Effects of Thinning on Hydrology and Water Quality of a Drained Pine Forest in Coastal North Carolina

A study was conducted to examine the effects of commercial thinning on hydrology and water quality of a 28-year old (in 2002) drained loblolly pine (Pinus taeda L.) plantation watershed (D3) using another adjacent watershed (D1) as a control. A paired watershed approach was used with data from two periods (1988-90 and 2000-02) for calibration and data from 2002-07 as the treatment period. Both of these 25 ha watersheds are located in Carteret County, North Carolina. Results showed a consistent rise in water table elevations on D3 compared to the control during the six months after 50% thinning. The water table rise of 12.6 cm on average decreased substantially by June 2007, indicating an increase in evapotranspiration caused by increased canopy closure. Similarly, flow rates increased immediately after thinning and decreased soon thereafter. Monthly outflows on thinned watershed were generally higher in the summer-fall months compared to the control. However, the total annual outflow from the treatment was lower than the control, except for the very wet years of 2003 with extremely high rainfall (>2300 mm) and 2005, possibly indicating a hydrologic recovery due to canopy closure approximately three years after thinning. Thinning did not affect nutrient concentrations and loading rates, except for NO3-N and total P. The increases in both of these nutrients were minor compared to the values obtained from fertilization of these stands. Results of the study indicate thinning may have only a short term (up to three years) effect on hydrology and water quality of drained pine plantation.