Water Yield Effects of Biofuel Intercropping in Loblolly Pine Plantations of the Southeastern US

As the biofuel industry develops, society is looking for a sustainable substitute for petroleum products without the problems associated with early biomass solutions.  A set of research studies has been developed to describe the effects of promising forest biomass scenarios on site productivity and sustainability, wildlife, and water.  The hydrology field studies consist of three sets of watersheds in Carteret County, NC, Calhoun County, MS, and Greene County, AL.  At each location there is an older, reference stand, an intercropped site - switchgrass (panicum virgatum) in between rows of loblolly pine, one or two pine sites, and a switchgrass only site.  Surface water quality and quantity will be collected by event and periodic sampling, groundwater will be monitored, and aquatic biology sampled. 

While these studies will give the detailed water balance needed to quantify field evapotranspiration, a short-term soil moisture study was conducted over the spring and summer of 2010 to give an early estimate of the water uptake by switchgrass relative to the typical loblolly pine plantation understory.  This study, at a scale large enough to integrate plant and layout variability but small enough to control for most other physical and climatic variables, took place in Clay County, MS in a five year old pine plantation. The soils are clay loam with a fragipan layer at around 1.5 m of depth. Two understory treatments were evaluated - an established switchgrass plot and a plot suppressed with herbicide.  Reference plots were in a nearby plantation with similarly-aged pine stands and an unmanaged understory. 

Soil moisture readings were taking regularly at multiple depths, and the water volume was estimated to give relative values of water use for each component.  Preliminary results suggest that soil moisture under the switchgrass is more similar to the plots with suppressed vegetation than that of natural understory, implying a water yield increase from the replacement of natural understory with switchgrass.  However, the switchgrass has not reached full site occupancy and maturity, and its ET will continue to increase.

A study was also completed to estimate additional sediment movement introduced by biofuel practices.  Operationally intercropped tracts with streams were selected across Mississippi and Alabama, and a field crew walked the riparian buffers to find sediment incursions.  These were described in the field and potential sediment delivery paths mapped.  The incursions, already occurring at a low rate, can be reduced further with operational guidance.