Effects of a native forest pathogen, Phellinus weirii, on Douglas-fir forest composition in western Oregon

The fungal pathogen Phellinus weirii (Murrill) Gilbertson (Family: Hymenochaetaceae) causes extensive rot in the rootsand bole of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and true firs, eventually leading to tree mortality. The nativepathogen spreads slowly via root grafts and root contacts between conifers. leaving behind areas of tree mortality commonlycalled infection centers. This study determines (i) whether the slow, systematic removal of the Douglas-fir overstory byP. weirii changes the community composition of old-growth and mature forests. (ii) if composition is significantly affected.to what degree P. weirii influences the composition, (iii) what effects the disease has on individual populations, and(iv) whether vascular plant diversity is affected by disease presence. The herb, shrub, and tree strata were randomly sampledwithin and adjacent to six P. weirii infection centers located in the low-elevation Cascade and Coast ranges of westernOregon. Statistically significant (P 5. 0.05) differences in species composition between infected and adjacent noninfectedforest were found across all sites. Ordination techniques showed that the distance of vegetation to the infection center edgewas a major factor underlying the forest community's structure. The average cover of all herbaceous species was higherinside infection centers as compared with outside, for all locations, though statistically significant at only two sites. In general.species differed in their responses to disease presence. Changes in diversity due to the presence of the root rot were statisticallysignificant in three of the six cases (P 5 0.05) but the patterns of change differed from site to site. The removal of Douglas-firoverstory has strong effects on the plant community, but the specific patterns depend on the species and site involved.