Water use within existing tree plantations of all ages resulted in average stream flow reductions of 38 percent, with losses increasing as the trees aged. Moreover, "13 percent of streams dried up completely for at least one year," the study said.
Overall, about 20 percent more of the water provided by precipitation was removed by current tree farming, the study estimated. And additional planting of trees for carbon mitigation will likely have large impacts on water resources of many nations that net less than 30 percent of what precipitation provides for their total annual supplies of fresh water, the authors predicted.
"The places that are most likely to grow trees for carbon sequestration are places where trees aren't growing now," Jackson said in an interview.
"One of the points of this paper is that those places tend to be relatively low rainfall areas. We predict we will see a decrease in stream flow particularly in these relatively drier spots that are targets for sequestration." Almost all plantation trees are heavy water using evergreen species such as pines and eucalyptus, he added.
However, the researchers said adding more of these plantations would also release more moisture into a region's atmosphere, as trees roots removed water from the soils and discharged some portions as water vapor emanating through leaf pores.
To predict whether this increased moisture might boost rainfall to counter tree water withdrawals, co-authors Bruce McCarl of Texas A&M University and Brian Murray of RTI International inResearch Triangle Park, N.C. first investigated where in the United States additional tree plantations would most likely locate.
Using a computer model that projected the likelihood of such conversions in return for payments of between $50 and $100 per ton of sequestered carbon, they estimated that 72 million hectares in the Southeast and