Despite their greater age, big, old trees do more than small, young ones to remove carbon dioxide from the atmosphere, said a study Wednesday that explores how well forests slow global warming.

The findings overturn the conventional view that old, large trees are relatively unproductive in absorbing heat-trapping greenhouse gases.

In more than 400 tree species studied, it was the bigger and older specimens that grew fastest and trapped most carbon, the scientists found.

"In human terms, it is as if our growth just keeps accelerating after adolescence, instead of slowing down," Nathan Stephenson of the US Geological Survey told AFP of the findings published in the journal Nature.

"By that measure, humans could weigh half a ton by middle age and well over a ton at retirement."

Trees absorb carbon dioxide (CO2) from the atmosphere and store it as carbon in their trunks, branches and leaves, thus slowing the rate of global warming.

This makes forests a so-called carbon sink, although their contribution is a hotly-debated issue in the climate-change saga.

"We already knew that old forests store more carbon than young forests. But old forests contain trees of all sizes and it was not clearly understood which trees grew the fastest, removing the most carbon dioxide from the atmosphere," said Stephenson.

The conclusion, he said, was clear.

"For reducing carbon dioxide in the atmosphere, more big trees are better!"

He and other researchers studied 403 tree species, using data taken from 673,046 individual trees on six continents in records dating as long as 80 years ago.

The terms "big" and "small" were relative to the tree species -- a giant sequoia might not be considered big until it reaches a diameter of 300 centimetres (120 inches), while for other trees a big specimen may be only 50 cm (20 inches) wide.