Responses to Pathogen-induced Disturbance: Decomposition, Nutrient Availability, and Tree Vigour

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Abstract
In the Oregon subalpine zone, extensive dieback occurs in relatively pure stands of 150 to 250-year-old mountain hemlock growing on very infertile soils. Tree death is caused by a root-rot fungus, Phellinus weirii . Young trees that become established following death of the original forest are apparently not reinfected by the pathogen until 80–140 years later. whereon mortality occurs again. We examined the effects.of this natural disturbance and subsequent regrowth on a number of ecosystem characteristics. Decomposition rates and nitrogen availability measured by in situ exchange resins increased in the zones of young regrowth, but dropped to values common for old growth as the forest aged and the canopy closed. Phosphorus and potassium accumulation on exchange resins showed trends opposite to nitrogen, and may have been associated with changes in biomass. Increased nitrogen concentrations and decreased lignin concentrations in fine roots in the zone of young regrowth suggested improved tree nutrition under conditions of higher N availability and lower leaf area index. Tree vigour, estimated as wood production per unit leaf area, also was significantly increased in the zones where young forests grew. Circumstantial evidence suggests that increases in nutrient availability and light following death of the mature forest improved photosynthesis leading to increased resistance of young trees against infection by the pathogen.