Older Trees Grow Faster

A study published today in the journal Nature notes that trees not only don't slow down as they age, like their human counterparts do, but actually accelerate in their growth.

"This finding contradicts the usual assumption that tree growth eventually declines as trees get older and bigger," said Nate Stephenson, the study's lead author and a forest ecologist with the United States Geologic Survey's Western Ecological Research Center. "It also means that big, old trees are better at absorbing carbon from the atmosphere than has been commonly assumed."

An international group of researchers collected growth measurements of 673,046 trees belonging to 403 tree species from tropical, subtropical and temperate regions across six continents.

The researchers calculated the mass growth rates for each species and then analyzed and compared them, looking for growth trends among the tree species.

The results revealed mass growth rate increases continuously with tree size in most species and, in some cases, larger trees produce the carbon mass equivalent of an entire smaller tree.

Assessing the tree growth in human terms, "it is as if our growth just keeps accelerating after adolescence, instead of slowing down," explained Stephenson. "By that measure, humans could weigh half a ton by middle age, and well over a ton at retirement."

The continuous and increasing growth rate means that large, old trees are better at absorbing carbon from the atmosphere. But, researchers also say, the rapid absorption rate of individual trees does not necessarily translate into a net increase in carbon storage for an entire forest.

"Old trees, after all, can die and lose carbon back into the atmosphere as they decompose," added Adrian Das, co-author of the USGS study. "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."

The study was a collaboration of 38 researchers from research universities, government agencies and non-governmental organizations from the United States, Panama, Australia, United Kingdom, Germany, Colombia, Argentina, Thailand, Cameroon, Democratic Republic of Congo, France, China, Taiwan, Malaysia, New Zealand and Spain.