Old, big trees like the giant redwood sequester carbon faster than young trees
The giant coastal redwoods of northern California are breathtaking—literally. They are the tallest trees on Earth, growing to more than 350 ft. (106 m), with trunks that can be more than 25 ft. (7.6 m) across. The oldest redwoods date back to the time of the Roman Empire, though few of that age still remain, since more than 95% of the original old-growth forest has been lost, mostly for lumber. And the trees are unparalleled living carbon banks—a large redwood can sequester a ton of carbon from the air in its trunk and roots.
Despite the redwoods’ beauty, though, scientists have long assumed that very old trees like them absorb less and less carbon as they age, slowing down like the rest of us as we get older. That idea has important implications for global warming: climate scientists assume that younger trees will take up carbon more rapidly than their older counterparts, which means youth is valued when it comes to using trees as carbon stores.
But according to a new study published in Nature, it turns out that the oldest trees are actually still growing rapidly, and storing increasing amounts of carbon as they age. An international research group led by Nate Stephenson of the U.S. Geological Survey Western Ecological Research Center reviewed records from forest studies on six continents, involving 673,046 individual trees and more than 400 species, going back as far as 80 years ago. For 97% of the species surveyed, the mass growth rate—literally, the amount of tree in the tree—kept increasing even as the individual tree got older and taller. Even though trees tended to lose leaf density as they aged—which, as a victim of male pattern baldness, I can sympathize with—the total amount of leaf cover kept increasing as the tree itself got bigger and older. In other words, the number of leaves per cubic foot fell off but the leafy surface area grew and grew. That enabled the tree to keep absorbing an increasing amount of carbon as it aged.
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For some species of trees, that increase could be enormous. A single big tree could sequester the same amount of new carbon in a year as might be contained in an entire mid-sized tree. For sports fans, it would be as if Jamie Moyer, a baseball pitcher who was a record 49 years old when he recorded his last win, could best Jose Fernandez, a Miami Marlins pitcher who won the National League Rookie of the Year in 2013 at the age of 21.
Of course, human beings don’t age like trees—except maybe for Peyton Manning—and that’s probably a good thing. “In human terms, it’s as if our growth just kept accelerating after adolescence, instead of slowing down,” Stephenson said in a statement. “By that measure, humans could weigh half a ton by middle age, and well over a ton by retirement.”
Still, on a forest by forest as opposed to tree by tree basis, youth does beat age, with younger stands of trees sequestering more carbon overall than ones near retirement age. That’s because as trees in an area of forest age, some of them will die, leaving older and bigger trees but fewer of them, sort of like the way a high school class will begin to thin out as the reunions pile up over the years. But on a tree by tree basis, elderly trees are carbon vacuums.That’s one more reason to appreciate—and conserve—these ancient, majestic forests.