Troublesome Tallow

Known for its heart-shaped leaves and white fruit, the Chinese tallow tree originated in Asia. The United States government introduced it to the Gulf Coast area around 1900 in hope of using the wax-covered seeds as an agricultural crop. The project was unsuccessful, and the trees escaped from cultivation and began proliferating, turning Gulf Coast grasslands into single-species forests. Now, Rice University ecologist Evan Siemann hopes to find out how this species has been able to “break all the rules.”

Siemann, an assistant professor of ecology and evolutionary biology, 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 fauna will not come back. “The incredible diversity of native plants in the coastal prairies is gone within 30 years after the Chinese tallow tree invades the area,” he says. “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.”

One of the reasons this tree has been able to grow so well is that insects have left it alone to munch on other foliage. Siemann says this is especially peculiar because the slow-growing tallow tree found in China has chemicals in its leaves that make them hard to digest. The American variety, on the other hand, does not produce this substance; instead, it appears to use the energy it saves to grow quickly, which promotes the development of forests.

Siemann is testing various methods of controlling tallow trees using land in Galveston County owned by the University of Houston Coastal Center. “If you knock down the tallow trees,” he says, “they just sprout from roots like crazy.” 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 out the tallow trees.

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 that fertilizing with nitrogen has on the trees’ growth. “Prairie grass is very efficient at using nitrogen,” Siemann says, “but the tallow tree uses nitrogen very inefficiently.”

Siemann believes the lessons learned from his research will be applicable to many areas. Chinese tallow trees are starting to sprout, for example, in the forests of East Texas. “This tree is gobbling up real estate everywhere,” Siemann says. “Once the canopy trees come down, they’ll be replaced by Chinese tallow trees.” Several experiments in the Big Thicket National Preserve are investigating 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 in 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.

—B. J. Almond