Pinus monticola

Although still considered a fairly common species in many areas of its range, populations of western white pine are affected by white pine blister rust, different species of bark beetles, and susceptible to pole blight from extreme weather conditions. Population declines have been noted since early in the century with a massive dieback in the Pacific region in 1935-1936 triggered by highly anomalous winter weather (Auclair et al 1990). Continuing threats of disease and extremes of climate are still affecting populations, however breeding programs have produced strains that are 65% resistant to white pine blister rust (Griffth 1992). Others consider populations will continue to decline as blister rust infection spreads and intensifies (Tomback 2001).

Western white pine occurs only in western North America from British Columbia to California, eastwards to Utah and Montana.

Although western white pine is a fire-adapted early seral species, trees are susceptible to fire which can damage the cambium layer. This usually results in the death of young trees and the creation of fire scars in older trees. Fire scars can provide an opening for disease, such as butt-rot fungi, to infect a tree. However, fire also removes the humus layer, preparing a favourable seedbed and eliminates competing plants. Large amounts of humus also can cause tree death from heating of the roots (Griffith 1992). White pine blister rust (Cronartium ribicola), dwarf mistletoe (Arceuthobium spp.) and various species of bark beetles also are serious threats affecting populations. Pole blight, a xylem dysfunctional condition related to extreme climatic variation, results in massive cavitation, yellow foliage, crown dieback and the death of many trees. One critical factor with this specific condition was untimely soil frost and winter thaw-freeze cycles, whereas air pollution was determined not to relate to tree dieback for Pinus monticola (Auclair et al. 1990). Weather extremes with climate change may increase the occurrence of this condition in this species.