In the southeastern U.S., loblolly pine (Pinus taeda) has been identified as a prime candidate species for bioenergy production. Intensive management of loblolly pine stands for bioenergy would involve frequent herbicide and fertilizer applications to achieve high yields. Further, pine would be grown over short rotations (8-12 years), resulting in a higher frequency of wood removal relative to pulp or sawtimber silviculture. The effects of growing short-rotation pine for bioenergy on water quality and the efficacy of forest Best Management Practices (e.g., minimizing bare soils, maintaining streamside management zones) for such management have not been examined at the watershed scale. In this study, we examined the water quality effects of growing short-rotation pine using a watershed-scale experiment in a before-after, control-impact design. Water quality metrics (dissolved and particulate nutrients, herbicides) were measured in streamwater, groundwater, and interflow (i.e., shallow subsurface flow) in 3 watersheds for a two-year pre-treatment period (2010-2012). In 2012, 40% of two watersheds (B, C) were harvested and managed for bioenergy production while the third watershed (R; minimally managed pine forest) served as a control. Water quality samples have been collected for 3.5 years post-treatment, and sample collection will continue until canopy closure (2018).
Results/Conclusions
In the post-treatment period, nitrate concentrations were high in concentrated flow tracks that developed in some planted areas (mean=539 µgN/L), and in interflow draining planted hillslopes (mean=4239 µgN/L). However, similarly high concentrations were not found in stream water in the treatment watersheds (mean nitrate in B=27 µgN/L, C=15 µgN/L), suggesting that dissolved nutrients were not moving to the stream via overland or subsurface flow, or that nitrate was being denitrified before entering the stream. Stream water total nitrogen (TN) and total phosphorus (TP) concentrations were also similar across watersheds post-treatment (mean TN in R=448 µgN/L, B=391 µgN/L, C=267 µgN/L; mean TP in R=11 µgP/L, B=12 µgP/L, C=8 µgP/L), suggesting that overland flow was not moving sediments and associated nutrients from planted areas directly to streams. Visual surveys confirmed that overland flow was rare. While nutrient concentrations were unchanged in streamwater, nitrate concentrations increased in groundwater in the treatment watersheds; however, concentrations were <2 mg N/L, and below the drinking water standard (10 mg N/L). Overall, forestry BMPs appear to protect stream water quality from intensive pine management for bioenergy in the short term. However, groundwater quality and transit times need to be examined in the long-term.