In the southeastern U.S., short-rotation woody crops are a significant part of a growing renewable energy supply. A USDA Forest Service study examines how growing different tree species for bioenergy may have impacts on water yield.
“Loblolly pine has long been considered the go-to woody bioenergy species in the South,” says Peter Caldwell, research hydrologist and lead author of the study. “We looked at sweetgum as a hardwood alternative, with a particular focus on tree water use and water yield — that is, how much excess precipitation, after accounting for water use by vegetation, becomes streamflow or recharges groundwater. We hypothesized that sweetgum would use more water during the growing season, and loblolly, as an evergreen species, would use more during the dormant season. We reasoned that the two would even out over the course of a year, and that there wouldn’t be a big difference in annual water yield.”
Scientists collected and compared data for two intensively managed loblolly pine (Pinus taeda) and sweetgum (Liquidambar styraciflua) stands at the US Department of Energy Savannah River Site in the sandhills of South Carolina, using a study site established to measure the effects of fertilization and irrigation on the productivity of several tree species.
They looked at annual growth in the two species, along with all of the components of evapotranspiration: transpiration, canopy interception of rainfall, and soil evaporation. Transpiration varies by tree species – leaf phenology (evergreen vs. deciduous), leaf water regulation, rooting depth, sapwood area, and xylem structure all affect the rate. This variation can mean differences in water use and yield at the forest stand level.
“This study was the first to quantify those effects in an environment where tree age and stocking density were comparable, and other environmental factors could be controlled. We were really able to isolate species effects,” says Caldwell.
The scientists found that sweetgum trees indeed transpired more during the growing season, a finding that supported their first hypothesis.
Annually, though, while the two stands produced similar aboveground biomass, sweetgum used 53 percent more water than loblolly pine in the process. As a result, annual water yield from loblolly was estimated to be more than three times greater than sweetgum.
“Sweetgum is a water hog when soil moisture is available. Loblolly is a more conservative water user by comparison,” says Caldwell. This is consistent with previous controlled studies that measured transpiration rates in sweetgum. In one closed chamber experiment, sweetgum used eight times as much water as loblolly pine.
Another reason for the difference takes place belowground. The research team measured fine roots near the soil surface and found sweetgum to have nearly twice that of loblolly pine, all of which contribute to total water use. In addition, the team speculated that the sweetgum trees may have used deeper taproots to access additional soil moisture, a theory that will be tested in a followup study.
Planting sweetgum across broad scales could result in lower downstream water availability by the end of the growing season. This timing is critical – that’s when water stored in streams, reservoirs, and groundwater is typically at its lowest.
“Our results suggest that species selection is of critical importance when establishing forest plantations for woody bioenergy production – especially potential impacts on downstream water yield,” adds Caldwell.
For more information, email Peter Caldwell at email@example.com.