Throughout the range of white spruce, fire has been an important, sometimes dominant factor in forest dynamics (25,136,162). Mature forests are easily destroyed because of their high susceptibility to fire. Under certain circumstances, in unmanaged forests white spruce may be eliminated; the probability increases with latitude because seed years are infrequent and seed quality poor in some years in the north (136,183). During early- and mid-succession, white spruce is more susceptible to fire than aspen, birch, black spruce, and lodgepole pine (182).
Fire frequency, intensity, and severity, and not simply the presence of fire, determine white spruce distribution and growth. Fire frequency may range from 10 years or less to more than 200 years; most commonly, it is from 60 to 200 years. If fires occur at short intervals (less than 40 or 50 years), the source of white spruce seed can be eliminated. The reduction in depth of organic matter depends generally on fire severity and is a critical factor because the organic substrate that remains following fire makes a poor seedbed. In general, even severe fires do not expose mineral soil on more than 40 or 50 percent of a burn, and this area is usually distributed in small patches.
On floodplains in the northwestern part of the range, floods and silt deposits provide a seedbed for germination and seedling establishment. Flooding is detrimental to young seedlings, however, and establishment of spruce stands may be prevented until the flooding frequency declines. Fifty years may be required after initial sandbar formation before sedimentation rate declines enough for white spruce to colonize (104). As much as 20 percent of the seedlings may be killed on moist and wet sites that have been scarified by tractor and bulldozer blade (94).
Slow initial root growth makes young seedlings and transplants particularly susceptible to frost heaving. The severity of damage generally is greatest on fine-textured and wet soils where water is adequate for ice crystal formation in the surface soil. Late fall and winter seeding and spring field planting are best in most cases (141). White spruce roots respond vigorously to pruning (146); spring planting with root pruning is likely to be of some protective value against frost heaving.
Depending on soil texture and drainage, white spruce may be prone to windthrow. Windthrow is common along stand edges and in heavily thinned stands on shallow or poorly drained soils where root systems are surficial. On soils where a strong taproot, strong descending secondary roots, or multi-layered root systems develop, the species is much more windfirm. In mixed stands in which white spruce is overtopped by hardwoods, the leader and upper stem of spruce are frequently damaged by hardwood branches whipping in the wind.
Snow and ice can break up to 70 percent of white spruce in stands and hail can cause defoliation, stem lesions, and leader or terminal bud mortality (31,52,156).
White spruce vegetative and reproductive growth are particularly susceptible to frost damage at the time of flushing (116,181). The risk of frost damage is less for late flushing genotypes (110,116). Damage by fall frost is uncommon but has been observed in 1-year-old seedlings, when plantations heavily damaged by spring frost have responded with regrowth in August. Damage from spring frost is less serious after trees reach from 4 to 6 m (13 to 19 ft) in height. Because the species is so susceptible to frost damage, sites exposed to late spring frost should be avoided in all white spruce regeneration efforts.
Young seedlings are damaged by rodents. The snowshoe hare can be a significant pest, but white spruce is not a preferred animal food (4,12).
Environmental factors such as frost, mammals, birds, insects, and disease reduce the number of cones and the number of dispersed seeds (101,181). The impact of squirrels can be substantial. In Alaska, they may harvest as much as 90 percent of the cone crop (144,193). Small mammals such as deer mice, red-backed and meadow voles, chipmunks, and shrews can be an important cause of failure of natural regeneration and artificial regeneration by direct seeding. Seed consumption by individual animals can be very high-2,000 white spruce seeds per day for caged animals of the species mentioned- and the population density substantial but highly variable. Estimates range from 7 animals per hectare (3/acre) to as high as 44/ha (18/acre). Even at the low density, the impact on regeneration would be unacceptably high (126,141). The impact on seed varies with the time of seeding: 50 percent for spring-sown seeds as compared to 19 percent or less for winter-sown seeds. Coating seeds with repellent is effective and has little influence on seed germination even when coated seeds have been stored for 5.5 years (125,127).
The impact of birds feeding on seeds is small compared to that of rodents (126), but chickadees, grosbeaks, crossbills, juncos, and sparrows feed on coniferous seeds.
Seed losses from insects can be a serious problem. The spruce cone maggot (Hylemya (Lasiomma) anthracina), the fir coneworm (Dioryctria abietivorella), and the spruce seed moth (Laspeyresia youngana) are most important. Hylemya leaves the cone in midsummer and, as a result, Laspeyresia is blamed for the damage it does; however, where the infestation is severe, Hylemya may destroy 100 percent of the seed (59). Damage by D. abietivorella is particularly severe in years of heavy cone crops and appears to be found when cones develop in clusters. The following insects also attack seeds and cones but do less damage: the spruce cone axis midge (Dasineura rachiphaga), the spruce seed midge (Mayetiola carpophaga), the seed chalcids (Megastigmus atedius and M. picea), the cone cochylid (Henricus fuscodorsana), and the cone moth (Barbara mappana) (59). The only disease associated with cone production is the cone rust Chrysomyxa pirolata (151). Seeds produced from infected cones are about half the weight but the same size as healthy seeds. Seeds are fragile because seed coats are poorly developed, and seed mortality is almost 100 percent in severely affected cones (101,151). Even if viable seeds are produced, they are not readily dispersed because cone malformation and resinosis prevent efficient opening of the cone scales (151).
White spruce seedlings are affected by disease during the dormant and growing seasons. Snow blight (Phacidium infestans) causes damage in nurseries and the field. Various species of Pythium, Rhizoctonia, Phytophthora, and Fusarium have been shown to be moderately to highly pathogenic to spruce seedlings in both pre- and post-emergent conditions (65). Pythium and Fusarium as well as Epicoccum and Phoma can also injure seedlings in cold storage; many of these damaged seedlings die when they are field planted (67). Nematodes have been shown to cause winterkill and reduce seedling vigor.
Needle and bud rusts are common throughout the range of white spruce. The most important rust causing premature defoliation in Canada is Chrysomyxa ledicola. Losses of up to 90 percent of the current year's needles have been observed in Western Canada. Other needle rusts that infect white spruce are C. weiri, C. empetri, C. ledi, and C. chiogenis. The witches' broom rust (C. arctostaphyli) frequently causes dead branches, abnormally proliferating branches, deformed boles, and reduced growth. A bud rust (C. woroninii) is more prevalent in far northern areas and infects seedlings and vegetative and female buds of mature trees (65,101,195).
Stem diseases of white spruce are not of major importance. A canker caused by Valsa kunzei has been reported. One of the most conspicuous and common stem and branch deformities is a tumor-like growth of unknown origin. These tumors occur throughout the range and may reach 0.6 to 0.9 m (2 to 3 ft) in diameter. In a small test of grafts of tumored and tumor-free trees, tumor growth was transmitted to some, but not all, ramets in some clones of tumored trees (44).
Root diseases of white spruce affect both seedlings and mature trees. Inonotus tomentosus is a major cause of slow decline and death of white spruce in patches of 0.4 ha (1 acre) or more in Saskatchewan. The disease has been called the "stand-opening disease." It develops slowly over a period of 20 to 30 years but the impact can be substantial- 87 percent of white spruce in mixed stands either dead or heavily rotted at the butt. Stand openings occur on soils of all textures but rarely on alkaline soils (174). Trees planted in infected areas are also damaged (175). Other root-rot fungi associated with white spruce are Coniophora puteana, Scytinostroma galactinium, Pholiota alnicola, Polyporus guttulatus, P. sulphureus, and Phaeolus schweinitzii.
Trunk rots affecting white spruce include Haematostereum sanguinolentum, Peniophora septentrionalis, and Phellinus pini. These species produce rot development beyond the tree base. Coniophora puteana, Fomitopsis pinicola, and Scytinostroma galactinium are associated only with butt rot. In general, cull percentage in white spruce caused by rot is low, particularly for trees less than 100 to 120 years old. Most trees older than 200 years have significant amounts of rot, however.
Although most spruce species are seriously injured by the European strain of scleroderris canker (Gremmeniella abietina), white spruce suffers only from tip dieback and eventually recovers (137). Dwarf mistletoe (Arceuthobium pusillum) is usually associated with black spruce, but it has killed white spruce in Minnesota (3), along the coast of Maine, and in the Maritime Provinces.
White spruce is attacked by a number of bark beetles in the genera Dendroctonus, Ips, Trypodendron, Dryocoetes, Scolytus, Polygraphus, and others. Although most of these species attack trees of low vigor, dying trees, windthrows, and slash, the spruce beetle (Dendroctonus rufipennis) attacks trees of normal vigor and has killed large areas of white and other spruces. In areas with transition maritime climates, such as western and south-central Alaska, prolonged extreme cold (-40° C or -40° F) kills large numbers of beetles. Where spruce beetle outbreaks are common, resistance of trees is greater in mature stands with stocking levels of 18m²/ha (80 ft²/acre) or less because of wide tree spacing and rapid growth (58). Dense stocking contributes to cold soils in the spring and thus tree moisture stress, which predisposes the trees to beetle attack (57). Bark beetles bore or mine in the phloem. or inner bark and girdle the tree. Symptoms of beetle attack are pitch flow tubes and fine wood particles on the bark or at the base of the tree. The foliage of the attacked tree changes color and dies, but this may not occur until after the beetle has left the tree. The best method of preventing beetle outbreaks is to remove or destroy desirable habitat such as slash and damaged trees; trees weakened by budworms are particularly susceptible.
Wood-boring insects (Monochamus spp., Tetropium spp., and Melanophila spp.) attack weakened or dead white spruce and are particularly attracted to burned areas. They can attack trees almost before the fire cools. The intensity of attack is determined by the condition of the individual tree (173). Lumber recovery from heavily infested trees declines rapidly because of extensive tunneling.
The spruce budworm. (Choristoneura fumiferana) and the western spruce budworm (C. occidentalis) feed and mine in old foliage, in developing reproductive and vegetative buds, and in new foliage of the expanding shoot. True firs are the principal hosts, but spruces are readily attacked and injured. Budworms are the most destructive conifer defoliators; severe defoliation for 2 years reduces growth, and sustained outbreaks have killed all spruce in some stands (48,81). Plantations are not usually subject to serious damage until they are about 6 m (20 ft) tall (141).
The yellowheaded spruce sawfly (Pikonema alaskensis), another defoliator, is not important in closed stands but can seriously reduce growth or kill plantation-grown trees if defoliation continues for 2 or more years (141). A number of other sawflies including the European spruce sawfly (Diprion hercyniae), also damage the species.
Spruce spider mites (Oligonychus spp.) build up in damaging numbers in early spring and summer and sometimes in fall. They are also common on young white spruce plants growing in greenhouses. Their feeding destroys the chlorophyll-bearing cells of the needle surface, causing a bleached look. Continuous attacks weaken and eventually kill the tree (81).
The European spruce needleminer (Epinotia nanana) causes unsightly webbing and kills needles on spruces in the Eastern United States. Heavy attacks cause severe defoliation, and weakened trees succumb to secondary pests. Other needleminers of less importance are in the genera Taniva and Pulicalvaria (122). Other insects damaging spruce needles include needle worms, loopers, tussock moths, the spruce harlequin, and the spruce bud scale.
The gall-forming adelgids (Adelges spp.), of which the eastern spruce gall adelgid (A. abietus) is the most prevalent, cause cone-shaped galls on the shoots. Other gall-forming insects belong to the Pineus and Mayetiola genera (122). Although not important for forest trees, these galls can deform and stunt the growth of seedlings, saplings, and ornamental trees (48,81).
Spruce buds are damaged by bud moths, Zeiraphera spp., the bud midge (Rhabdophaga swainei), and bud and twig miners (Argyresthia spp.). None of these insects causes serious damage (122), but killing of the terminal leader by Rhabdophaga results in multiple leaders and thus poor tree form.
White spruce is considered lightly susceptible to damage by the white pine weevil (Pissodes strobi) and certainly is much less damaged than either black or Norway spruce (Picea abies). However, an ecotype of the insect, sometimes called the Engelmann spruce weevil, is an important pest in plantations in interior British Columbia and on natural regeneration in British Columbia and Alberta (141).
Warren's collar weevil (Hylobius warreni) does cause appreciable damage on spruce. Small trees may be girdled and killed; on older trees, the wounds are entries for root rots such as Inonotus tomentosus (122). In controlled experiments, 4-year-old white spruce has shown high radio-sensitivity when exposed to chronic gamma irradiation. The trees were most sensitive in mid-July when the central mother-cell zone was enlarging.