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July 29, 2011

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DNA clears Ben Franklin in invasive tree mystery

July 28, 2011
Courtesy of Rice University
and World Science staff

Falsely ac­cused mur­der­ers and rapists aren’t the only peo­ple get­ting their good names re­stored thanks to DNA test­ing these days.

So per­haps is U.S. found­ing fa­ther, states­man and sci­ent­ist Ben Frank­lin, whose his­tor­i­cal reputa­t­ion had fall­en un­der the shad­ow of, well, a tree. 

Hun­dreds of mil­lions of trees, ac­tu­al­ly. 

Chinese tallow trees have overrun thous­ands of acres of tall grass coas­tal prairie on the US Gulf Coast. (Courtesy Rice U.)
In­va­sive Chin­ese tal­low trees are over­run­ing swaths of the U.S. Gulf Coast. Frank­lin has been sus­pected of hav­ing brought their an­ces­tors to the coun­try, be­stowing on a nation he helped forge a gift it doesn’t want.

Indeed, the man on the $100 bill im­ported such trees to Amer­ica. But new ge­net­ic tests in­di­cate his trees weren’t those whose de­scen­dants are pro­lif­er­at­ing wild­ly.

Cu­ri­ous­ly, the re­search shows Frank­lin’s trees have been rel­a­tively well be­haved, per­haps more so than their no­to­ri­ously woma­nizing own­er.

“It’s widely known that Frank­lin in­tro­duced tal­low trees to the U.S.,” said bi­ol­o­gist Ev­an Sie­mann of Rice Uni­vers­ity in Tex­as, co-author of the study pub­lished in this mon­th’s Amer­i­can Jour­nal of Bot­a­ny. “Frank­lin was liv­ing in Lon­don, and he had tal­low seeds shipped to as­so­ci­ates in Geor­gia.” The year was 1772.

What Frank­lin could­n’t have known was that tal­low trees—al­so known as chick­en trees or pop­corn trees—would over­a­chieve in the New World. To­day, they’re clas­si­fied as an in­va­sive spe­cies. Like Asian carp in the Great Lakes and kud­zu vines in the east­ern U.S., they’re spread­ing so fast that they’re de­stroy­ing na­tive habi­tats and caus­ing eco­nom­ic dam­age.

Each tal­low tree can pro­duce up to a half mil­lion seeds a year. That fer­til­ity is one rea­son Frank­lin and oth­ers were in­ter­est­ed in them. Each seed is cov­ered by a waxy, white tal­low that can be pro­cessed to make soap, can­dles and ed­i­ble oil. 

The plant is “an or­na­men­tal tree with col­or­ful au­tumn fo­li­age that can sur­vive full sun­light and shade, flood­ing, drought, and in some cases fire,” said a U.S. Ge­o­log­i­cal Sur­vey re­port from Oc­to­ber 2000. “To hor­ti­cul­tur­al­ists this kind of tree sounds like a dream, but to ecol­o­gists, land ma­nagers, and land own­ers [it] can be a night­mare.” In­vad­ing tal­low “even­tu­ally mo­nop­o­lizes an ar­ea, cre­at­ing a for­est with­out na­tive an­i­mal or plant spe­cies.” A 2010 U.S. Forest Service re­port cited esti­mates indi­cating a total po­pu­lation of around 600 mil­lion tallow trees for Texas, Loui­siana and Mis­sis­sip­pi.

Sie­mann and two col­leagues col­lect­ed and froze leaves from more than 1,000 tal­low trees at 51 sites in the U.S. and a doz­en in Chi­na. The re­search­ers con­ducted hun­dreds of ge­net­ic scans on the leaves, and spent more than two years an­a­lyz­ing the re­sults.

There were a few sur­prises. First, the re­search­ers said, the tal­low trees run­ning amok in ma­ny U.S. states aren’t from the batch Frank­lin im­ported; the de­scen­dants of those trees are con­fined to a few thou­sand square miles of coast­al plain in north­ern Geor­gia and south­ern South Car­o­li­na. 

All oth­er U.S. tal­low trees the team sam­pled were found to have de­scended from seeds fed­er­al bi­ol­o­gists brought to the U.S. around 1905.

As far as Frank­lin’s trees, the ge­net­ic pic­ture “is mud­dled; we may nev­er know where they orig­i­nat­ed,” Sie­mann said. “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.”

Be­yond his at­tempts to solve the hor­ti­cul­tur­al who­dun­it, 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 and col­leagues have found that the U.S. trees in­vest far less en­er­gy in pro­duc­ing chem­i­cals that ward off in­sects. They’ve al­so found that U.S. trees grow about 30 per­cent faster than their Chin­ese kin.

“This raises some in­ter­est­ing sci­en­tif­ic ques­tions,” Sie­mann said. “Are tal­low trees in the U.S. un­der­go­ing ev­o­lu­tion­ary se­lec­tion?” he asked, re­fer­ring to adapta­t­ions that spe­cies un­dergo in re­sponse to en­vi­ron­men­tal pres­sures. “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 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 said. “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.” 

Sie­mann said that with ma­ny new spe­cies of foreign plants and an­i­mals still be­ing in­tro­duced to the U.S. yearly, it’s cru­cial for sci­ent­ists to bet­ter un­der­stand what makes some cross the line and be­come dan­ger­ous in­va­sive pests.

* * *

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Falsely accused murderers and rapists aren’t the only ones getting their good names restored by DNA testing. So is U.S. founding father, statesman and scientist Ben Franklin, whose historical reputation of late has fallen under the shadow of, well, a tree. Thousands of them, actually. Invasive Chinese tallow trees are overruning swaths of the U.S. gulf coast, and Franklin has been suspected of having brought their ancestors to the country. The man on the $100 bill did indeed import such trees to the republic he was helping to forge, but the new genetic tests indicate his trees weren’t those whose descendants are proliferating wildly. Curiously, the new research indicates Franklin’s trees have been relatively well behaved, perhaps more so than their notoriously womanizing owner. “It’s widely known that Franklin introduced tallow trees to the U.S. in the late 1700s,” said biologist Evan Siemann of Rice University in Texas, co-author the study published in this month’s American Journal of Botany. “Franklin was living in London, and he had tallow seeds shipped to associates in Georgia.” What Franklin couldn’t have known was that tallow trees—also known as chicken trees or popcorn trees—would overachieve in the New World. Today, they’re classified as an invasive species. Like Asian carp in the Great Lakes and kudzu vines in the eastern U.S., they’re spreading so fast that they’re destroying native habitats and causing economic damage. Each tallow tree can produce up to a half million seeds a year. That fertility is one reason Franklin and others were interested in them. Each seed is covered by a waxy, white tallow that can be processed to make soap, candles and edible oil. The plant is “an ornamental tree with colorful autumn foliage that can survive full sunlight and shade, flooding, drought, and in some cases fire,” said a U.S. Geological Survey report from October 2000. “To horticulturists this kind of tree sounds like a dream, but to ecologists, land managers, and land owners this kind of tree can be a nightmare… when tallow invades, it eventually monopolizes an area, creating a forest without native animal or plant species.” Siemann and two colleagues collected and froze leaves from more than 1,000 tallow trees at 51 sites in the U.S. and a dozen sites in China. The researchers conducted hundreds of genetic scans on the leaves, and spent more than two years analyzing the results. There were a few surprises. First, the researchers said, the tallow trees running amok in many U.S. states aren’t from the batch Franklin imported; the descendants of those trees are confined to a few thousand square miles of coastal plain in northern Georgia and southern South Carolina. All other U.S. tallow trees the team sampled were found to have descended from seeds federal biologists brought to the U.S. around 1905. As far as Franklin’s trees, the genetic picture “is muddled; we may never know where they originated,” Siemann said. “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.” Beyond his attempts to solve the horticultural whodunit, 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 and his colleagues have found that the U.S. trees invest far less energy in producing chemicals that ward off insects. They’ve also found that U.S. trees grow about 30 percent faster than their Chinese kin. “This raises some interesting scientific questions,” Siemann said. “Are tallow trees in the U.S. undergoing evolutionary selection?” he asked, referring to adaptations that species undergo in response to environmental pressures. “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 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 said. “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.” Siemann said that with many new species of plants and animals still being introduced from foreign environments into the U.S. each year, it is vitally important for scientists to better understand the circumstances that cause introduced species to cross the line and become dangerous invasive pests.