Environmental Effects of Short-Rotation Loblolly Pine Production for Bioenergy and Evaluation of Current Forestry Best Management Practices
The objective of this research project was to assess whether standard forestry best management practices (BMPs) are sufficient to protect stream water quality from intensive silviculture associated with short-rotation woody crop (SRWC) production for bioenergy. Forestry BMPs are designed to prevent the movement of deleterious quantities of nutrients, herbicides, sediments, and thermal energy (sunlight hitting stream channels) from clear-cuts and plantations to surface waters. Until now, there have been no watershed-scale studies examining the effectiveness of traditional forestry BMPs as applied to SRWC production for bioenergy. The demand for woody bioenergy feedstocks is expected to increase, especially in the southeastern United States where the climate, topography, and land ownership are favorable for wood production. Therefore, it is important to evaluate the environmental effects of SRWC production for bioenergy and the efficacy of BMPs.
This study used a watershed-scale experiment in a before-after, control-impact design to examine the environmental effects of short-rotation loblolly pine (Pinus taeda) production for bioenergy and evaluate the efficacy of BMPs for protecting surface water quality. Environmental measurements included water and soil quality (i.e., nitrogen, phosphorus, suspended solid, pesticide concentrations in water, nitrate leaching, nitrogen mineralization, denitrification, ecosystem nitrogen budget, conservative tracer modeling), hydrology (i.e., overland flow and concentrated flow tracks, interflow [shallow lateral subsurface flow], groundwater dynamics), and productivity and stand-level ecophysiology (i.e., tree growth, carbon, water, and energy fluxes). Most of these environmental metrics were measured before (for ~2 years) and after (for ~6 years) harvest, planting, and managing short-rotation loblolly pine for bioenergy on more than 50% of the land area in two treatment watersheds and also in one mature timber reference watershed. The three study watersheds are located in the Upper Fourmile Creek watershed at the Savannah River Site in South Carolina. All silviculture practices in the two treatment watersheds followed South Carolina Forestry BMPs (e.g., minimized soil compaction and bare ground exposure; inhibited hydraulic connections between bare ground and surface waters; provided forested buffers around streams).
The silvicultural plan used in the watershed-scale experiment was designed to achieve high yields of loblolly pine over a short rotation (10–12 green tons/acre/year at 10–12 years), and we intentionally pushed the system in terms of high rates of fertilizer applied. Tree growth and net ecosystem exchange (carbon flux) data demonstrated that the objective of accelerating growth was achieved. In the fourth growing season, aboveground biomass of trees averaged 12,000 kg/ha and carbon sequestration was 466 g C/m2/y. The carbon sequestration rate of the loblolly pine was 1–8 years ahead of conventional southern pine stands grown for pulp production. However, our plot-scale study that manipulated levels of fertilizer and herbicide applications found that the most efficient production system based on the ecosystem N budget was a silvicultural treatment of herbicide without fertilizer; tree growth was 90% of that achieved with operational-scale fertilizer additions and nitrate leaching was lower than in the fertilized treatments. At the operational (watershed) scale, only 30–60% of the nitrogen applied in fertilizers was sequestered in pine after the fourth growing season. Overall, some components of the silvicultural treatments were efficient (i.e., early control of competing plants) and some aspects were not (i.e., early fertilization). These results suggest that nitrogen fertilizers were applied in excess in the first three years and highlight the importance of evaluating water quality responses and efficacy of BMPs under these intensive silvicultural applications.
Despite the high fertilizer applications in the watershed-scale experiment, there were minimal effects of SRWC production on stream water quality, suggesting that forestry BMPs appear to be effective at protecting surface waters. However, nitrate concentrations were elevated in shallow subsurface flow (interflow) and in concentrated flow tracks. Nitrate concentrations also increased in groundwater following harvest and the first fertilizer application. The highest nitrate concentrations measured in groundwater were <2 mg N/L, which is below the US Environmental Protection Agency regulatory limit of 10 mg N/L. These low-gradient watersheds are dominated by groundwater flow paths, and there are several lines of evidence suggesting that some of the elevated nitrate in groundwater should have reached the streams during the 6-year-long posttreatment monitoring period. Groundwater modeling suggests that although transport times to the stream might be on the order of a decade, transport from near-stream portions of the plantations are shorter (1–3 years). Conservative (i.e., non-reactive) tracer modeling also suggests that nitrate concentrations would be elevated in streams following the silvicultural treatments if nitrate travelled conservatively (i.e., nitrate is not taken up or transformed along the groundwater flow path). Estimates of denitrification suggest that this microbial process is important in removing nitrate in groundwater both in the sandy upland areas and in the organic-rich riparian zones (streamside management zones) that are characteristic of this region. Overall, the magnitude of these processes suggests that BMPs in these low-gradient, Coastal Plain watersheds are sufficiently robust to mitigate a relatively low nitrogen fertilizer use efficiency. Phosphorus-based fertilizers were also applied as part of the watershed-scale study, but there were no changes in soluble reactive phosphorus concentrations in stream or groundwater, likely because phosphorus is much less mobile than nitrate and the subsoils contain clays that bind phosphorus.
Aside from fertilizer fate, other important water quality parameters are the fate of applied pesticides and the transport of sediments and associated nutrients to streams. We found little evidence of pesticide movement as none of the stream water samples collected posttreatment had detectable levels of pesticides. The pesticides applied in this study are commonly used in southeastern US silvicultural operations and have low mobility and are moderately persistent. We also found very little evidence of sediment transport to streams via overland flow. Concentrated flow track surveys found that the most likely path of solutes by overland flow was from variable source areas that expanded into the plantations during periods with elevated water tables. The greatest sediment input was from an interior ditch of a paved road and was unrelated to silvicultural management of the site. There were no effects of SRWC production on total nitrogen, phosphorus, or suspended solid concentrations in stream water. Therefore, forestry BMPs were effective with respect to pesticide applications, and overland flow and associated sediment transport.
Overall, the lack of effect of short-rotation loblolly pine production for bioenergy on stream water quality suggests that current forestry BMPs are effective at protecting surface waters in the Coastal Plain landscape even with high levels of fertilization and herbicide application associated with SRWC production. These results should be applicable throughout the southeastern Coastal Plain, in watersheds that are characterized by low-gradient uplands with sandy soils and organic-rich riparian zones. Hydrologic processes in the Piedmont differ sufficiently from those in the Coastal Plain that caution should be used when extrapolating these findings to the Piedmont.