Franklin, then in Europe, sent Chinese tallow tree seedlings to an associate in Georgia. Known for its heart-shaped leaves and white fruits, the Chinese tallow tree originated in Asia. The U.S. government brought it to the Houston area around 1900 in hope of using its wax-covered seeds as an agricultural crop. Rice University biologists Evan Siemann and William Rogers have discovered surprising evidence that the trees in Texas are genetically different from their Asian forebears.
With two new grants from the United States Department of Agriculture (USDA) and the National Science Foundation (NSF), Siemann, assistant professor of ecology and evolutionary biology, and Rogers, Faculty Fellow, plan a detailed comparison of Chinese tallow trees from the U.S. Gulf Coast, Hawaii and China. In field studies and lab experiments, Siemann, Rogers and colleagues will chart the genetic differences between the regional varieties of tallow trees, and they'll study how well the trees from each region survive when they are transplanted to other regions. Ultimately, they hope to find out whether the genetic changes they have already documented are occurring as a result of natural selection.
"This is the single best example of post-introduction genetic change by any plant species," said Siemann. "What we don't know is whether those genetic changes are important for the Chinese tallow trees' success as an invader."
On the coastal plains of the U.S., Chinese tallow trees are a scourge. In a scant 30 years, they can reduce an ecologically diverse prairie to a single-species forest. Recent studies, in areas including the Big Thicket National Preserve, indicate the invading tallow trees are just as successful at taking over native forests.
One reason the trees thrive in Texas is that they're ignored by the local insect population. Siemann and Rogers have shown that Texas trees produce lower amounts of chemicals that deter insects, which allows them to grow faster than Asian varieties.
"It could be these chemicals are produced in lower quantities because they aren't needed, which allows the trees to put more energy into rapid growth," said Siemann. "It could also be that the genetic differences we've documented don't reflect changes that have taken place in North America but are due to the fact that trees came from different parts of China."
To study this, Siemann and Rogers this month received a $390,000 grant from the NSF to compare the genetic traits of tallow trees in China and the United States. The study will include trees from Texas, Hawaii and 10 provinces of China.
"One reason we're including Hawaii in our NSF research program is that tallow tree cultivation failed miserably there, largely because the Chinese rose beetle got to the islands long before the trees did," said Siemann. "During the '70s energy crisis, a privately funded group tried to grow Chinese tallow trees in Hawaii as a source of biomass for fuel, but the introduced Asian beetles just decimated the trees, especially those which were brought from Texas."
Reuniting Texas varieties of Chinese tallow tree and its native herbivores in China may help researchers understand its success in the U.S. If the combination of low defenses, fast growth and low insect attack is the secret to Chinese tallow tree's success in Texas, varieties collected in Texas and grown in China should exhibit lowered insect defenses and be easy prey for insects. But, when protected from insects, the invasive North American varieties should outperform the Asian varieties.
Next month, Siemann's group will be awarded a three-year $195,000 grant from the USDA for a comparative analysis of tallow trees on the U.S. mainland. This study in Texas, Louisiana, Florida and Georgia will examine the trees' tendency to let down their defenses against insects.
In contrast to Texas, some data suggests that Chinese tallow trees in Georgia are targeted by indigenous insects. Siemann and Rogers think this may indicate the trees are going through a cycle where they are initially ignored by local insects, let down their defenses and consequently become easy targets. Such a process of evolution -- lower resistance by alien plants in the absence of appreciable natural enemies followed by rapid colonization by native insects -- may play an important role in plant invasions and their eventual integration into the local flora.
To test this theory, Siemann, Rogers, and Saara DeWalt, the Huxley Instructor of Ecology and Evolutionary Biology, are growing seedlings from seeds collected from states across the southern U.S. Working with a large group of graduate and undergraduate students, they'll mount a half-dozen road trips to Georgia each year. Along the way, they'll stop at test sites in each state to plant seedlings from all regions. If the theory is correct, the trees from Texas should have the least insect defenses and therefore be targeted more often by insects in every region. Trees from each region should suffer higher damage in Georgia where local insects have had more time to develop a taste for Chinese tallow trees. If confirmed, their results could have profound implications for the field of biological control. They may change the underlying assumptions that are used to predict future invaders and aid in the development of new control strategies for existing invaders.