BS in Soil Science
Natural Resources Management and Environmental Sciences Department
Thinning of forested lands and timber stands in the Pacific Northwest have taken place for centuries with a limited understanding of how the alterations may affect ecosystem functions. The goal of this study was to examine the soil climate and microbial activity on a seasonal timescale of thinning practices examined at different stages of succession. Two timber stands in Southwestern Oregon within the Grayback Creek Watershed were chosen because of identical forest management techniques separated by a 10-year treatment interval (40% variable density thinning). Field methods and equipment measured canopy coverage, soil moisture and temperature at 3 depths (5, 15, 30 cm), as well as snow and precipitation events. Laboratory analysis included particle size analysis (PSA), determination of total %C and %N, and CO2 respiration. The least diurnal flux was at the deepest (30cm) and the greatest temperature flux near the surface (5cm) The covered stand showed a smaller magnitude of diurnal flux compared to the thinned stand (±3°C and ±7°C respectively). The driest time of the year was identical for both stands, however the thinned stand had twice as much (10.8%) water by volume at the 5cm level compared to the covered stand (5.2%). Snowpack acted as a buffer from air temperature limiting the diurnal flux, and the greatest addition to soil water occurred when snow melted on both stands. The thinned stand reached 40% water content (qs) throughout the profile after the first snowmelt, the covered stand rarely approached that level at any depth and time. CO2 respiration, total carbon, and total nitrogen were significantly less on the thinned site compared to the covered site (12 v 45 ppm CO2 at the surface, 2 v 5% carbon, 7 v 20% total nitrogen respectively). The lack of vegetation density (therefore decreased transpiration demands) on the thinned site may account for the differences between (1) water content differences at the peak of the dry season, (2) differences in the magnitude of diurnal flux, (3) and amount of precipitation required to reach qs. The CO2 respiration differences can be attributed to the covered understory vegetation significantly adding more organic matter for microbe decomposition compared to the relatively bare ground on the thinned stand.