This excerpt is from A Forest Journey: The Role of Trees in the Fate of Civilization.
ASTRONOMERS, FOR THE LONGEST TIME, regarded Venus as the planet most resembling Earth. Venus, being nearly the size of Earth and almost as close to the sun, has even led many to call it Earth’s twin. The clouds that always covered the Venusian landscape gave another compelling reason to believe in Venus’s affinity to Earth. Pioneering astronomer Svante Arrhenius, in the early part of the twentieth century, hypothesized that rains pouring from these clouds nurtured lush vegetation below. But, when various space probes penetrated the Venusian atmosphere, they promptly dispelled this belief. Astronomers found an inferno rather than a tropical paradise. Here they discovered the ultimate greenhouse effect. Although the carbon dioxide–laden atmosphere of Venus is transparent enough to allow the shorter wavelengths of the incoming sunlight, when it hits the interior surfaces, it changes into heat, whose wavelength is too long to escape through the clouds of carbon dioxide. In this way heat accumulates at the surface of Venus, causing temperatures to reach almost nine hundred degrees Fahrenheit.
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Interestingly, Earth has as much carbon dioxide as Venus. But unlike the carbon dioxide blanketing the sky, as happens on Venus, much of Earth’s carbon dioxide has been captured in inorganic rock and in the organic sedimentary rock called coal. The sequestration of carbon dioxide on Earth accelerated in late Mid-Devonian times—385 million years ago—with help from the emergence of Archaeopteris, the world’s earliest modern tree. Archaeopteris had all the anatomical features of today’s trees—a woody trunk protected by an exterior of bark, with deep and extensive roots, as well as branches from which grew leaves. Its roots had a maximum spread of thirty-six feet from the base of the tree, with the largest roots having a circumference of nineteen inches.13 Such a robust root system had no difficulty in breaking formerly impenetrable rock into soil, allowing Archaeopteris to spread beyond swampy environments— where most tree-like plants had been confined—to cover a larger portion of the terrestrial world. Roots also provided the anchorage that allowed the trunk to grow up to ninety feet above ground, dominating the surrounding flora. Archaeopteris’ height, combined with a wide swath of branches, maximized the leaves’ exposure to sunlight-optimizing photosynthesis while the tree’s roots mined essential nutrients that photosynthesis could not provide.
According to Dr. Christopher Berry, a paleobotanist at Cardiff University in the United Kingdom, “Roots maximize [a tree’s] physiological capacity. An efficient rooting system is key to being a successful tree.”
The robust roots of Archaeopteris also took up huge amounts of water, of which the tree could use only 2 or 3 percent. But none was wasted. The remainder escaped through the pores in the leaves as water vapor, which kept the foliage exposed to the sun from overheating, sending large quantities of aqueous vapor into the air, which irrigated distant landscapes in the form of rainfall to perpetrate even greater vegetal florescence. The flat leaves of Archaeopteris protected the carbon-rich soil below by shading the earth from the beating sun, breaking the force of the winds that would otherwise have blown the soil away, and by shielding the dirt from the direct hits of raindrops. In these ways, the overstory protected the soil from the ravages of the elements. Shade provided by its foliage also greatly reduced the daily fluctuations of temperature at the soil surface, as well as evaporation from the soil. In short, the spread of trees and forests established more favorable microclimates that allowed the spread of other plants, invertebrates, and ultimately vertebrates on land.
As ARCHAEOPTERIS RECEIVED ONLY A PORTION of the minerals mined by its roots, much of the remaining minerals mixed with carbonic acid, primarily formed by rainwater that had infiltrated the carbon-laden soil created in an understory of fallen leaves and branches. This slurry flowed into nearby rivulets and streams that eventually reached the ocean where, in time, it became limestone and magnesite, which locked large quantities of carbon dioxide, sequestering it for millions of years. Phosphorus released in the same fashion poured into the oceans, too, fertilizing plankton blooms that absorbed additional carbon dioxide and released more oxygen into the air. Roots additionally stored large amounts of carbon as did the trunk and branches. Much of this carbon would have returned to the sky once the tree died had not Archaeopteris’ wood contained microbially resistant material called lignin that helped prevent decay. Thanks to the lignin, the dead roots, branches, and trunks were eventually buried deep in the bowels of Earth, keeping the carbon they contained from reentering the air. And, of course, when the trees were alive, their leaves absorbed carbon dioxide from the atmosphere and released oxygen to the sky. In these ways the rise of Archaeopteris and its successors have helped to significantly remove carbon dioxide from the atmosphere while increasing the amount of oxygen, causing temperatures on land to fall to temperate levels. As Dr. Stephen Scheckler, one of the world’s experts on Devonian plants, affirmed, “The Earth’s atmosphere was changing rapidly, going from perhaps 10 percent to 1 percent CO₂ and from about 5 percent to 20 percent oxygen over a 50-million year period in the [late] Devonian period. All plants were responsible for the transformation, but Archaeopteris was important because it made up 90 percent of the [Earth’s] forests during the last 15 million years when these changes accelerated.”
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Logs, large branches, and other organic matter, a goodly portion coming from Archaeopteris forests, flowed seaward and eventually mixed with proliferating plankton blooms, and clogged the shallow Devonian seas. Fishlike creatures with newly developed limbs could propel themselves through the organic obstructions better than their counterparts that depended on fins alone for locomotion.19 Some of these limbed animals also had lungs and could gulp air. When shallow waters experienced decreasing amounts of oxygen due to the added organic matter, creatures capable of breathing, as well as walking, escaped death by making their ascent to land.
Largely due to changes brought about by Archaeopteris along with other large plants, a mild climate, sufficient oxygen, and a plentitude of smaller vegetation, which harbored small herbivorous insects and spiders that served as food, now welcomed these oceanic refugees. These first four-limbed animals, called tetrapods, also found additional advantage in not having to compete in waters full of voracious, carnivorous fish. In time—measured in millions of years—the cooling and oxygenated landscape enabled relatively large creatures to survive and flourish. At the same time, the enlarging ozone layer above Earth, caused by the increased supply of oxygen provided by photosynthesis and the coincident removal of carbon dioxide, shielded the new terrestrial arrivals from lethal doses of ultraviolet radiation. And so it was that Archaeopteris and its successors contributed to the chain of events that has permitted animals of significant size—including you and me—to flourish on land.
This excerpt is from A Forest Journey: The Role of Trees in the Fate of Civilization ©2022 by John Perlin. Reprinted with permission by Patagonia.