Effects of Phellinus weirii on plant community composition and succession of mature and old-growth Douglas-fir forests

Graduation date: 1992Presentation date: 1991-09-19Disease is often overlooked as a natural disturbance agent in plant communities. This study examines what effects, if any, a disease-mediated disturbance has on the plant community as a whole in old-growth and\ud mature forests of western Oregon. Phellinus weirii (Murrill) Gilbertson (Family: Hymenochaetaceae) is a native root-rotting pathogen that has co-existed with its conifer hosts for thousands of years. E. weirii can infect the roots of nearly all conifers,\ud infection eventually leading to mortality for Douglas-fir, mountain hemlock, and true fir species. As the pathogen grows slowly via root contacts and grafts of conifers, areas of mortality are left in its wake, areas\ud commonly called infection centers. Herb, shrub, and tree species presence and percent abundance were noted inside and outside six infection centers located throughout the\ud Cascade and Coast ranges. Douglas-fir is the major species experiencing mortality due to the disease in this region. The vegetation inside infected areas was compared to that found immediately adjacent to infection\ud centers. A Multi-Response Permutation Procedure (MRPP), a non-parametric\ud multivariate analysis of variance technique, was used to test for significant differences in the composition as a whole between the two areas of each site. An ordination technique, Detrended Correspondence Analysis (DCA), was used to examine if the effect the disease had on the plant community was a major factor underlying composition patterns. Common herb, shrub, and tree species, excluding Douglas-fir, were examined separately for\ud significant differences in cover between the infected and non-infected areas for each site. Differences in the abundance of late-successional species and their regeneration between the two areas were tested in order to assess possible past and future impacts the disease had on succession in these forests. All six infection centers had significantly different overall species\ud composition compared to the composition of the adjacent non-infected areas. The effect of the disease on the forest composition was a major agent influencing community composition patterns for the six areas. The responses of herb, shrub, and tree populations to disease presence was species-specific and varied across sites, responses varying especially between Cascade and Coast range sites for some species. Effects of the disease on overall plant diversity appear to be dependent on site characteristics. Though generalizations are often made about disease as a diversifying agent in communities, vascular plant diversity was not significantly enhanced by P. weirii. with the exception of one site.\ud In terms of successional impacts, the succession rates may be accelerated in Cascade forests because the growth of late-succesional species is promoted within infection centers