Feb. 25, 2002 The Chinese tallow tree has been turning Gulf Coast grasslands into single-species forests, and Rice University ecologist Evan Siemann hopes to find out how this tree has been able to "break all the rules."
"The incredible diversity of native plants in the coastal prairies is gone within 30 years after the Chinese tallow tree invades the area," said Siemann, assistant professor of ecology and evolutionary biology. "By studying how this tree has been able to thrive, we should be able to learn more about the rules that govern a biological community and the interactions among species within that community."
Known for its heart-shaped leaves and white fruits, the Chinese tallow tree originated in Asia. The U.S. government brought it to the Gulf Coast area around 1900 in hope of using the wax-covered seeds as an agricultural crop. That project was unsuccessful, and the trees escaped from cultivation. Siemann is concerned about the spread of Chinese tallow trees, because once they replace bluestem grasses, sunflowers, blazing stars and other plants found in the prairies, those species and their associated animal fauna will not come back.
One of the reasons this tree has been able to grow so well is that insects have left it alone and munched on other foliage. Siemann said this is particularly peculiar because unlike the slow-growing tallow tree found in China, the American variety lets its defenses down. The Chinese variety has chemicals in its leaves that makes them hard to digest. The American variety does not produce this substance; instead, it appears to use that energy to grow quickly, which promotes the development of forests.
Siemann is testing various methods of controlling the tallow trees using land in Galveston County owned by the University of Houston Coastal Center. "If you knock down the tallow trees, they just sprout from roots like crazy," he said. But fire can kill small tallow trees when they’re vulnerable. During a six-year experiment, Siemann is studying how frequently a prairie needs to be burned to keep the tallow trees out.
He is also flooding sections of prairies and pumping water out of others to determine whether wet or dry conditions can make the prairies more vulnerable to invasion. Another study involves examining the effect fertilizing with nitrogen has on the trees’ growth. "Prairie grass is very efficient at using nitrogen, and the tallow tree uses nitrogen very inefficiently," Siemann said.
Because the Chinese tallow trees are starting to sprout in the forests of East Texas, Siemann believes the lessons learned from his research will be applicable to many areas. "This tree is gobbling up real estate everywhere," he said. "Once the canopy trees come down, they’ll be replaced by Chinese tallow trees." Several experiments in the Big Thicket National Preserve investigate whether the same factors are responsible for the Chinese tallow tree’s success in the forests as in grasslands.
Siemann and William Rogers, the Huxley Instructor of Ecology and Evolutionary Biology at Rice, have grants from the Environmental Protection Agency, the National Park Service and the National Science Foundation to study the biology of Chinese tallow trees and methods of controlling it.
Photos and more information about Siemann’s work can be found at http://www. ruf.rice.edu/~siemann.
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