Forest soil characteristics in a chronosequence of harvested Douglas-fir forests

This study was designed to measure the microbiological and chemical characteristics of forest soils in a chronosequence of harvested Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stands in different climatic settings. Mineral soil samples were collected along transects running from old-growth (OG) forests into harvested stands of ages 5, 15, and 40 years (5YS, 15YS, and 40YS, respectively) in the H.J. Andrews Experimental Forest in the central Oregon Cascade Mountains. We took litter depth measurements and cores to test for the presence of mycorrhizal mats at each sampling location. Denitrification potential was significantly lower in OG than in 5YS, and litter depth, forest floor respiration rate, and concentration of ectomycorrhizal mats were significantly greater in OG than in 5YS. Values were intermediate in 15YS and similar to those measured in OG in 40YS. No significant stand-age differences occurred in soil organic matter, soil moisture, pH, mineralizable N, laboratory soil respiration rate, or extractable ammonium. Sample variability was generally lowest in OG forests and highest in 5YS, and no consistent autocorrelations were observed for any of the variables at lags of 5 m or greater. We found no second-level interactions between stand age and location in ANOVA analyses, suggesting that, within the limits of this study, climate did not influence soil response to disturbance and subsequent recovery; however, several soil properties were affected by site location and, therefore, climate.