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Downed Bole-Wood Decomposition in Lodgepole Pine Forests of Central Oregon

USDA-Forest Service, Pacific Northwest Research Station, Bend, OR 97701

*Corresponding author.

ABSTRACT

Input of decaying wood from natural disturbances may benefit nutrient cycling processes in lodgepole pine [Pinus contorta var. murrayana (Grev. & Balf.) Engelm.] forests of central Oregon by supplementing detrital C and nutrient pools. This study was conducted to determine the decomposition rate and nutrient retention of downed lodgepole pine boles, and to characterize their effect on soil biological and chemical properties. Three sites, in stands recently damaged by insects, had an average downed wood biomass of 38.7 Mg ha^–1. Bole-wood decay was rapid once in contact with the soil surface. The decomposition rate constant based on the single-exponential decay model was 0.027 yr^–1 and the turnover time of C was 37 yr. Nitrogen content per unit volume was 75% greater for downed boles after 38 yr of decay compared witb their initial content. A net release of Ca (38%), Mg (54%), and K (42%) was found, but P content remained constant during decomposition. With the exception of Mg, however, downed boles comprised <3% of the combined mineral soil, O horizons, and downed wood nutrient pool. In comparison, downed boles comprised 23.5% of the detrital C. Microbial biomass C (C_M) and the ratio of microbial C to total organic C (C_M/C_T) were significantly greater in the surface 0 to 4 cm of soil beneath rapidly decaying boles than in soil without a bole-wood component. The C_M/C_T ratio remained elevated throughout the advanced stages of wood decomposition, indicating a long-term change in the efficiency of C utilization by soil microbial communities associated with decaying wood.

This research was performed in cooperation with the USDA-Forest Service, Deschutes National Forest, Bend, OR.

Received for publication July 10, 1992.

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