Like a book and a cover, you can't always judge a tree by how it looks. Fungus can rot a living tree from the inside, leaving behind a healthy-looking but hollow trunk. Typically the rot is only seen when the tree is cut down.

When a tree decays, it releases carbon dioxide back into the atmosphere, contributing to climate change. And the tropics are home to 96 percent of the world's tree diversity, according to researchers, and those trees store a quarter of the world's terrestrial carbon.

To get a better read on the health of tropical forests, researchers are now using sound to measure decay in trees.

The research, published in the journal Applications in Plant Sciences, was conducted collectively by a group of college professors, grad students and high school teachers and students, who used a new way to examine 1,800 live trees in the Republic of Panama.

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The group used a method for sending a sound wave through a tree and measuring how fast the sound wave travels — a process called sonic tomography. Sounds waves travel faster through solid trees, a measure of their health. In a decayed tree or one with cavities, where molecules are packed less tightly, sound waves travel more slowly.

The process provides insights into how rotting affects tree mortality overall in a forest, said Greg Gilbert, lead author of the article and chair of the Department of Environmental Studies at the University of California, Santa Cruz, in a statement.

"Most of the decay is hidden," Gilbert said. "The tomography now allows us to see how many apparently healthy trees are actually decayed inside."

And while the method won't work with some trees — including palms or any species that uses internal tissue to store water — it has benefits outside of the forest. The team used sonic tomography to perform a checkup on trees in Panama City, in part to highlight which ones might snap off in storms or heavy winds and cause injury or damage to property.

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