Trees Are Growing Rocks Inside Themselves, and It’s Incredible

ArlenSci/Tech2025-07-193430

Here’s what you’ll learn when you read this story:

Planting trees has often been seen as a way to combat climate change, but recent research suggests that this method of carbon sequestration isn’t enough to offset what we’re doing to the planet.

Now, planting certain trees is getting a second look, as scientists have found evidence of food-producing trees capable of turning CO2 into limestone.

The trees in question—three species of figs found in Kenya—could be well-suited for agro-forestry, as they’d provide both fruit and long-lasting carbon sequestration.

Planting trees has long been touted as a major tool in the fight against climate change. And on the surface, that makes a lot of sense. After all, there’s a lot of carbon dioxide in the atmosphere, and trees store that carbon in their leaves and woody bark.

But planting trees alone can’t get us out of this mess. For one, large dark forests can negatively impact the planet’s albedo (the ability to reflect sunlight back into space), and once those trees die, they release that carbon back into the atmosphere. Now, a new study presented at the Goldschmidt conference earlier this month in Prague shows that three fig tree species—Ficus wakefieldii, F. natalensis, and F. glumosa—found in the basaltic soils of Samburu County, Kenya, have a carbon-sequestering superpower.

Of course, like most trees, these figs use carbon resources to build their leaves and other woody bits. But they also display a natural sequestration technique known as an “oxalate carbonate pathway.” These are the first fruit trees ever discovered that take carbon and turn it into stone—specifically, calcium carbonate (a.k.a. limestone)—which is then threaded throughout its trunk. This makes these fig trees particularly adept at storing carbon, because even after these trees die, some of that carbon remains in these calcium carbonate deposits.

“We’ve known about the oxalate carbonate pathway for some time, but its potential for sequestering carbon hasn't been fully considered,” Mike Rowley from the University of Zurich, who was involved in the experiment, said in a press statement. “If we’re planting trees for agroforestry and their ability to store CO2 as organic carbon while producing food, we could choose trees that provide an additional benefit by sequestering inorganic carbon also, in the form of calcium carbonate.”

So, how and why do these trees go through all of the trouble? Well, the trees first convert CO2 into calcium oxalate crystals, and then rely on specialized bacteria and fungi to transform it into calcium carbonate. According to the researchers, this increases the pH of the surrounding soil, which makes different types of nutrients available.

To confirm how this limestone formed within the tree, the researchers relied on the Stanford Synchrotron Radiation Lightsource—a division of the SLAC National Accelerator Laboratory—to investigate the nano-architecture of these trees and discern what microbial communities contributed to this sequestering superpower.

“The calcium carbonate is formed both on the surface of the tree and within the wood structures, likely as microorganisms decompose crystals on the surface and also, penetrate deeper into the tree,” Rowley said in a press statement. “It shows that inorganic carbon is being sequestered more deeply within the wood than we previously realized.”

Scientists have known about this oxalate-carbonate pathway for decades. The first tree found to exhibit such a behavior is called the Iroko (Milicia excelsa), a large hardwood native to the tropical western coast of Africa. But these figs are the first food-producing specimens to have this limestone-making ability.

And they’re likely not the only ones.

“We believe there are many more,” Rowley said in a press statement. “This means that the oxalate-carbonate pathway could be a significant, underexplored opportunity to help mitigate CO2 emissions as we plant trees for forestry or fruit.”

Planting trees just became cool again.