Influence of Thinning Loblolly Pine (Pinus taeda L.) on Hydraulic Properties of an Organic Soil

The impact of forest operations on soil properties has been a concern in forest management over the past 30 years. The objective of this study was to evaluate the impact of forest thinning operations on soil hydraulic properties of a shallow organic (Belhaven series) soil in the Tidewater region of North Carolina. Soil physical properties were evaluated in a nested design by collecting soil cores from an unthinned control and following a 40 ha fifth-row thinning with selection performed on a 14-year-old loblolly pine plantation in April 2001. Thinning decreased saturated hydraulic conductivity and drained volumes for a given water table depth; however, changes in bulk density were not detected. Saturated hydraulic conductivity determined by the constant head method before thinning was 100 cm h-1. Thinning resulted in a 3-fold decrease (from 100 to 32 cm h-1) in saturated hydraulic conductivity. The thinned watershed had less drainage at low pressures and greater retained water contents under increased soil water tensions in comparison with the control. Drained volume on the thinned watershed for a water table depth of 200 cm under drained to equilibrium conditions was reduced by 60% in comparison to drained volume for the control watershed. The reductions in ksat, drained volumes, and drainable porosity will likely result in shallower water tables and increased runoff for the thinned watershed.