Tree DNA clears Ben Franklin

RICE U. (US) — Tallow trees imported from China are overrunning thousands of acres of U.S. coastal prairie, but new research has found Benjamin Franklin not guilty of aiding and abetting the enemy.

“It’s widely known that Franklin introduced tallow trees to the U.S. in the late 1700s,” says Evan Siemann, professor of ecology and evolutionary biology at Rice University. “Franklin was living in London, and he had tallow seeds shipped to associates in Georgia.”

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The trees are classified today as an invasive species—and like Asian carp in the Great Lakes and kudzu vines in the eastern U.S., they are spreading so fast that they’re destroying native habitats and causing economic damage.

Each tallow tree can produce up to a half million seeds per year—a prodigious fertility that caught the attention of Franklin and others. Each seed is covered by a waxy, white tallow that can be processed to make soap, candles, and edible oil.

Siemann has spent more than 10 years compiling evidence on the differences between U.S. and Chinese tallow trees. For example, the insects that help keep tallow trees in check in Asia don’t live in the U.S. and Siemann has found that the U.S. trees invest far less energy in producing chemicals that ward off insects. U.S. trees also grow about 30 percent faster than their Chinese kin.

The research is reported in the American Journal of Botany.

“This raises some interesting scientific questions,” Siemann says. “Are tallow trees in the U.S. undergoing evolutionary selection? Did those original plants brought from China have the traits to be successful or did they change after they arrived? Does it matter where they came from in China, or would any tallow tree do just as well in the U.S.?”

In 2005, Siemann and study co-authors William Rogers, now at Texas A&M University, and Saara DeWalt, now at Clemson University, collected and froze leaves from more than 1,000 tallow trees at 51 sites in the U.S. and a dozen sites in China, conducting hundreds of genetic scans on the leaves, and spending more than two years analyzing and correlating the results.

There were a few surprises. First, the tallow trees that are running amok in most of the U.S. aren’t from the batch that Franklin imported, but from seeds brought to the U.S. by federal biologists around 1905. The descendants of Franklin’s trees are confined to a few thousand square miles of coastal plain in northern Georgia and southern South Carolina.

“The genetic picture for Franklin’s trees is muddled; we may never know where they originated,” Siemann says. “But the genetic evidence for the other population—the one that’s problematic in the Gulf Coast—clearly points to it being descended from eastern China, probably in the area around Shanghai.”

In controlled tests in China, the researchers found the U.S. trees even grew and spread faster than their Chinese forebears, despite the lack of chemical defenses to ward off insects.

“They suffered twice the damage from insects that the natives did, but they grew so much faster that they still retained a competitive edge,” Siemann says.

“In some ways, this raises even more questions, but it clearly shows that if you are going to explore control methods for an invasive species, you to need to use appropriate genetic material to make certain your tests are valid.”

The research received funding from the National Science Foundation and the Department of Agriculture.

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