Fire and fungus in a mountain hemlock forest

Abstract

Two mortality factors create large-scale pattern in forests of Tsuga mertensiana in subalpine central Oregon Cascade Mountains. Half of an 18000- ha study area has experienced stand-destroying fire during the last 500 years. These fires varied in size from 1 to 3200 ha. Individual Phellinus weirii infestations are smaller than most fires and collectively cover less total area, but they are more numerous and fine-grained in their dispersion. Postfire stands are colonized by Pinus contorta, which persists for 2 centuries before being replaced by mountain hemlock. In stands older than 200 years, Phellinus weirii becomes apparent as it spreads from centers and forms patches where it infects trees and alters the plant community. Fungal isolates were collected from 61 infestations; these were subjected to clonal analyses. Spatial dispersion of ramets and genets supports the inference that many infestations are sibling ramets of genets that have survived stand-destroying fire. The age distribution of genets yields the inference that infestations have been initiated or modified by basidiospore infection durig the past 1300 years. Several genets are older than 1000 years. Fire has reduced the visible area of infestation of the fungus. It probably has done so by favoring less susceptible host species and by reducing the modal size of dead roots and logs. While repeated fire could reduce the level of fungal infestation, infestations may enhance the probability of stand-destroying fires.