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Thinning Effects on Microbial Biomass, N Mineralization, and Tree Growth in a Mid-Rotation Fire-Origin Lodgepole Pine Stand in the Lower Foothills of Alberta, Canada

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We examined the effects of precommercial thinning on microbial biomass C and N, rates of microbial respiration and net N mineralization, and tree growth 24 years after thinning of a 22-year-old fire-origin lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) stand near Edson, Alberta. Treatments were control (unthinned) and manual-thinned (even spacing) to 1,600 trees ha−1 with thinning residue removed. Forest floor and mineral soil (0–10 and 10–20 cm) were sampled on June 15, July 26, and September 25 of the 2002 growing season for the analyses of soil chemical and biological properties, with the last set of incubated samples for determining net N mineralization rates retrieved on May 23, 2003. Forest floor mass was 16 Mg ha−1 lower in the thinned (59 Mg ha−1) than in the unthinned (75 Mg ha−1) plots. Soil moisture content in the forest floor and net N mineralization rates and available N in the mineral soil on most of the sampling dates were higher in the thinned than in the control plots. In the thinned plots, basal area and relative density were 27.7 m2 ha−1 and 54, respectively, and these were still significantly lower than values in the unthinned plots 24 years after treatment. Current annual diameter increment of the largest 1,000 trees ha−1 was greater in the thinned than in the unthinned plots in 2002–2003. Diameter growth was positively correlated with net N mineralization and soil available N. In this stand, thinning still influenced N availability and soil processes with the trees continuing to respond 24 years after thinning.

Keywords: forest floor accumulation; microbial biomass C and N; net N mineralization; precommercial thinning; soil microbial respiration

Document Type: Research Article

Publication date: August 1, 2008

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