Responses to pathogen-induced disturbance: decomposition, nutrient availability, and tree vigor

In the Oregon subalpine zone, extensive dieback occurs in relatively pure stands of 150to 250-year-old mountain hemlock growing on very infertile soils. Tree death is causedby a root-rot fungus, Phellinus weirii. Young trees that become established followingdeath of the original forest are apparently not reinfected by the pathogen until 80-140years later, whereon mortality occurs again. We examined the effects of this naturaldisturbance and subsequent regrowth on a number of ecosystem characteristics.
Decomposition rates and nitrogen availability measured by in situ exchange resinsincreased in the zones of young regrowth, but dropped to values common for oldgrowth as the forest aged and the canopy closed. Phosphorus and potassiumaccumulation on exchange resins showed trends opposite to nitrogen, and may havebeen associated with changes in biomass. Increased nitrogen concentrations anddecreased lignin concentrations in fine roots in the zone of young regrowth suggestedimproved tree nutrition under conditions of higher N availability and lower leaf areaindex. Tree vigour, estimated as wood production per unit leaf area, also wassignificantly increased in the zones where young forests grew. Circumstantial evidencesuggests that increases in nutrient availability and light following death of the matureforest improved photosynthesis leading to increased resistance of young trees againstinfection by the pathogen.