Recently published research by U.S. Forest Service scientists provides important first-time analyses of the potential impacts of introducing plantations of freeze-tolerant Eucalyptus into the South.
Eucalyptus, a fast-growing tree native to Australia and Indonesia, is planted across large areas of Asia, Africa, and South America as a major source of hardwood fiber for paper and biofuels. Because of its sensitivity to freezing temperatures, Eucalyptus hasn’t been planted extensively in the U.S., where fiber markets are dominated by softwood from pines grown in the Southeast.
Some paper mills require hardwood fiber, and interest in using small-diameter hardwoods for biofuel pellets is growing. In the southern U.S., hardwood fiber mostly comes from natural stands, and sources are scarce, especially in localized markets. There is increasing interest in the South in the development of a freeze-tolerant Eucalyptus species to grow in plantations as a hardwood fiber source.
The prospect of Eucalyptus plantations also raises concerns, since Eucalyptus is known for using large quantities of water. Will landowners convert their forest land to Eucalyptus to get better returns? If they do, how could this affect southern water resources?
Forest Service Southern Research Station (SRS) scientists recently published companion articles in the journal Forest Science: one estimates the acres and types of land southern landowners might convert to freeze-tolerant Eucalyptus, while the other evaluates possible impacts on the region’s water resources.
Wear and North Carolina State University collaborators first defined a study area based on hardiness zones and water availability that spans the southeastern U.S. from east-central Texas to South Carolina. Then they compared net economic returns from potential Eucalyptus plantings with those of existing land uses, exploring how Eucalyptus plantations might develop under different market futures.
They found that Eucalyptus would probably be most competitive with planted pine, as opposed to agricultural and other land uses, and that conversions from pine could be expected on 0.8 to 1.4 million acres, 5 to 9 percent of the current area of planted pine.
“Extending the analysis to the current area of naturally regenerated pine results in as much as 2.8 million acres,” said Wear. “But actual adoption depends on uncertain future markets for cellulose, especially for bioenergy feedstock.”
Vose and fellow scientists from the SRS Center for Watershed Science and the SRS Eastern Forest Threat Assessment Center used the study area developed by Wear to analyze potential impacts on water resources at stand and regional scales. “With one of the highest evapotranspiration rates among tree species, Eucalyptus plantings could have negative effects on water resources and aquatic ecosystems, especially at local scales,” said Vose. “To our knowledge, this study is the first critical analysis of the potential impacts of Eucalyptus on water resources in the U.S. South.”
To analyze effects at the stand level, researchers used a process-based model tied to climate data at five specific locations across the study area, applying the model to a hypothetical Eucalyptus plantation at each site from initial planting through a full rotation.
Precipitation varied widely among the five locations, from about 31 inches per year in the Texas location to about 61 inches per year in Mississippi. Stand-level results showed that Eucalyptus plantations could potentially lower water flow drastically in some locations.
“These effects will depend on the hydrologic conditions of the planting site and on the amount of land planted in Eucalyptus,” said Vose. “Negative impacts might occur in areas with low rainfall or where dry years are likely.”
For the regional analysis, the researchers applied the large-scale water balance model WaSSI at the watershed level, identifying the HUC-12 watersheds across the entire South-wide planting area identified in the Wear study. They simulated replacing different proportions of the existing conifer land cover with Eucalyptus.
The researchers found that at the level of conversion indicated by Wear’s research, effects at the watershed level would be negligible – even lower if plantations were on early rotation schedules. In contrast, if economic conditions promoted large-scale conversion of existing land cover (such as 50 percent of current conifer cover), there could be regional effects on water availability in areas such as the Florida Panhandle, south Alabama, southwest Georgia, and Mississippi.