Switchgrass in Pine Plantations

Effects on Water Quality and Quantity

In the southeastern U.S., loblolly pine plantations cover about 37 million acres of land. “Growing switchgrass in loblolly pine plantations could provide a sustainable source of biomass for cellulosic energy,” says U.S. Forest Service research hydrologist Devendra Amatya.

Switchgrass can potentially be grown in between rows of loblolly pine trees as a bioenergy crop. Photo by Devendra Amatya, USFS.

“Growing the two species together could also help maintain the economic and environmental benefits of a forest managed for timber production.”

Switchgrass is a native perennial grass that uses water very efficiently and prevents erosion. By growing  switchgrass in between rows of loblolly pine trees, energy crops could possibly be boosted – without having to remove land from food production.

However, growing loblolly pine and switchgrass together would have to be implemented over large scales in order to meet global demand for biofuel. There is a critical need for information about the water quantity and quality effects of biofuel production on forest landscapes.

Amatya and his colleagues recently conducted a seven year watershed-scale study to evaluate the effects of growing switchgrass on managed pine plantations in coastal North Carolina. The research team included scientists from North Carolina State University, the University of Georgia, Weyerhauser, and many others, and has already resulted in a number of peer-reviewed journal articles, with more to come.

Overall, results from the project suggest that growing switchgrass and loblolly pine together has no significant effect on the water table or on the water yield to nearby streams.

“Silvicultural operations like harvesting, shearing, bedding, raking, and other processes that are part of growing pine and switchgrass can lead to increases of nitrogen,” says Amatya. “However, the increase was lower than usually observed on agricultural drainage waters in the region and did not exceed the drinking water standards.”

Bales of switchgrass harvested on the study site. Photo by Weyerhaeuser.

The scientists also found that the amount of nutrients in streams was affected by a number of factors, including management operations, changes in vegetation cover, and storms. Augustine Muwamba, a post-doctoral researcher at the University of Georgia in Athens, led the study on water quality, which was published in the Journal of Environmental Quality.

Nutrient contamination in streams was worst soon after rain storms that broke long dry periods. However, a clearcut on the site where pine and switchgrass were growing also led to higher levels of nitrogen in the streams.

The clearcut occurred in 2009, and the elevated nitrogen levels persisted through early 2010. By late 2011, nitrogen levels were reduced in all the sites where switchgrass was growing, whether it was the only crop or was grown with loblolly pine.

Data from the period since full growth of switchgrass suggested that planting switchgrass with pine improved downstream water quality by reducing nitrate and phosphate concentrations and loads compared to the traditional pine forest.  However, the watershed planted with only switchgrass yielded higher nitrate concentration and loads than the intercropped site.

Amatya and his colleagues also tested a new method for calibrating pairs of watersheds. The method, called moving sums of recursive residuals, or MOSUM, can help reduce uncertainty in the model. The calibration study was led by Herbert Ssegane, a researcher currently with Monsanto, and preliminary results were published in the journal Hydrology and Earth System Sciences. The new method does not replace the classic paired watershed approach, but can be used as an alternative when data for a paired watershed is not available.

“Using this technique, we were able to detect relationships between the water table and silvicultural operations,” says Amatya. “Our approach also minimized the uncertainty of the relationships between the pairs of similar watersheds.”

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For more information, email Devendra Amatya at damatya@fs.fed.us

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