Carbon Storage and Forest Soils: Trees as Geochemical Agents
Until recently, scientists focused primarily on inorganic rather than biologic interactions with the Earth’s surface to explain how most atmospheric carbon dioxide gets captured. Only in the last several decades has science discovered that trees’ deep roots and their helpful mycelium have also played a significant role over time in locking up CO2 safely in the Earth’s crust. Roots physically break up rocks, invading small cracks and enlarging them with their growth as time goes by. These same roots attack the most commonly found rocks underground with their acidic and enzymatic juices, releasing certain nutrients—calcium, magnesium, and phosphorous. While the main portion of these elements feeds the tree, the remainder eventually drains from the soil and winds up in the ocean. Similarly, when a tree dies, its roots surrender the remaining calcium, magnesium, and phosphorus as ions in solution along with dissolved carbon.
Some of this feeds new plants; the rest again finds its way into rivers that flow into the sea. In both cases chemical reactions ensue between the dissolved calcium, magnesium, and carbon and nascent mollusks, corals, and phytoplankton in the ocean to help give them their protective exteriors. When these organisms die, much of the former carbon dioxide and calcium becomes limestone, removing a portion of this greenhouse gas from the atmosphere for millions of years. The excess phosphorus fertilizes the plankton to enhance their growth, which increases their intake of carbon dioxide through photosynthesis while magnesium turns into a mineral called magnesite, which also locks in carbon dioxide.
In fact, recent research has found that “roots act like a thermostat, drawing more carbon dioxide out of the atmosphere when it is warm and less when it is cool,” according to Dr. Christopher Doughty, lead author of a study published in Geophysical Research Letters. The importance of roots, long underrated, has become of great interest, requiring scientists to reassess their role in the global carbon cycle. As a consequence of this new interest, it has been discovered that on average, roots comprise about 28 percent of the biomass in a tree.
Dead roots also give up CO2 as bacteria, protozoa, nematodes, insects, and probably many other life-forms feed on them, as well as on the decomposing leaf fall. Since most of these activities happen underground, the majority of the carbon dioxide cannot return to the atmosphere. Instead, it turns into an acidic form that leaches rock and removes similar minerals as live roots do. Again, much of this leached material flows down waterways into the ocean, eventually ending up as limestone, adding to the long-term extraction of carbon dioxide from the air. As far back as the latter part of the eighteenth century, the French chemist Lavoisier noticed the great amount of carbon dioxide “which is neutralized by a particular earth called lime.”
While roots break down preexisting rock to “make” soil, the forest canopy protects it from the wind and sun and from the buffeting and erosive force of direct hits by rain. Fallen branches and leaves mix with the soil to act as a sponge that soaks in the carbonic acid—diluted carbon dioxide (commonly known as rainwater)—rather than allowing it to rush down in torrents to eventually acidify the oceans. Instead, the trapped water joins that carbon-sequestering slurry that finally ends up as limestone deep in the seabed.
Let us not forget, the vast beds of coal are in large part buried forests from an earlier day. Lignin—the material that makes trees rigid and woody—results in logs very resistant to decomposition. Their burial in swamps and in the ocean led to the permanent removal from the atmosphere of huge amounts of carbon dioxide and the addition of oxygen, a removal that we are reversing with our use of coal. All those nutrients that forests released to lakes and especially the sea fueled the growth of phytoplankton and thus the zooplankton that fed upon them, both of which, upon burial and compression and heating, became oil. Had they been left alone, we would not be facing the existential catastrophe presented by climate change that the maintenance of the undisturbed ecology of the forest can help alleviate. As world-renowned carbon dioxide experts concluded in their study of the geochemistry of trees, “plant evolution … has been a major factor in the atmosphere and of climate over geological time.” As a striking empirical proof of this assertion, the recent deforestation of the island of Borneo resulted in, according to NASA researchers, the greatest increase in atmospheric carbon dioxide emissions over the last two millennia, as a consequence of both burning the trees and the underlying peatlands they had formed that contain great amounts of stored carbon.
Over the long-term, conditions get even worse. With both the trees cut down and the understory cleared, the destroyed forest system no longer can perform the services required to maintain a temperate climate as they have for almost four hundred million years, beginning with the advent of Archaeopteris.
Conversely, large-scale reforestation has the opposite effect. The great dying of American Indigenous peoples in which 90 percent of the sixty million native Americans succumbed over the first two centuries after the start of the European intrusion of the Western Hemisphere, allowed forests to take over more than two hundred thousand square miles of land formerly farmed by native Americans. This also spared fifty-six million tons of wood per year from being cut for everyday fuel demands. As a consequence, scientists have found a detectable reduction of atmospheric CO2 and global surface air temperatures over the following two centuries.