Aging organisms tend to slow down as the years roll on, right?

Not so when it comes to trees, according to forest ecologists, who were surprised to discover that most species of trees actually undergo accelerated growth as they get older.

Scientists measure the girth of a tree as part of a research project tracking the growth rates of older trees versus younger trees. ( Andrew Larson/University of Montana)

The findings, which contradict long-held beliefs about large trees being relatively unproductive as far as pulling carbon dioxide from the air, are published in a letter in the science journal Nature.

An international research group led by Nate L. Stephenson of the U.S. Geological Survey Western Ecological Research Center analyzed 403 tropical and temperate species. The scientists found that 97 per cent of the species monitored were growing at increasingly faster rates as time progressed.

Compared to smaller trees, the older ones also sucked in and converted more carbon dioxide from the atmosphere to create wood and bulk up.

"Large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees," the paper states.

"At the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree."

Initial confusion

That might seem counterintuitive, given that humans go through growth spurts that stop once we reach a point of maturity.

"We're sort of primed to think that's how living things grow," Stephenson told CBC News.

By that measure, humans could weigh half a ton by middle age, and well over a ton at retirement - Nate L. Stephenson

The findings initially confused his team, too. It had long been presumed that younger trees were the more robust growers and absorbed carbon dioxide faster.

The issue is one of perception and scale.

"A big tree doesn't look like it's growing very fast because it's already so huge," Stephenson said.

"If you were able to take one year's worth of growth off of a really big tree, just peel off that outer ring and add it all up, that ring may be very narrow, but the tree is so huge that the surface area is enormous."

A sapling over the years might grow to a towering height, but at a certain point, it will reach a peak and continue expanding in diameter at the trunk.

"They eventually quit getting taller, but they're getting wider and they might be adding more bulk in their branches and adding more leaves," Stephenson said.

'Global norm'

The researchers found that this was not just typical in species like eucalyptus trees, also known as Australian mountain ash, as well as coast redwoods.

“Rather, rapid growth in giant trees is the global norm and can exceed 600 kilograms (1,300 pounds) per year in the largest individuals,” the researchers wrote.

A tree weighing that much might have a trunk that's about 25 centimetres in diameter, or the circumference of a dinner plate, Stephenson said.

Translated into human terms, it's as if growth keeps accelerating beyond adolescence.

“By that measure, humans could weigh half a ton by middle age, and well over a ton at retirement,” Stephenson said.

The study, which was a collaboration of 38 scientists around the world, was based on measurements of 673,046 individual trees, and used data collected over the last 80-plus years.

The researchers hope the revelation could help improve future forest management practices.