Hydrologic and Water Quality Effects of Harvesting and Regeneration of a Drained Pine Forest
Data on precipitation, weather, water tables, outflows, and nutrient concentrations from two paired watersheds (D1 - control and D2 - treatment) on a pine forest in Coastal North Carolina were measured during 1988-90 calibration period to characterize the pre-treatment hydrology and water quality. Similarly, measured data from 199 5 (D2 harvested) to 2004 (seven years after planting in 1997) were then used for evaluating the effects of harvesting and regeneration (D2) using a paired watershed approach. Annual rainfall varied widely during the study period with 2388 mm in a very wet year (2003) to as low as 85 1 mm in a very dry year (2001). Harvesting resulted in substantial increases of as much as 20 cm in the average water table and 9 1 mm in outflow from D2 compared to the control (D1) in the first six months after harvest. The increase in water table was mainly attributed to decrease in ET losses as a result of reduced canopy. The water table increase declined substantially after 1998 (trees two years old), except during some dry summer months. However, by 2002 (trees five years old), the difference in water tables between the regenerated and control watersheds was reversed, consistent with the pre-treatment levels. The increase in measured annual outflows on D2 varied from 260 mm in a wet year 1996 (first year after harvest) to 56 mm in a near normal year 1999 (two years after planting). Peak flow rates from the harvested watershed for a summer event after harvest were nearly seven-fold higher than the control. The monthly and annual data indicated that the outflows on the harvested watershed returned to base line levels by 2003, nearly six years after planting. Although both the nutrient concentrations and loadings (except for total P) on D2 were substantially elevated after harvesting, they were only short-lived (< 3 years). The measured NO3-N, TKN, and TP loadings on the harvested watershed varied from 0.01 - 4.5 kg ha-1, 0.18 - 4.7 kg ha-1, and 0 - 0.4 kg ha-1, respectively. The minimum loadings occurred in the driest year 2001 (rain = 850 mm, outflow = 51 mm). Harvesting also increased sediment levels, but for only three years.