Chinese tallow leaf litter negatively affects frogs

The white fruits of the Chinese tallow tree, which give it the common name popcorn tree, were used by Daniel Saenz in his childhood as sling shot ammunition. The oily fruits are also used in soap making. USDA Forest Service photo by Daniel Saenz.

“Where I grew up in the Houston area, Chinese tallow was the main tree in forests near my house. They were a beautiful crimson color in the fall and great for climbing,” says USDA Forest Service researcher Daniel Saenz. “But most importantly, they were the best source of ammunition. Tallow fruits were the perfect size for a slingshot, and in the fall the seeds blew nicely through a straw.”

Chinese tallow tree (Triadica sebifera) is a nonnative, invasive tree whose decaying leaf litter alters water quality and the microbial community in the wetland habitats. This negatively impacts the lifecycle of semi-aquatic species like frogs.

Saenz’s early relationship with Chinese tallow matured overtime and ultimately led him to spend a decade of his career studying how Chinese tallow affects aquatic communities, especially frogs.

Chinese tallow, often called the popcorn tree, is native to east Asia and was introduced in the U.S. in the 1770s as an ornamental and for seed oil. Outside its home range and without the system of checks and balances it evolved with – like insects, the tree grows unchecked and forms thick, dense stands. Combined with people continuing to plant it for its fall colors, for making soap, for kid ammunition, tallow trees’ invasive potential increases.

“I lived in Nacogdoches for ten years before I noticed a tallow tree in the woods at the Stephen F. Austin Experimental Forest. Then I began seeing more and more seedlings. As a scientist, I knew what was happening: this was the expansion and invasion of a nonnative species. I was studying amphibians by this point in my career and noticed a distinct change in the watercolor after tallow leaves fell. I began to wonder how this was affecting the aquatic community.”

Saenz began conceiving project ideas related to Chinese tallow and aquatic environments. “I closely advised a series of graduate students at Stephen F. Austin University to see the projects through,” says Saenz.

Taylor Cotten led a study comparing tadpole survival in mesocosms – basically outdoor kiddie pools – with Chinese tallow leaves versus mesocosms with native species leaves, like swamp chestnut and red maple. In early winter, tadpole survival rates were lower in mesocosms with Chinese tallow leaf litter, but later in the season hatchlings did equally well with Chinese tallow and native species leaf litter.

Wildlife biologist Cory Adams led a study looking at the effect of tallow leaf litter on frog hatchling success. The study, published in Canadian Journal of Zoology, found eggs exposed to tallow leaves in the earliest stages of development died. More developed eggs hatched. The cause of death was very low dissolved oxygen and pH levels in the water.

Kiddie pool mesocosms were used to test effects of Chinese tallow leaf litter on water quality, as well as tadpole survival and behavior. USDA Forest Service photo by Daniel Saenz.

In another study, published in Herpetological Review, Saenz and Adams found tallow tree caused premature hatching of frog eggs. This led to reduced size and reduced development of hatchlings, making them more susceptible to predation.

Erin Fucik Childress then led a study investigating the combined effect of tallow and climate change on frog survival. Researchers staggered when tallow leaves were introduced to the mesocosms. The frogs breeding closest to leaf introduction had the worst survival – due to the low dissolved oxygen and pH levels.

Temperature triggers breeding in frogs. Climate change is expected to cause leaves to fall later and frogs to breed earlier. This could have real consequences on tadpole survival and thus frog populations. These results were published in Ecology and Evolution.

Raymond Montez led a study with Saenz to find out why tallow leaf litter caused dissolved oxygen to drop so quickly. Their results were published in Aquatic Ecology.

Tallow leaves have very little lignin – plants’ rot resistant support material – and are tissue paper thin when dried out. Thus, they decay very fast (much faster than native species) and dump a load of nutrients in the water all at once. More nutrients mean more microbial activity, which uses up all the oxygen. Tadpoles can’t get enough oxygen from the water, and they die.

Some tadpoles have behavioral traits that help them survive in a low dissolved oxygen environment – they surface and gulp air. Saenz and Adams found that frogs in mesocosms with high tallow concentrations surface and gulp air more frequently than those without tallow. This may affect frog’s ability to get food and avoid predators.

Chinese tallow litter treatments had less diverse fungi communities than native plant litter treatments, the study found. We know nonnative species can lower the diversity of native vascular plant communities, but it is interesting that they also impact fungal communities in aquatic systems.

The population size of nearly half of all amphibian species are currently in decline. Understanding the causes of declines can help us halt and hopefully reverse this trend. Managing nonnative, invasive species, like Chinese tallow trees, will help aquatic communities recover and maintain species diversity.

For more information, email Daniel Saenz at dan.saenz@usda.gov.

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