Stone-making fig trees offer surprising climate solution
New research shows that wild fig species in Kenya turn partially to stone, locking away carbon effectively forever and boosting soil health, making them prime candidates for agroforestry and reforestation initiatives.
The findings will be presented by Mike Rowley of the University of Zurich at the Goldschmidt geochemistry conference in Prague next week.
All trees turn carbon dioxide in the air into organic carbon compounds in their tissues. While this removes carbon from the atmosphere, thereby helping to mitigate global warming, the carbon is eventually released when the trees die and rot.
The fig trees that Rowley’s team studied do this, but they do something else too. They convert carbon dioxide into crystals of calcium oxalate that form on the surfaces and inside their trunks, leaves, roots and branches. Bacteria and fungi that live on the trees convert the calcium oxalate into calcium carbonate — the same mineral as limestone.
Whereas most tree-stored carbon returns to the atmosphere within decades, the calcium carbonate can persist in soils for tens or even hundreds of thousands of years.
“Our fig trees in Samburu County, Kenya have precipitated a significant amount of calcium carbonate in the aboveground biomass, where the trees were coated in it, but also in the soils surrounding them,” says Rowley.
The researchers don’t yet know exactly how much carbon the fig trees are locking away through this process, which is technically termed an oxalate carbonate pathway — or OCP. Related species like the iroko tree (Milicia excelsa) have been shown to lock away up to one tonne of carbon as calcium carbonate during their lifetimes, though these trees are taller than the Kenyan figs.
Rowley told me that including the fig trees in agroforestry initiatives could be an effective strategy for increasing carbon sequestration at the same time as producing fruit and improving conditions for crops growing beneath the trees. That’s because as well as locking away carbon, the calcium carbonate increases soil alkalinity (pH) and fertility.
“Increasing the pH of the soil could have beneficial effects on the yields of certain intercropped species within an agroforestry system,” says Rowley.
The fig species that Rowley’s team studied were Ficus natalensis, Ficus glumosa and Ficus wakefieldii, the latter of which was the most effective at storing carbon as calcium carbonate. It occurs in the Democratic Republic of the Congo, Kenya, Tanzania, Uganda and Zambia and its figs are edible to humans.
“Calcium oxalate deposits or calcium carbonate deposits are reportedly quite common in figs,” Rowley told me. “I’m sure there are many more fig species with active OCPs that are waiting to be studied and identified worldwide.”
If confirmed across other fig species and regions, this hidden talent could reshape how scientists and land managers assess the climate value of trees.
This is a beautiful bonus to the already exceptional bounty of this type of tree. I hope to read more about this from you as more studies emerge. It could be revolutionary to storing carbon long term, which we are in need of right now! Thanks for your contribution on substack!
Very cool! And this tree can grow across most of tropical Africa, from West Africa to Zambia. Multiple wins: food, enhanced carbon storage, soil improvement, habitat etc.