A Tree Made of Stone: Milicia excelsa

If you’ve heard of the iroko (Milicia excelsa) tree, it’s most likely because of the attractive timber it produces. It’s sometimes called African teak because of its similarity in strength and appearance to teak wood, making it highly desirable in woodworking.

But iroko, widespread in tropical Africa, has a more profound function: it can turn sunlight into stone.

The process begins with photosynthesis and the resultant absorption of carbon dioxide from the air. In iroko trees, like other plants, this carbon dioxide is combined with water and light to make carbohydrates that provide them with food. When most plants die, the carbon is eventually released as they decompose. But iroko is more complex.

Oxalate-Carbonate Pathway

The tree uses something called the “oxalate-carbonate pathway.” In basic terms, oxalate is an organic substance1 partly made of carbon and is found in many plants. Carbonate is a kind of salt2.

Oxalate is a component of iroko’s wood and bark. The tree’s litter—or dead plant material that falls to the ground—creates an abundance of oxalate on the soil. When detritus feeders like fungi, termites, and bacteria consume the litter, they produce calcium oxalate (or oxalate crystals) as a waste product by combining oxalate with calcium already in the soil.

Then, oxalotrophic (oxalate-consuming) bacteria feed on the calcium oxalate, and in the process they oxidize it. This means they combine it with oxygen, and the resulting products are carbonate and calcium ions in the soil.

Finally, the substances return to the iroko tree when its roots draw up moisture from the soil. Combined, they are precipitated (made solid) into the mineral calcite, a form of calcium carbonate. The stone-hard calcite becomes embedded in the tree’s tissue and the surrounding soil.

This is an example of biomineralization, the process through which living organisms create minerals. Iroko is not the only plant capable of this feat, but it is relatively uncommon, particularly the impressive volume of mineralization that iroko can produce. It’s also a rare case of this process taking place in the acidic soil of its tropical habitat.

The Importance of Biomineralization

From an evolutionary perspective, it makes sense why the oxalate-carbonate pathway could have been selected for. It make trees less attractive to termites, and to grazing by other animals, because of its impenetrable mineral deposits.

From an environmental perspective, iroko represents an important carbon sink—that is, a reservoir to remove carbon dioxide from the atmosphere indefinitely. Carbon in the form of calcite endures for up to a million years. This is significant in the context of climate change and the increased atmospheric concentration of carbon dioxide due to human activity.

While Milicia excelsa is certainly not the only solution to these issues, its deforestation and threatened status raise environmental concern. Biomineralization increases the importance of its conservation, along with other plants with this capability.

Solutions

One proposed answer to its decline is agroforestry—the use of trees in farmland. Iroko has a positive effect on soil fertility and interacts advantageously with crops, as is often the case when biodiversity in agriculture increases. Its benefits can also be as simple as preventing soil erosion, providing shade to livestock, or producing timber as long as it is sustainably managed.

Studies have made iroko’s importance to the environment very clear. It is up to us to be proactive in its conservation, if not for the pragmatism of growing this species, then for its particular majesty, that slow-growing beauty which stands as firm as stone.

Milicia excelsa seeds are available for purchase from The Seedy Business.

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1More precisely, oxalate is an organic acid that can bind to minerals like calcium.

2Carbonate is a salt of carbonic acid, which is known for being a component of carbonated water.

 

References

Braissant O, Cailleau G, Aragno M, Verrecchia EP (2004) Biologically induced mineralization in the tree Milicia excelsa (Moraceae): its causes and consequences to the environment. Geobiology 2:59–66.

Cailleau G, Braissant O, Verrecchia E (2011) Turning sunlight into stone: the oxalate–carbonate pathway in a tropical tree ecosystem. Biogeosciences 8:1755–1767

BotanyWah Mohn