Like other animals and many living things, we humans grow when we're young and then stop growing once we mature. But trees, it turns out, are an exception to this general rule. In fact, scientists have discovered that trees grow faster the older they get.
Once trees reach a certain height, they do stop getting taller. So many foresters figured that tree growth — and girth — also slowed with age.
"What we found was the exact opposite," says Nate Stephenson, a forest ecologist with the U.S. Geological Survey, based in California's Sequoia and Kings Canyon national parks. "Tree growth rate increases continuously as trees get bigger and bigger," Stephenson says.
There have been hints before that mature trees grow faster than they age, but the idea had been controversial, he says. So he got together with 37 scientists from 16 nations to answer the question on a global scale.
They examined nearly 700,000 trees that have been the subject of long-term studies. Their conclusion, published in this week's issue of the journal Nature: While trees did stop getting taller, they continued to get wider — packing on more and more mass the older they got. And we're not talking about the tree-equivalent of an aging crowd with beer guts — old trees are more like active, healthy body-builders.
Because, in the world of trees, that means the oldest members of the forest are doing the most to pull carbon dioxide out of the air and to store it as carbon in their wood. Stephenson says that's another argument for preserving old-growth forests.
"Not only do they hold a lot of carbon, but they're adding carbon at a tremendous rate," Stephenson says. "And that's going to be really important to understand when we're trying to predict how the forests are going to change in the future — in the face of a changing climate or other environmental changes."