Recovery Plan for the Northern Spotted Owl - DRAFT

InsectsDiseasesInsects in this subregion usually act as secondary agents of disturbance aftermajor disturbances caused by fire and wind. Insects tend to have less influencein this subregion because physiological stress from drought and othercauses is relatively low.After fire Douglas-fir beetles will attack weakened trees and freshly down trees.Ambrosia beetles will also attack boles of standing dead and down conifers.Trees weakened by root diseases, particularly by Phellinus weiriL, are very susceptibleto Douglas-fir beetle attack.Occasional outbreaks of the western hemlock looper in extensive old-growthhemlock stands have occurred every 15 to 20 years from the 1890s to the1960s, but no other insects have caused large scale defoliation. Sitka spruceweevil causes leader deformation in young trees (Furniss and Carolin 1977).Sitka spruce tends to be least impacted by weevils in the fog zone along thecoast and up major river valleys. Outside this zone it is commonly attacked bythe Sitka spruce weevil.Diseases, especially root rots, stem decays, and dwarf mistletoes, in the WestCascades subregion appear to be a greater agent of disturbance than insects.Foliage diseases, stem cankers, and rusts play a minor role. Diseases appearto occur in a relatively random pattern across the landscape, although somesite factors such as moisture no doubt influence disease expression. Theyhave considerable influence on forest succession and biodiversity. Manydiseases have specific hosts such as Douglas-fir. Thus, diseases can changespecies composition in affected stands.After a large-scale stand-replacement fire (Figure F.6, A-C) Douglas-fir tends toinvade sites except in wetter areas near the coast. In the early stages ofsuccession where Douglas-fir dominates, root diseases are important causes ofmortality. Rhizina root rot can be important in the seedling stage. Phellinusweirit is important from about age 15 to 125 years (Childs 1970) whileArmillaria ostoyae is most important from about age 5 to 25 years (Hadfield etal. 1986). These ages are relative since in many cases the actual establishmentperiod in natural stands may be as long as 20 years. In old-growth forests,butt and bole decay fungi dominate although root rots are still present and cancontribute to mortality. Dwarf mistletoes are not important in forests dominatedby Douglas-fir because it is rarely attacked west of the Cascade crest(Bega 1979). As the hemlock component increases with succession, dwarfmistletoes also increase.In drier areas of the West Cascades subregion away from the coast, fires arenot always of stand-replacement intensity. Such fires may not kill large, oldDouglas-fir trees, and these fires typically create uneven-aged stands.Rhizina undulata is a fire-loving fungus. Dormant spores in the soil germinateonly in the presence of heat from fires. The fungus is only active for a fewyears after fire then it once again goes dormant (Morgan and Driver 1972).Plellinus weirii tends to create large expanding root rot centers. Northwesternconifers appear to vary in their susceptibility to P. weirii (Hadfield et al. 1986).Douglas-fir and true firs are highly susceptible. Western hemlock and Sitkaspruce are intermediately susceptible (infected but seldom killed). Westernwhite pine (Pinus monticola) and western redcedar are tolerant or resistant. All448