Ramped Up Risk for Frogs When Chinese Tallow Interacts with Climate Change

Chinese tallow tree leaf fall could adversely affect amphibian breeding. Photo by Dan Saenz.

The timing of mating and egg-laying in many amphibians is directly related to temperature. Due to climate change, spring warming comes sooner in many areas, and a new study led by U.S. Forest Service researcher Daniel Saenz suggests that the changed timing of breeding could cause native amphibians such as the southern leopard frog and Chinese tallow tree (tallow) to interact in new, unexpected ways that would adversely affect amphibians.

Introduced to the United States in the late 1700s, tallow has spread aggressively through the Southeast, frequently growing near the streams and temporary wetlands where amphibians lay their eggs. In 2012, Saenz, a wildlife biologist at the Forest Service Southern Research Station Southern Pine Ecology unit, co-authored a study that showed that decomposing tallow leaf litter can kill amphibian eggs. His new study, published in Ecology and Evolution, shows that the shorter winters predicted by future climate scenarios could amplify this effect.

Tallow sheds its leaves after frost, and when winters are warmer, leaves shed much later and have less time to decompose before spring.  Meanwhile, earlier warm spring temperatures cue many amphibian species to lay eggs. (Some species, such as the southern leopard frogs that Saenz used in the study, will lay eggs any time in the year when temperatures are warm enough.) Because amphibian breeding patterns are so closely tied to temperature, earlier breeding could collide with later tallow litterfall and cause high mortality of amphibian eggs and larvae.

“To look at the interaction between climate and tallow, we simulated five winter scenarios, ranging from cold to warm,” says Saenz. “For each scenario, we set up 20 tanks of water, and altered the relative timing of when we introduced leaf litter and amphibian larvae.” To mimic warmer winters, Saenz and his colleagues added tadpoles to tanks where the leaf litter was fresher, and had only been soaking for 8 days. They found that few tadpoles survived exposure to freshly submerged tallow leaf litter.

“We found very low levels of dissolved oxygen in tanks with fresher leaves,” said Saenz. “Oxygen levels may have been low enough to kill the tadpoles.” Other researchers have come to similar conclusions, and some have also found that tannins leaching from fresh leaves can be toxic to aquatic life.

Water chemistry is the primary way invasive plants kill amphibians, but ultimately the impact of Chinese tallow will be determined by the timing of tallow leaf fall and amphibian breeding. “Warmer temperatures could allow tallow to hold its leaves later into the winter, making overlap between leaf fall and amphibian breeding more likely,” says Saenz. “If this happens frequently, it could place the amphibians breeding where tallow invades in a consistent state of high risk for mortality.”

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For more information contact Daniel Saenz at dsaenz@fs.fed.us

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