Not all trees are created equal. A forest full of young, small trees seems like it might be a great natural outdoor space, but compared to a grove full of big, ancient stock, it doesn’t stand up, at least on an ecological level. Big trees provide more space for sheltering animals as well as more food, for instance, and a large tree obviously holds onto more carbon than a smaller one.
But does the rate at which a tree sucks up carbon change as it gets older? Scientists had assumed that it did, that trees slowed down their growth as they aged. And they had some circumstantial evidence of this from comparisons of stands of trees in which all the trees in a forest were of similar age. Stands of small, young trees appeared to be more productive.
Big, old trees have moved back to the top of the heap, however, with new evidence that they grow much faster and, thus, sequester a lot more carbon than young trees. The study was published January 15 in Nature.
Nate Stephenson of the U.S. Geological Survey in Three Rivers, California, and an international group of colleagues estimated growth rates of 403 species of trees by comparing repeated measures of trunk diameter of 673,046 individual trees. The trees came from tropical, subtropical and temperate areas of the world and represented samples from every continent except Antarctica (which of course has no forests there to measure).
“For all continents, aboveground tree mass growth rates (and, hence, rates of carbon gain) for most species increased continuously with tree mass (size),” the researchers write.
The largest trees, those with a trunk diameter of a meter or more, tended to be the most heavyset, adding about 10 to 200 kilograms of mass each year. That’s about three times as much mass that a tree half that diameter accumulates in a year. Or, put another way, that's the equivalent of growing an entire tree of 10 to 20 centimeters in diameter from the ground up—something that takes years of growth.
This “is not a phenomenon limited to a few unusual species,” the team notes. “Rather, rapid growth in giant trees is a global norm, and can exceed 600 kg per year in the largest individuals.”
"In human terms, it is as if our growth just keeps 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 at retirement."
On a forest scale, it’s possible for densely packed stands of young trees to win out on total carbon stored, however, simply because they can pack in more trees in the same space, the researchers note.
But big, old trees are definitely important—the researchers explain that, in forests in the western United States, trees with trunks more than a meter wide comprise only about six percent of all trees but contribute a full third of forest growth.
What should worry us, though, is that these old timers are fast disappearing. “Large old trees are declining in forests at all latitudes,” David Lindenmayer of the Australian National University and colleagues noted in Science in December 2012. Grazing lands have edged out old trees in regions of Australia, Costa Rica, Spain and the United States, for instance, but even protected areas aren’t safe. In Yosemite National Park, the density of the largest trees, they note, declined by nearly a quarter between the 1930s and the 1990s.
Standing at the base of one of these old trees, it may appear to be a permanent fixture, but it’s vulnerable to a list of threats, Lindenmayer and his colleagues explain. Droughts, wildfires, air pollution, insect attacks and competition with invasive plants can all kill these trees, but the trees are also frequently intentionally removed to make way for agriculture, as part of efforts to manage fire risk and to turn the trees into valuable timber.
Planting new trees as replacement isn’t enough. Not only do these smaller individuals not have the same ecological and climate benefits, but few of them will live so long as to reach old age. With large, old trees having such a benefit disproportionate to their number, scientists say that more needs to be done to make sure they don’t disappear any more quickly than they already are.