We all know that trees are an important part of our climate efforts. But we usually envision this as lush forests soaking up carbon dioxide through their leaves and storing it as wood. What if trees could go a step further, not just locking away carbon in their trunks, but converting it into actual stone?

That’s exactly what a new study has uncovered about certain fig trees in Kenya. These trees aren’t just carbon sinks. They’re advanced chemists. They get a bit of help from microbes, and together, they pull carbon dioxide from the air and turn it into calcium carbonate (the same material in chalk or limestone). This crystallized material then gets stashed deep inside the wood and in the surrounding soil. It’s a slow and hidden process but its implications could be huge.

The tree that grows its own rock

All trees absorb CO₂ from the air. Many of them then convert it into organic carbon and use it in their branches, and roots. But these fig trees go further. They form microscopic crystals of a mineral called calcium oxalate, which is a common crystal that shows up in kidney stones. Over time, as parts of the tree die or are broken down by fungi and bacteria, these crystals transform into calcium carbonate. This is the same substance that forms limestone cliffs and seashells.

This process is known as the oxalate-carbonate pathway, and it’s especially promising for our climate efforts. The regular carbon dioxide that trees store is still in the carbon cycle; the wood eventually rots or burns and the CO₂ goes right back into the atmosphere. Essentially, trees just store CO₂ temporarily. But fig trees take carbon out of the system, essentially locking it in long-lasting mineral deposits.

“We’ve known about the oxalate carbonate pathway for some time, but its potential for sequestering carbon hasn’t been fully considered. 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,” says Mike Rowley, senior lecturer at the University of Zurich and study author.

The team focused on three fig species native to Samburu County in Kenya. The most promising was Ficus wakefieldii, which seemed particularly good at forming calcium carbonate both inside its trunk and in the nearby soil.

The researchers then used a powerful synchotron to analyze the internal structure of the fig trees. They found that carbon was mineralized both at the surface and deep within the tree’s tissues. This suggests an active process where trees (and their microbe allies) continuously produce rocky crystals and then incorporate them in their structure.

Yes, you can also eat its fruits

Whenever we talk about a way to take carbon out of the atmosphere, the question is always scale. We emit so much CO₂ into the atmosphere (over 37 billion tons every year) that in order for something to really work, it needs to be scalable. There’s some good news on this side: Ficus wakefieldii and its cousins are fruit trees.

That means they offer a potential triple win for carbon capture, soil improvement, and food production. As the calcium carbonate builds up, it makes the surrounding soil more alkaline. That’s not just a side effect — it actually helps unlock key nutrients, potentially boosting local agriculture. So, these trees may not only be climate allies, but also quiet agronomists, enriching the land around them. You could even give farmers a bonus for growing trees that pull CO₂ out of the atmosphere and enrich local soils.

This won’t solve climate change on its own. But it could be a useful tool.

Tree planting is bound to be a key strategy. But not all trees are equal. While organic carbon in wood might last decades or centuries, it’s eventually released again. Calcium carbonate, by contrast, can stay buried for millennia. It’s geological time. And if food-bearing trees like figs can do this while feeding people, they might be ideal for reforestation, especially in tropical or semi-arid regions where conventional strategies struggle.

What’s more, this is likely just the beginning. Researchers suspect that there are plenty of other trees out there with the same ability that we haven’t discovered yet.

“So far, numerous species of tree have been identified which can form calcium carbonate. But we believe there are many more. 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,” Rowley concludes.

The results were presented at the Goldschmidt Conference, a joint congress of the European Association of Geochemistry and the Geochemical Society. It takes place in Prague, Czech Republic, from July 6-11, 2025.