Short-rotation woody crops

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.

Growing interest in renewable and domestically produced energy motivates the evaluation of woody bioenergy feedstock production. In the southeastern U.S., woody feedstock plantations, primarily of loblolly pine (Pinus taeda), would be intensively managed over short rotations (10-12 years) to achieve high yields. The primary differences in managing woody feedstocks for bioenergy production vs for pulp/sawtimber production include a higher frequency of pesticide and fertilizer applications, whole-tree removal, and greater ground disturbance (i.e., more bare ground during stand establishment and more frequent disturbance). While the effects of pulp/sawtimber production on water quality are well-studied, the effects of growing short-rotation loblolly pine on water quality and the efficacy of current forestry Best Management Practices (BMPs) have not been evaluated for this emerging management system. We used a watershed-scale experiment in a before-after, control-impact design to evaluate the effects of growing loblolly pine for bioenergy on water quality in the Upper Coastal Plain of the southeastern U.S. Intensive management for bioenergy production and implementation of current forestry BMPs occurred on ~50% of two treatment watersheds, with one reference watershed in a minimally managed pine forest. Water quality metrics (nutrient and pesticide concentrations) were measured in stream water, groundwater, and interflow (i.e., shallow subsurface flow) for a two-year pre-treatment period, and for 3.5 years post-treatment. After 3.5 years, there was little change to stream water quality. We observed a few occurrences of saturated overland flow, but sediments and water dissipated within the streamside management zones in over 75% of these instances. Stream nutrient concentrations were low and temporal changes mainly reflected seasonal patterns in nitrogen cycling. Nitrate concentrations increased in groundwater post-treatment to <2 mg N L-1, and these concentrations were below the U.S. drinking water standard (10 mg N L-1). Applied pesticides were almost always below detection in streams and groundwater. Overall, these findings highlight that current forestry BMPs can protect stream water quality from intensive pine management for bioenergy in the first 3.5 years. However, groundwater quality and transit times need to be considered in these low-gradient watersheds of the southeastern U.S. that are likely to become an important location for woody bioenergy feedstock production.

Nitrogen (N) is an important nutrient as it often limits productivity, but in excess can impair water quality. Most studies on watershed N cycling have occurred in upland forested catchments where snowmelt dominates N export; fewer studies have focused on low-relief watersheds that lack snow. We examined watershed N cycling in three adjacent, low-relief watersheds in the Upper Coastal Plain of the southeastern United States to better understand the role of hydrological flowpaths and biological transformations of N at the watershed scale. Groundwater was the dominant source of nitrified N to stream water in 2 of the 3 watersheds, while atmospheric deposition comprised 28% of stream water nitrate in one watershed. The greater atmospheric contribution may have been due to the larger stream channel area relative to total watershed area or the dominance of shallow subsurface flowpaths contributing to stream flow in this watershed. There was a positive relationship between temperature and stream water ammonium concentrations and a negative relationship between temperature and stream water nitrate concentrations in each watershed suggesting that N cycling processes (i.e., nitrification, denitrification) varied seasonally. However, there were no clear patterns in the importance of denitrification in different water pools possibly because a variety of factors (i.e., assimilatory uptake, dissimilatory uptake, mixing) affected nitrate concentrations. Together, these results highlight the hydrological and biological controls on N cycling in low-gradient watersheds, and variability in N delivery flowpaths among adjacent watersheds with similar physical characteristics.

This dataset reports the pre-treatment hydrology and pre- and post-treatment water quality data from a watershed-scale experiment that is evaluating the effects of growing short-rotation loblolly pine for bioenergy on water quality and quantity in the southeastern U.S. The experiment is taking place on the Savannah River Site, near New Ellenton, South Carolina, USA.  Beginning in 2010, water quality and hydrology were measured for two years in 3 watersheds (R, B, C). At the end of February 2012, 40% of two treatment watersheds (B, C) were harvested and loblolly pine seedlings were planted and managed for bioenergy (including multiple applications of herbicides and fertilizers). Water samples were collected from stream water (weekly), riparian groundwater (monthly), groundwater beneath the uplands (monthly), throughfall (weekly), and trenches that collected shallow subsurface flow (during storms), and these data are available for the pre- and post-treatment periods. Water samples were also collected from three concentrated flow tracks that formed in watersheds B and C in the post-treatment period. Water samples were analyzed for nitrate-N, ammonium-N, soluble reactive phosphorus (SRP), and dissolved organic carbon (DOC) concentrations. Stream water samples only were analyzed for total nitrogen and total phosphorus concentrations, and select samples (usually collected seasonally) were analyzed for pesticide concentrations. Water samples were also analyzed for stable isotopes of nitrate (δ15N, δ18O), and these data are available for the pre-treatment period. Stream flow and trench flow were measured every 10-15 minutes, and these data are available for the pre-treatment period.

The pre-treatment data were presented in a manuscript (Griffiths et al. 2016) that utilized stable isotope of nitrate data to describe hydrological and biological drivers of watershed N cycling and sources of stream water nitrate in the 3 study watersheds. Both the pre-treatment and post-treatment water quality data were presented in a manuscript (Griffiths et al. 2017) that examined the water quality responses to short-rotation pine production for bioenergy.

Griffiths, N.A., C.R. Jackson, J.J. McDonnell, J. Klaus, E. Du, and M.M. Bitew. 2016. Dual nitrate isotopes clarify the role of biological processing and hydrologic flowpaths on nitrogen cycling in subtropical low-gradient watersheds. JGR-Biogeosciences 131:422-437.

Griffiths, N.A., C.R. Jackson, M.M. Bitew, A.M. Fortner, K.L. Fouts, K. McCracken, and J.R. Phillips. 2017. Water quality effects of short-rotation pine management for bioenergy feedstocks in the southeastern United States. Forest Ecology and Management 400:181-198.

Acknowledgements: This research was supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Bioenergy Technologies Office. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the US Department of Energy under contract DE-AC05-00OR22725.

Forest industry experts were consulted on the potential for hardwood tree species to serve as feedstock for bioenergy in the southeastern United States. Hardwoods are of interest for bioenergy because of desirable physical qualities, genetic research advances, and growth
potential. Yet little data is available regarding potential productivity and costs. This paper describes required operations and provides a realistic estimate of the costs of producing bioenergy feedstock based on commercial experiences. Forestry practitioners reported that high productivity rates in southeastern hardwood plantations are confined to narrow site conditions or require costly inputs. Eastern cottonwood and American sycamore grow quickly on rich bottomlands, but are also prone to pests and disease. Sweetgum is frost hardy, has few pest or disease problems, and grows across a broad range of sites, yet growth rates are relatively low. Eucalypts require fewer inputs than do other species and offer high potential productivity but are limited by frost to the lower Coastal Plain and Florida. Further research is required to study naturally regenerated hardwood biomass resources. Loblolly pine has robust site requirements, growth rates rivaling hardwoods, and lower costs of production. More time and investment in silviculture, selection, and breeding will be needed to develop hardwoods as competitive biofuel feedstock species.  Because of existing stands and fully developed operations, the forestry community considers loblolly pine to be a prime candidate for plantation bioenergy in the Southeast.