Older Trees Absorb More Carbon
Humans' growth spurts stop by late adolescence, but trees accelerate their growth and get bigger as they age. According to a global study by 38 international researchers published in Nature, these findings could have implications for how the world's forests are managed to contain the ill-effects of climate change.
Nate Stephenson, the study's lead author and a forest ecologist with the U.S. Geological Survey (USGS), said: "This finding contradicts the usual assumption that tree growth eventually declines as trees get older and bigger. It also means that big, old trees are better at absorbing carbon from the atmosphere than has been commonly assumed."
Stephenson added, "in human terms, it is as if our growth just keeps accelerating after adolescence, instead of slowing down. By that measure, humans could weigh half a ton by middle age, and well over a ton at retirement."
According to the USGS, a global team of researchers took measurements of more than 670,000 trees from more than 400 species across tropical, subtropical, and temperate regions in six continents. They used the same methods to calculate "the mass growth rates for each species" and then analyzed trends across all species. The study found that nearly 99 percent of trees accelerate growth rates as they age. "For most tree species, mass growth rate increases continuously with tree size — in some cases, large trees appear to be adding the carbon mass equivalent of an entire smaller tree each year."
With their miraculous, ever-increasing growth rate, older trees are then better at adding weight faster than younger ones. The Guardian writes that an older trees can add up to 600 kilograms of weight each year. These older, faster-growing trees, account for a "disproportionately important role in forest growth." For example, "trees of 100 centimeters in diameter in old-growth western U.S. forests comprised just 6 percent of trees, yet contributed 33 percent of the annual forest mass growth." This is because older trees continue to add girth as well as branches and leaves. More leaves mean more carbon absorption.
However, as USGS noted, it's important to not lose sight of the forest for the aged trees. The accelerated rate of carbon absorption by some older, larger trees doesn't mean "a net increase in carbon storage for an entire forest." As living systems, forests are constantly in a state of flux — and that's a good thing. A mix of older and younger trees in a forest may help limit the amount of carbon returning the atmosphere at any given time from the death of older trees.
Adrian Das, a USGS coauthor, said: "old trees, after all, can die and lose carbon back into the atmosphere as they decompose. But our findings do suggest that while they are alive, large old trees play a disproportionately important role within a forest's carbon dynamics. It is as if the star players on your favorite sports team were a bunch of 90-year-olds."
Stephenson told The Christian Science Monitor that these findings would ideally lead to more effort put into saving old growth forests. "Maybe if environmental changes are hitting your biggest trees the hardest, this is sort of an added impetus to go: 'Oh my gosh, we need to mitigate that.'" Doug Boucher, director of tropical forest and climate initiative at the Union of Concerned Scientists, added that "it reinforces the value of old-growth forests for the storage of carbon in the biosphere."
One key take-away for local officials, planners, and design professionals: Do as much as you can to keep those old trees in place. It will be much harder to accomplish the same positive climate impact with younger trees.
Image credit: Old Tree by Andrew Danielsen / Fine Arts America