When asked which tree uses more water, the native, industry favorite loblolly pine or the ultra-fast growing immigrant from Australia, Eucalyptus, U.S. Forest Service biological scientist Chris Maier had a quick answer: both. “Growing wood requires water,” says Maier.
Loblolly and slash pines currently serve as the main sources of wood fiber in the South, much of which goes to make paper products or to fuel burgeoning biomass and biofuel markets. “Forest industry and private landowners are interested in Eucalyptus because it grows so fast. Intensively managed Eucalyptus can grow 500 cubic feet per acre per year, whereas loblolly and slash pine can grow around 350,” says Maier.
But Eucalyptus is sensitive to cold and frost and was not seen as a feasible crop in the southern U.S. — outside of Florida, anyway — until recent hybrids and frost-tolerant species came on the scene. In particular, Eucalyptus benthamii is frost tolerant and fast growing, with a short rotation length of six to eight years — thus an excellent subject for Maier’s recent study.
Maier, along with SRS plant physiologist Kurt Johnsen, project leader James Vose, and other colleagues, compared the water use of loblolly pine (Pinus taeda) and Eucalyptus benthamii in a warm, humid area of South Carolina that was, at the time, MeadWestVaco land, another partner on the project.
Over the course of one year, the scientists measured tree growth, water use, and environmental conditions like temperature, precipitation, humidity, and soil water. Their results were published in Forest Ecology and Management.
The two species grew differently over the course of 2013, with the Eucalyptus trees continuing to add biomass into the late fall and winter. This difference is in part due to Eucalyptus’ greater potential for growth and higher photosynthesis rates. “As long as temperatures are favorable and water is available, Eucalyptus will grow year-round,” adds Maier.
Maier and his colleagues found that the Eucalyptus trees used almost a third more water than the pines over the observation period. However, “Eucalyptus had forty percent greater water use efficiency,” notes Maier.
Previous studies have shown Eucalyptus trees to use water more efficiently than other, slower growing trees. Water use efficiency describes the relationship between the water used or lost by a tree with the amount of new growth it produces over the same period.
Because of Eucalyptus’ greater water use efficiency, it actually used less water than loblolly pine to produce the same amount of woody biomass. The scientists recorded above-average rainfall at the site that year and didn’t get a chance to test how the two species would respond to summer droughts that often occur in the region.
But could greater overall water use have negative impacts where Eucalyptus are commercially planted? “Over multiple rotations, Eucalyptus plantations would use more water than loblolly pines,” adds Maier. “Three rotations of Eucalyptus would yield a lot more wood than one rotation of loblolly pine, and water is needed to grow all of that wood.”
“The keys are locating intensively managed Eucalyptus in areas where water availability or sensitive wetland species are not a concern and limiting plantation size to avoid major impacts to the water table,” concludes Maier.
For more information, email Chris Maier at email@example.com.