These Fruit Trees Turn CO₂ Into Stone – And Might Just Save the Planet

In a surprising twist of nature, certain fig trees in Kenya are doing more than just bearing fruit—they’re capturing carbon dioxide from the air and turning it into stone.

Scientists discovered these trees can form limestone-like deposits in their trunks and surrounding soil, locking carbon away in a highly durable form. This little-known carbon pathway, previously studied mainly in non-fruit trees, could offer a powerful, natural tool in the fight against climate change, especially if scaled through agroforestry.

Stone-Making Fig Trees Discovered

New research reveals that certain fig trees in Kenya have a remarkable ability: they can absorb carbon dioxide (CO₂) from the air and lock it away by forming calcium carbonate—essentially creating stone inside their trunks and in the surrounding soil.

This research, led by a team of scientists from Kenya, the U.S., Austria, and Switzerland, is being presented at the Goldschmidt geochemistry conference in Prague.

The trees are among the first known fruit-bearing species to use a process called the oxalate-carbonate pathway. This means they don’t just store carbon the way most trees do—by growing wood and leaves—they also lock carbon into solid mineral form, deep in their trunks and in the soil around them.

Hidden Carbon Pathway Explained

All trees use photosynthesis to pull CO₂ from the air and turn it into organic matter like wood, leaves, and roots. That’s why forests are often seen as a natural climate solution.

But some trees go a step further. They use CO₂ to create tiny crystals called calcium oxalate. When parts of the tree die and decay, microbes and fungi convert these crystals into calcium carbonate. This mineral not only stores carbon for much longer than organic matter, but it also makes the soil around the tree more alkaline and nutrient-rich.

Dr. Mike Rowley, senior lecturer at the University of Zurich (UZH) is presenting the research at the Goldschmidt conference. He said: “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.”

Synchrotron Insights and Microbial Role

The team, from UZH, the Nairobi Technical University of Kenya, Sadhana Forest, Lawrence Berkeley National Laboratory, University of California, Davis, and the University of Neuchatel studied three species of fig tree grown in Samburu County, Kenya. They identified how far from the tree the calcium carbonate was being formed and identified the microbial communities involved in the process. Using synchrotron analysis at the Stanford Synchrotron Radiation Lightsource, they found that calcium carbonate was being formed both on the exterior of tree trunks and deeper within the wood.

Dr. Rowley explained: “As the calcium carbonate is formed, the soil around the tree becomes more alkaline. 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. It shows that inorganic carbon is being sequestered more deeply within the wood than we previously realised.”

Of the three types of fig tree studied, the scientists found that Ficus wakefieldii was the most effective at sequestering CO₂ as calcium carbonate. They are now planning to assess the tree’s suitability for agroforestry by quantifying its water requirements and fruit yields and by doing a more detailed analysis of how much CO₂ can be sequestered under different conditions.

Agroforestry Promise & Fig Performance

Most of the research into the oxalate-carbonate pathway has been in tropical habitats and focused on trees that do not produce food. The first tree to be identified as having an active oxalate-carbonate pathway was the Iroko (Milicia excelsa). It can sequester one ton of calcium carbonate in the soil over its lifetime.

Calcium oxalate is one of the most abundant biominerals, and the crystals are produced by many plants. The microorganisms that convert calcium oxalate to calcium carbonate are also widespread.

Unlocking a Global Tree Solution

“It’s easier to identify calcium carbonate in drier environments,” explained Dr. Rowley. “However, even in wetter environments, the carbon can still be sequestered. 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.”

Meeting: Goldschmidt 2025

The Goldschmidt Conference is the world’s foremost geochemistry conference. It is a joint congress of the European Association of Geochemistry and the Geochemical Society (US) and 4000 people are expected to attend. It takes place in Prague, Czech Republic, from July 6 to 11, 2025.

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