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Scientists worry that vines are taking over the American tropics

Feb. 15, 2011
Courtesy Smithsonian Tropical Research Institute
and World Science staff

Sleeping Beauty's kingdom was overgrown by vines when she fell into a deep sleep. Now,  scientists worry that real vines are taking over the American tropics.

Data from each of eight sites tested show that vines are overgrowing trees, according to a research group.

Stefan Schnit­zer ex­a­m­ines a li­ana, a type of woody vine, in Pa­na­ma. (Cred­it: Beth King, STRI)


"We are witnessing a fund­amental struct­ural change in the phy­sical make-up of for­ests that will have a pro­found im­pact on the ani­mals, hu­man com­muni­ties and busi­nesses that de­pend on them," said re­search team mem­ber Ste­fan Schnit­zer of the Smith­son­ian Trop­ical Re­search In­sti­tute in Pa­na­ma and the Uni­versity of Wis­con­sin at Mil­wau­kee.

Tropical forests hold more than half of the Earth's land spe­cies and much of the plan­et's carbon. If vines take over trop­i­cal for­ests, re­search­ers say, the rain­forests' role in the the larger en­viron­ment, such as through water cycl­ing and car­bon storage, may change in ways that are hard to pre­dict.

"In 2002, Oliver Phillips, a professor at the Uni­vers­ity of Leeds in the U.K., pub­lished a con­trov­er­sial study claim­ing that vines were becoming more com­mon in the Ama­zon," said Schnit­zer. "By pul­ling together data from eight diff­erent stud­ies, we now have irre­fut­able evi­dence that vines are on the rise not only in the Ama­zon, but through­out the Amer­ican trop­ics," added the in­vesti­ga­tor, whose team has re­ceived a grant of more than $1 mil­lion from the U.S. National Sci­ence Found­ation to study the prob­lem.

On Barro Colorado Island in Panama, the proportion of vines in tree crowns has more than doubled over the past 40 years, researchers said. In French Guiana, liana vines increased 60 percent faster than trees from 1992 to 2002. Similar reports came from from Brazil, the Bolivian Amazon and subtropical forests in South Carolina in the United States.

Trees have stout trunks that take a lot of time and energy to produce. Vines exploit trees, growing quickly on slender stems up into the forest canopy, where their leaves may compete for light with those of the supporting trees.

Scientists are unsure as to why lianas are gaining the upper hand. They may survive seasonal droughts that are becoming more common as climate becomes more variable, researchers speculated. They may recover more quickly from natural disturbances such as hurricanes and El Niño events and from human disturbances like logging, clearing land for agriculture and road building. In several experiments, lianas were found to respond more quickly than associated trees to an increase in atmospheric carbon dioxide, such as what scientists believe occurs with human-induced global warming.

In North American forests, invasive vines such as kudzu, oriental bittersweet, English ivy and Japanese honeysuckle often reduce native tree regeneration and survival, although there is no obvious trend as there is in the American tropics, according to researchers. Two studies of forests in tropical Africa did not detect vine overgrowth.

The authors propose to take advantage of the widespread network of large-scale, long-term monitoring plots-the Smithsonian Institution Global Earth Observatory network coordinated by the Center for Tropical Forest Science-combined with experiments to reveal what gives vines a competitive edge over trees.

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