Environment
An in-depth look at how trees work to keep us alive and the value of old-growth forests
Excerpted from A Forest Journey: The Role of Trees in the Fate of Civilization ©2022 by John Perlin. Reprinted with permission by Patagonia.
An article in Science magazine that appeared in 1990, the same year as the spotted owl controversy proved the timber industry’s denigration of old-growth to be wrong, the authors first addressed the contention of the industry and their academic apologists’ derogation of old-growth: “It has been suggested the CO2 content of the atmosphere could be reduced if slowly growing ‘decadent’ old-growth forests were converted to faster growing, younger, intensely managed forests.” The paper disagreed, arguing that the critical factor is “the amount of CO2 stored in the forest, not the rate of C uptake” and that “logging dramatically takes down the carbon [dioxide] sequestration capability.” As a consequence, “the conversion of old-growth forests to younger forests has been a source of increasing CO2 over the last century.” Researchers found that “rotations of tree crops” over a fifty-, seventy-five-, or one-hundred-year period would store only 38 percent, 44 percent, and 51 percent, respectively, of the carbon that an old-growth stand would retain. In fact, according to this breakthrough study, the harvesting of old-growth is even more damaging to CO2 storage than if a fire had swept through the same forest.
Twenty-four years later, scientists learned that old-growth trees not only act as carbon dioxide reservoirs but “actively fix larger amounts of carbon compared to smaller trees” due to the fact that the amount of leaves (including needles)—the epicenter of photosynthesis—increases as the square of trunk diameter. In other words, if the trunk’s diameter increases tenfold, it will undergo a one-hundred-fold increase in total leaf mass, enhancing the tree’s ability to remove carbon dioxide from the atmosphere and add oxygen to the air.
Studies concerning old-growth have aged well. A major paper, appearing also in Science magazine, published on March 21, 2022, shows that monoculture plantations, that is, growing trees like crops, do not perform very well when it comes to carbon storage and soil erosion control, as compared to old-growth. The study also reveals just how sterile the surrounding environment becomes in plantations that have replaced natural forests, hosting far less animals and plants than did the formerly untouched tree cover.
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.
Forests do more to keep the Earth temperate than remove carbon dioxide from the atmosphere. Like fans, treetops cause the air surrounding them to circulate to moderate temperatures in their vicinity. The leaves of trees also emit great amounts of water into the atmosphere, which, in the process of changing into vapor, absorbs heat, cooling the surrounding air in the same fashion as perspiration keeps our bodies from overheating, and cools the surrounding land. As an example, without the forest canopy, seasonable temperatures in the Amazon basin could get up to 41 degrees warmer. Leaves, especially those of conifers, also release packets of particles called aerosols into the air, reflect sunlight back into space, and help form clouds that block incoming sunlight from hitting the Earth and converting to heat. They seed clouds, which reflect a goodly portion of the incoming sunlight back into the sky, preventing the sun’s rays from striking the Earth, which would then convert into heat. Climate-change scientist Gordon McFiggans calls the phenomena “One of the main climate feedback cycles.” Or as BBC environment correspondent Matt McGrath quips, the “smell of forest pine can limit climate change.”
Old-growth forests also rate as the best cover for watersheds. A report titled “Running Pure” by the World Wildlife Fund, a study of the world’s top 105 cities (twenty-five each from Africa, the Americas, Asia, and Europe, and five from Australia), demonstrated the global importance of forests to urban water supplies. Rainfall and runoff data obtained in the study from the Melbourne area, for example, showed that water yield correlates directly with the age of a forest. Deforestation can cause the mean annual runoff in a watershed to decline up to 50 percent. It takes up to 150 years of regeneration to reach optimum water yield. Mature forests excel at regulating the flow of water throughout the year, at preventing sedimentation, and at filtering out impurities. Researchers have developed ways to value a forest’s worth in protecting the supply of water throughout the world, calculating its value at $2.3 trillion, or nearly six times the worth of today’s timber trade.
Corporations and municipalities have begun to recognize the economic value of sustaining forests for keeping water supplies clean and available. In one example, heavily farmed watersheds threatened the integrity of the aquifers that supplied the bottlers of beverage brand Perrier. The company found that reforesting was far cheaper than building filtration plants. Likewise, the City of New York found it far less expensive to buy up conservation easements along its watersheds, guaranteeing the presence of forests to purify the water to the same quality as would proposed new facilities costing $6 to $8 billion with annual operating costs ranging between $300 and $500 million.
Over the last hundred years, the City of Seattle has spent large sums buying land piece by piece to protect the integrity of the Cedar River, its water source. The city found that the quality of water had suffered on lands already clear-cut and where logging was going on. In 1998, Paul Schell ran for mayor on a platform advocating modest rate increases to end logging and implement Seattle’s Habitat Conservation Plan for the ninety thousand-acre watershed. Schell was elected. The city no longer permits logging and has also implemented its pioneering plan to ensure the survival of endangered species in the watershed. Having a plan that various regulators have approved for the next fifty years gives the city the certainty that it will not lose the right to manage its water operations over a reasonable period. As a consequence, Mami Hara, the general manager and CEO of Seattle’s public utilities, could report that the city’s drinking water “is among the best in the nation, both in purity and taste.” Other major urban areas in America focusing on protecting their water supplies by maintaining the forest health of their watersheds include Boston, as well as West Coast cities of Portland, San Francisco, and Tacoma. Leaving watersheds naturally forested often becomes a compelling, cost-effective approach.
But do forests actually generate rainfall? For the longest time, most believed evaporation of large areas of water, primarily oceans, produced the world’s rain. But in 1977, geologist Dr. Irving Friedman observed that the Amazon receives vast quantities of water “derived from recycled transpired moisture” primarily produced by the jungle’s canopy. Two years later Brazilian meteorologist Dr. Eneas Salati quantified Friedman’s account that proved that half of the rainfall in the Amazon comes from the forest. An article, whose title says it all —“Angiosperms Helped Put the Rain in the Rainforest”—demonstrates that the loss of tree cover in the Amazon would result in the diminution of more than five feet of rainfall per year and an extension of the current dry season by eighty days in the Amazon basin!
Recent research not only confirms Friedman and Salati’s work but builds on it, conclusively demonstrating that intact forests also supply rain to areas distant from their locations. Deeply rooted trees, like powerful pumps, pull up water from the ground into their leaves to photosynthesize and release excess water as vapor into the air. The resulting vapor travels as clouds, which then condense in some far-away area to become rain. For example, rain originating in the West African rain forest—primarily the Congo basin—provides the Nile River with about 40 percent of its water supply. The main source of rainfall for southeastern South America comes from the Amazon. Sixty-seven million people living in southeastern South America, including urban centers such as São Paulo, Montevideo, and Buenos Aires, rely on evaporation by the Amazonian canopy for 70 percent of their rainfall. The majority of rainfall in China originates from moisture generated by the Atlantic Ocean and relayed by the boreal forests of Scandinavia and Russia. The physical principles underlying these new discoveries have led many climatologists, foresters, and hydrologists to further speculate that forests may take on the role of being a major driver of atmospheric circulation of our planet, challenging long-held concepts of climate science. It takes little imagination to consider the catastrophe that would occur should these agents of water transfer, and possibly air circulation, disappear. If not for the forest, interiors five hundred miles or more away from large bodies of water would likely become deserts!
“Trees absorb carbon dioxide. They give us oxygen. They help to make rain. So they are a gift,” concluded renowned environmentalist Jane Goodall in her campaign to plant a trillion trees by 2030
Are you passionate about Canadian geography?
You can support Canadian Geographic in 3 ways:
Environment
Conservation photographer TJ Watt advocates for the protection of old-growth ecosystems by documenting the loss of giant trees
Travel
An insider’s account of the modern-day gold rush
Environment
Timber poaching has become “a problem in every national forest,” with an estimated $1 billion worth of wood poached yearly in North America
Environment
How a niche British Columbia-based community is working to bring attention to the importance of old-growth forests