Old trees continue to grow and store more carbon

An international study has found that trees do not slow in their growth rate as they get older and larger. In some cases, large trees appear to be adding the carbon mass equivalent of an entire mid-sized tree each year and are thus better at absorbing carbon from the atmosphere than previously thought.

The researchers compiled growth measurements of 673,046 trees belonging to 403 species from tropical, subtropical and temperate regions across six continents, calculating the mass growth rates for each species and analysing the trends. For most species, mass growth rate increased continuously with tree size.

“This finding contradicts the usual assumption that tree growth eventually declines as trees get older and bigger,” said lead author Nate Stephenson of the US Geological Survey (USGS).

“In human terms, it is as if our growth just keeps accelerating after adolescence, instead of slowing down.”

This continuously increasing growth rate means that on an individual basis, large, old trees are better at absorbing carbon from the atmosphere. Carbon that is absorbed through natural processes reduces the amount of carbon dioxide in the atmosphere and can help counter-balance the amount of CO₂ people generate.

“However,” noted contributing author Associate Professor Patrick Baker of the University of Melbourne, “the rapid carbon absorption rate of individual large trees does not necessarily translate into a net increase in carbon storage for an entire forest.”

As Dr Adrian Das, a USGS co-author explained, “Old trees … can die and lose carbon back into the atmosphere as they decompose. 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 favourite sports team were a bunch of 90-year-olds.”

The study was a collaboration of 38 researchers from Argentina, Australia, Cameroon, China, Colombia, Democratic Republic of Congo, France, Germany, Malaysia, New Zealand, Panama, Spain, Taiwan, Thailand, the UK and the US. It was published in the journal Nature.