Chinese tallow trees, after being introduced to the southern United States, have evolved in North America to outgrow tallow trees in their native Asian habitat. In the absence of enemies, tallow trees in the US have become less resistant to browsing, but they now also compete more aggressively against other plants.

Tallow trees from China were first introduced to Georgia state in the late 1700s and again about 100 years ago in Texas. The invasive tree has since naturalized in the southeastern US, ousting native plants to form monocultures. Without predators in North America, the tree has flourished.

In this study, seeds from US tallow trees were sent back to China to be grown amid the species' natural enemies. Leaf-eating insects and snails more intensively browsed the US trees than tallow trees from Chinese seed sources. Still, leaf damage set back growth of the US trees less than for the Chinese trees.

After four months, the US tree shoots were heavier and had more leaves than their Chinese counterparts. Trees from both US states grew equally well, consistently outperforming seedlings from the northern and southern reaches of its range in Asia.

The study demonstrates the hypothesis that “evolution of increased competitive ability” occurs when plants invade new habitat. In the US, tallow trees have evolved reduced resistance to but greater tolerance of leaf damage from herbivores, so that browsing has less impact on its growth rate.

This suggests that the outcome of biological control measures may be disappointing. Insects introduced from tallow tree's original range would likely be ineffectual at controlling its spread in North America.

Reference

Jianwen Zou, William E. Rogers and Evan Siemann. 2007. Increased competitive ability and herbivory tolerance in the invasive plant Sapium sebiferum. Biological Invasions. In press.

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