<i>Phellinus weirii</i> Infestation of Two Mountain Hemlock Forests in the Oregon Cascades

Abstract

Abstract Root rot by Phellinus weirii was studied in two coniferous stands at 1,650 m elevation in Central Oregon. The fungus, by growth from centers of infection, forms circular areas that are rather devoid of trees at their margins and occupied by successively older vegetation centripetally. Number, species, age, and circumference at breast height of trees were obtained in transects of three infection centers per stand. Some trees escape death or are killed slowly. These "resistant" trees were discriminated by size and age from adjacent regrowth trees. The rate of spread of P. weirii and relative resistance of trees on the basis of escape frequency were determined at ten infection centers in each stand. P. weirii spread vegetatively at 23 cm/yr in a mixed mountain hemlock-other conifer stand and 34 cm/yr in a less diverse stand dominated by mountain hemlock. The order of resistance from most to least was western white pine, lodgepole pine, Pacific silver fir, noble fir, Englemann spruce, and mountain hemlock. Successional tree vegetation that developed after passage of the advancing mycelium was more diverse than that being attacked. Thus, mortality caused by the fungus creates diversity, but the inverse relationship between rate of advance and tree diversity suggests that diversity may suppress the rate of mycelial advance. Reinfestation of successional trees occurred within infection centers after 88 to 165 yr. Forest Sci. 26:23-29.