Impacts of a native root-rotting pathogen on successional development of old-growth Douglas fir forests

Because Pseudotsuga menziesii (Douglas fir), an early seral dominant in western Oregon forests, is particularly susceptible to the pathogenic root rot caused by the fungus Phellinus weirii, it was hypothesized that successional development in affected forests would be accelerated. The basal area of late successional tree species and common shrubs were compared inside and outside Phellinus weirii"infection centers". Future successional impacts indirectly caused by disease presence were assessed by comparing the abundance of regenerating tree species inside and outside of these centers. Results for old-growth sites in the lower Cascades supported the initial hypothesis. The presence of Phellinus weirii at these sites appears to push changes in the late successional species basal area along the same trajectory. At the Coast Range sites, however, shrub growth was greatly enhanced rather than the growth of Tsuga heterophylla (western hemlock), the only late successional tree species at these sites. Regeneration of trees was sparse in all areas and was negatively correlated with the basal area of existing western hemlock, and not directly correlated with the presence of the pathogen. However, because the basal area of western hemlock is influenced by the presence of disease in the Cascade sites, Phellinus weirii may be an influential indirect factor in the future successional direction of the sites. This study demonstrates the potential importance of native pathogens in determining the successional status of a community, as well as the importance of such biotic agents in the physical structure of multi-strata forested communities as a whole.