Trees That Turn CO2 into Limestone

Certain Species Could Become Warriors in the Climate Change Fight

By Mal Cole*

An African fig tree (Ficus wakefieldii). It forms bits of calcium carbonate (limestone) in its woody tissues. Note the man at the right side of the trunk for size comparison. Sadhana Forest/Courtesy of Mike Rowley

In the struggle against climate change, it’s easy to wish for some technology—or even magic!—that could remove Earth-warming carbon dioxide (CO₂) from the atmosphere and turn it into something useful. But this mechanism already exists and is much older than the climate changes of the past two centuries.

Trees and other plants act as carbon sponges, taking carbon dioxide from the air and, through photosynthesis, storing it in their tissues as carbohydrates (cellulose, sugars, and starches). The forests around the world are a major “sink” for carbon, and even the oceans play a role in capturing CO₂. In fact, the oceans, with their vast numbers of phytoplankton and deep waters, are a gigantic carbon sink, absorbing 31% of CO₂ emitted into the air. However, all of this carbon can rather quickly find its way back into the atmosphere when plants die and decay or are harvested and consumed.

These fascinating trees, many of which exist in threatened ecosystems, are able to create a long-term carbon sink by a remarkable biochemical process, the oxalate–carbonate pathway.

But some special trees—African fig trees and iroko hardwoods among many others—have been found to be able to ingest atmospheric CO₂ and turn it into durable, alkalizing limestone, “fixing” the carbon for much longer than in plant tissue. These fascinating trees, many of which exist in threatened ecosystems, are able to create a long-term carbon sink by a remarkable biochemical process, the oxalate–carbonate pathway (OCP). Scientists are currently working to understand more about the OCP’s untapped potential in fighting climate change, and a recent study may reveal more about this underexplored mechanism.

Trees with Stones in Their Wood

A group of iroko trees growing in Uganda. Fenrith/Wikimedia Commons

In the 1930s, a group of scientists investigated a puzzling phenomenon involving the iroko tree, a species in the fig and mulberry family (Moraceae). Lumberjacks in the African forests where the trees grow had noticed small stones within the tissues of the trees that dulled their saws.  Also, the soil beneath the trees contained bits of limestone. This was strange because the soils in that part of Africa were known to be acidic—limestone and other forms of alkaline calcium carbonate (CaCO₃) were not expected to be present. It seemed like the limestone was coming from the iroko trees—but how was that possible?

The iroko trees were sinking carbon in a way that had never been studied before. They were drawing CO₂ from the air to fuel photosynthesis and then producing oxalate from the by-products of the process. The oxalate was then transformed into limestone by symbiotic bacteria and fungi and stored in the woody portions of the tree and in the surrounding soil. This process is the oxalate–carbonate pathway.

Scientific interest in this fascinating biogeochemical process has increased within the last 25 years, and as the implications of climate change become more pronounced, scientists are investigating the OCP as an untapped source of carbon sequestration. The limestone that the iroko trees deposit in the soil is a much more stable way to store carbon and cannot be as easily re-released into the atmosphere as would be the case with decaying wood and leaves. One iroko tree is estimated to be able to store 1,160 kilograms (2,552 lbs) of carbon as calcium carbonate over its lifetime, assuming storage of 5.8 kg/year and an average lifetime of 200 years.

The OCP in India

It’s unknown exactly how many plants use the OCP, but the number could be substantial, as about 80% of plants produce the oxalate that facilitates the process. But several more species have been identified since the process was discovered, especially in the past 25 years.

In a scientific paper currently under review, Dr. Mike Rowley and his colleagues at the University of Zurich identified several OCP trees in the tropical dry evergreen forests (TDEF) of Tamil Nadu, India. This study, supported by the Agassiz Foundation at the University of Lausanne, Switzerland, and the Sadhana Forest organization in India, was the first of its kind in the TDEF. Rowley, Camille Rieder, and their colleagues tested dozens of trees before selecting their subjects for the study. The initial selection process involved field-testing trees for surface evidence of calcium carbonate.

“We were looking for calcium carbonate precipitation on the bark,” said Rowley in an interview with The Earth & I. “When we apply a weak acid to the tree on these precipitates, it releases the CO₂ that’s trapped within the calcium carbonate.” When the scientists applied a dilute solution of hydrochloric acid to the bark of a selected tree, Rowley said, they could observe a bubbling chemical reaction—an effervescence indicating they were on the right track for finding the OCP in that species.

Once the trees with the precipitate were selected for the study, samples were taken for more testing. Biomineral deposits within the tissues were identified using electron microscopes. The team was looking for calcium oxalate crystals that would help them find more evidence of the OCP.

Other samples were sent to the University of Neuchatel to identify oxalate-consuming microorganisms—the bacteria and fungi that facilitate the OCP process. The team also determined that the effect of the trees on their local soil chemistry demonstrated there was an alkalizing effect, particularly in the trunks of the trees. Evidence of the OCP was present in all of the sample species Rowley and his team studied.

An Ecosystem at Risk  

The presence of the OCP in the Tamil Nadu forest shows the process’s potential as a long-term carbon sink and could help preserve the highly threatened ecosystem. The TDEF is the rarest type of forest ecosystem on the Indian subcontinent.

Many of the tree species identified as having an active OCP have potential uses besides carbon sequestration, Rowley told The Earth & I. Agroforestry may be one way to maximize the benefits. Many trees in the fig and mulberry family have been shown to exhibit the pathway, and more species of fruit-producing trees could be found. Abandoned agricultural sites with exhausted, acidic soils could potentially host this research and benefit from the alkalizing effects of OCP trees and the associated beneficial microorganisms.

Rowley is optimistic about the OCP’s potential for agroforestry projects and would like to study how much calcium carbonate an OCP tree can move into soils.

Rowley is optimistic about the OCP’s potential for agroforestry projects and would like to study how much calcium carbonate an OCP tree can move into soils, and the effects of that process.

“I’d love to have a long-term experiment to really look at the change in soils,” he said. “Models indicate that it could take up to 20 years before carbon starts to precipitate as calcium carbonate.”

If, as Rowley and others suspect, it takes two decades for OCP trees to start fully precipitating carbon in the form of CaCO₃, it’s urgent to get more experiments underway to study the effect it might have on climate change.

Despite the obvious potential of OCP trees, more research is needed to fully understand this fascinating process. While conducting the TDEF study, Rowley’s team observed calcium carbonate deposits deeper than had been seen before in the trunk tissues of OCP trees in Kenya.

“We were mapping what types of calcium we had and where,” said Rowley. “What was surprising to us is that the calcium carbonate wasn’t just at the surface of the tree or in the cracks of the bark, but had also penetrated more deeply into the wood.”

Rowley can only speculate as to why this might happen. But this mechanism could be another key to how the OCP performs a biogeochemical process that could add just the touch of “magic” needed to tip the scale against climate change.  

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*Mal Cole is a freelance science and nature writer based in Massachusetts.