Recovery Plan for the Northern Spotted Owl - DRAFT

__hardwoods are immune. Shade tolerant species in the understory which areless susceptible to P. weini, such as western hemlock, are usually favored thusspeeding up succession (Figure F.6A). In some cases, especially in very youngstands, hardwoods such as vine maple (Acer circinatunm), big leaf maple (Acerniacrophyllutm, and red alder (Alnus rubra) may establish in P. weirti pockets(Figure F.6B). These species are immune to P. weiiii and this process effectivelysets succession back in time. Brush species also may establish in diseasepockets devoid of trees (Figure F.6C). Thus P. weirti can create significantspecies and structural diversity at the landscape level.The spread of P. weirir is almost exclusively by vegetative growth along root contactsbetween neighboring trees. Spread by spores is rare (Hadfield et al.1986). Clones of P. weu-i can occupy the same relative location on the landscapefor 1,000 or more years moving very slowly (12 inches per year: Dickman1984). Tree species change is associated with spread of the infection center.The fungal clone usually is not destroyed by fire since it resides in large woodyroots and the base of trees. It also infects old-growth trees, but large, old treesoften survive infection for many years. Production of adventitious roots assiststhis process. In an old-growth stand dominated by Douglas-fir in the OregonCoast Range, Tkacz and Hansen (1982) estimated that 19 percent of the susceptiblespecies were healthy, 30 percent were live and infected, 36 percentwere killed by the fungus, and 15 percent were killed by other causes. There isno doubt some degree of genetic resistance to the fungus exists and someclones appear more pathogenic than others (Driver and others 1972). Afterfire, P. weirii can stay alive in large woody root systems for as long as 100years, thus influencing the structure of the post-fire stand.The area of western Oregon and Washington heavily infected by P. weirli isestimated to be about 10 percent (Hansen and Goheen 1989). Clones of P.weirti however, are not distributed evenly across the landscape. The incidenceof P. weirii seems to be higher on moisture and/or nutrient stressed sites. Incidenceappears to be higher in sites with dry gravelly soils and or lower rainfalland ridges and upper slopes (Kastner 1991). Disease incidence is not stronglyrelated to aspect. P. weirti incidence seems to be particularly high in the PugetSound region, the Cascade Mountains foothills, the Oregon Coast range, andmountain hemlock (Tsuga mertenstana) forests of the Oregon Cascades.Armrllaria ostoyae attacks a wider range of conifers than P. wetit, but in thissubregion it is not thought to be as important as P. weiriL It usually does notoccur in large pockets but attacks individual or small clusters of trees. Thesetrees are usually under environmental or competitive stress. Mortality causedby Armlllaria seldom occurs in stands older than 25 years west of the Cascadecrest unless the trees are undergoing extreme stress. Occasionally trees onvery moist sites are attacked. Armnilaria also may attack Douglas-fir alreadystressed by P.RweiriLBlack stain root disease has become important in young-growth managedstands in southern Oregon, but the role of this disease in older forests andforest succession is not known at this stage. In later stages of succession, buttrot and bole decay become increasingly important as shown in Figure F.6 A-C.Dwarf mistletoe is also important in areas with a lot of western hemlock.In coastal areas where western hemlock and Sitka spruce dominate, and wherewind is the primary disturbance rather than fire, root rots do not seem to be asimportant as they are in Douglas-fir dominated forests (Figure F.6D). Incoastal forests decay fungi tend to be the dominant disturbance agents alongwith hemlock dwarf mistletoe. Heterobastdion annosum is the dominant diseaseorganism in western hemlock, and commonly acts as butt and root rot.Trees do not seem to develop significant butt rot until they are more than 100450