Fig trees could turn CO₂ into stone, study finds

Fig tree in Africa. Credit: Mike Rowley.

Scientists have discovered that some fig trees in Kenya can absorb carbon dioxide (CO₂) from the air and turn it into stone-like material inside their trunks and the soil around them.

This unusual ability could offer a new way to help fight climate change by locking away carbon for long periods of time.

The research, led by a team from Kenya, Switzerland, Austria, and the United States, was presented at the Goldschmidt geochemistry conference in Prague.

They focused on three species of fig trees growing in Samburu County, Kenya.

These trees use a natural process called the oxalate-carbonate pathway, which is known to exist in some plants but has rarely been studied in fruit trees.

All trees absorb CO₂ through photosynthesis and store it as organic carbon in their leaves, trunks, and roots.

That’s why planting trees is seen as a way to reduce CO₂ levels. But fig trees go a step further. They also use CO₂ to make tiny crystals of calcium oxalate.

When the tree’s parts decay, certain bacteria and fungi break down these crystals and turn them into calcium carbonate—the same material found in chalk and limestone.

Unlike organic carbon, which can return to the atmosphere as trees decay, calcium carbonate is a stable form of inorganic carbon that stays locked in the soil for much longer. This makes it a powerful form of long-term carbon storage.

Dr. Mike Rowley, a senior lecturer at the University of Zurich, said this pathway has been known for some time, but its potential to fight climate change hasn’t been fully appreciated.

He believes that if we’re planting trees for food and reforestation, we should also consider their ability to store carbon in stone form.

The team used advanced imaging tools at Stanford University to study the trees in detail. They found that calcium carbonate was forming not just on the outside of the tree trunk but also deep inside the wood.

As this mineral formed, it made the surrounding soil more alkaline, which can improve nutrient availability for plants.

Among the three species studied, Ficus wakefieldii was the most effective at turning CO₂ into calcium carbonate.

The team now plans to study how much water this tree needs, how much fruit it produces, and how much CO₂ it can store under different conditions. This will help determine if it’s a good choice for agroforestry—growing trees that produce both food and environmental benefits.

While this research focused on trees in dry regions, Dr. Rowley says the process can also happen in wetter areas.

Since many plants create calcium oxalate, and the microbes that help complete the process are widespread, the potential for more trees to store CO₂ this way is likely much larger than we realize.