spring frosts. Sun-scalding following thinning is rare in mature trees, although young, thin-barked trees are susceptible. When white fir boles are injured, recovery is slow (9).
Compared to its associated species, white fir is moderately susceptible to ozone damage. Although fir grows faster than associated species in southern California, diameter growth is affected by oxidant damage as much as that of Ponderosa pine (43). White fir is more resistant to fluoride damage than Douglas-fir or ponderosa pine (37).
As intensive management of this productive species increases, so will the importance of mechanical injury. Studies in Oregon and California have shown that conventional logging techniques for thinning or partial cutting damaged 22 to 50 percent of the residual stand. Seventy-five percent of these wounds were at ground level, where infection by some decay-causing fungus is almost certain (3). Loss of volume by time of final harvest can be considerable.
Two parasitic plants, white fir mistletoe (Phoradendron bolleanum subsp. pauciflorum), a true mistletoe, and white fir dwarf mistletoe (Arceuthobium abietinum f. sp. concoloris), cause major damage to white fir (9). In Arizona, Mexico, and the central to southern Sierra Nevada of California, white fir mistletoe is a serious problem on large old trees. Heavy infections cause spike tops, loss of vigor, and increased susceptibility to bark beetle attack. Dwarf mistletoe is a major problem from the southern Sierra Nevada north into Oregon. It is found elsewhere throughout the native range of white fir in coastal and southern California, Nevada, and Arizona (39,63).
One-third of the white fir stands in California are severely infested by dwarf mistletoe and the parasite is present in other forest types that contain white fir. Heavily infected trees suffer significant growth losses and are prone to attack by Cytospora abietis, a fungus that kills branches and further reduces growth. Because of reduced vigor, infected trees are more susceptible to bark beetle attack and various diseases (50,51). Heart rots, entering through open mistletoe stem cankers, increase mortality of old-growth trees through stem breakage.
Changes in wood structure in the large stem bulges caused by dwarf mistletoe infections reduce the strength of lumber produced. Current lumber grading practices, however, are not adequate to identify the affected wood (61).
Dwarf mistletoe need not be a problem in young managed stands because three factors make damage subject to silvicultural control. The parasite is host specific: white fir can be infected only by A. abietinum f. sp. concoloris, which in turn can parasitize only one other fir, grand fir. Small trees (less than 1 m [3.3 ft] tall) are essentially free from infection even in infested stands. Infected young firs free from new overstory infection outgrow the spread of mistletoe if height growth is at least 0.3 m (1 ft) per year (50).
Annosus root rot (Heterobasidion annosum) is present in all conifer stands and may become a major disease problem as management of white fir increases. Once established, the disease affects trees within a slowly expanding, circular infection center. Spread from tree to tree is through root contacts. New infection centers begin by aerial spread of spores and infection of basal wounds and freshly cut stumps. In true fir, annosus root rot usually does not kill directly but produces considerable moisture stress and loss of vigor that predispose the tree to attack by bark beetles, notably Scolytus. Direct damage resulting from infection is restricted primarily to heart rot of butt and major roots, leading to windthrow and stem breakage (9). Some degree of control is available through silvicultural means and use of borax on freshly cut stumps.
Other rots of major significance include the yellow cap fungus (Pholiota limonella), Indian paint fungus (Echindontium tinctorium), and white pocket rot (Phellinus pini) (9). Yellow cap fungus causes heavy losses from butt rot and enters through fire scars and basal wounds (9). Indian paint fungus is a major heart rot organism. This fungus probably infects fir in the same manner it does western hemlock (3). Entry is through branchlets less than 2 mm (0.08 in) in diameter. The fungus can remain dormant for up to 50 years before being activated by injury to the tree (18). Rot commonly extends 3 m (11 ft) below and 6 m (20 ft) above each characteristic fruiting body (4). No effective control is known although trees less than 40 years old are relatively free of rot because they have so little heartwood. In the white fir-grand fir complex of Idaho, the fungus was found in 97 percent of the trees that had decay. Almost 80 percent of the decay in old-growth grand fir-white fir stands of eastern Oregon and Washington is caused by Indian paint fungus; in California, it is much less common (9).
Insects from seven genera attack white fir cones and seeds. Two cause damage with considerable loss of seed. Seed maggots (Earomyia spp.) are the most abundant and damaging. The fir cone looper (Eupithecia spermaphaga) covers almost the entire range of white fir and periodically causes considerable local damage (27).
Although many insects feed on white fir foliage, few cause significant damage as defoliators. The most destructive of these is the Douglas-fir tussock moth (Orgyia pseudotsugata). Over most of its range the tussock moth shows equal preference for true fir and Douglas-fir foliage. Epidemic outbreaks, although sporadic, are explosive and damaging. In California, white fir is the preferred host, but outbreaks have not reached the severe levels sustained elsewhere (27). Occasionally, localized outbreaks result in increased stand growth as mortality of subordinate trees "thin" an overdense stand (59,60).
The western spruce budworm (Choristoneura occidentalis) is the most destructive defoliator in western North America, causing serious damage in Canada and the Rocky Mountains and Pacific coast regions of the United States. Some outbreaks are short lived, but some continue for 20 years or more. Although initial damage is to new foliage and buds, trees can be completely defoliated in 4 to 5 years. Ultimate damage ranges from minor growth loss to major tree mortality over extensive areas, depending on severity and duration of the outbreak (27).
A similar species, the Modoc budworm (Choristoneura retiniana [= viridis]), is endemic to the Warner Mountains of northeastern California and southeastern Oregon. Damage to California white fir in the Warner Range has been sporadic and light (27).
The New Mexico fir looper (Galenara consimilis) is restricted to New Mexico and can be a serious problem locally on white fir. Weevils of the genus Agronus attack foliage of young trees and may cause concern with intensive forest management. Sawflies (Neodiprion spp.) are generally not a problem-but are potentially damaging in dense stands of young fir. In California, a species of Neodiprion sawfly has reached epidemic levels locally on white fir. White fir needleminer (Epinotia meritana) covers the full range of white fir and can cause extensive branch kill predisposing trees to bark beetle (Scolytus) attack (27).
Cutworms (Noctuidae) can be a problem in nurseries and, more especially, in natural regeneration areas. Cutworms have been responsible for more than 30 percent of the seedling mortality in California (21,28).
The most damaging white fir pest is the fir engraver beetle (Scolytus ventralis). This bark beetle is found over the entire range of white fir and causes serious damage nearly everywhere. Mortality equivalent to an estimated 2.4 million m³ (430 million fbm) of growing stock is caused each year in California alone. Losses during epidemics are even larger (27). The fir engraver can attack any tree, but those suffering from root rot infections or tussock moth attack are especially vulnerable. In general, anything that reduces tree vigor, such as mistletoes, Cytospora, drought, or fire, increases susceptibility to attack (20). Several other bark beetles-including one species of Pseudohylesinus and two species of Scolytus, the roundheaded borer (Tetropium abietis) and the flatheaded fir borer (Melanophila drummondi)- frequently join the fir engraver in attacking and killing individual trees. In epidemic conditions, however, mortality is primarily caused by the fir engraver. Maintenance of stand health and vigor is the only known control (27).
Locally, small rodents can cause significant loss of seed and occasionally girdle seedlings. Pocket gophers limit regeneration in many areas, particularly clearcuts, by feeding on fir seedlings during winter and spring. Pocket gophers in combination with meadow voles and heavy brush can prevent conifer establishment for decades (21,37). Pocket gopher damage occurs on trees of all ages and sizes. Feeding on root tissues at the root crown has girdled saplings up to 12.7 cm (5 in) in diameter at breast height (d.b.h.). In at least one place, such feeding has resulted in death of mature trees up to 93.7 cm (36.9 in) d.b.h. (32). Direct control of pocket gopher is difficult and expensive. Indirect control by habitat manipulation offers some possibilities.
Spring browsing of succulent growth by deer and other big game animals can retard height growth for many years. Normally, trees are not killed, and most can grow rapidly once browsing pressure is removed. In managed stands, however, reduced height growth can result in significant economic loss. Damage by big game can be severe in the Southwest. Damage from livestock grazing is limited primarily to trampling and appears to be decreasing as the number of cattle on the open range decreases (37).