A carbon dioxide analyzer watches over a forest in Metolius, Oregon. Credit: Cristoph Thomas

Old forests continue to accumulate carbon at a much greater rate than researchers had previously thought, making them more important as carbon sinks that must be factored into global climate models, researchers say.

Until recently, it was assumed that very old forests no longer absorbed carbon. The only new growth occurred in the small spaces that opened up when large old trees died and decomposed, releasing their accumulated carbon. The forests at large were therefore considered to be carbon neutral, and accounted as such in climate models.

In the past decade or so, murmurs of disagreement with this idea have grown louder, and individual projects have found that even very old forests are capable of storing carbon thanks to tree growth, the addition of new trees and a decreased rate of respiration in old trees.

Since the mid 1990s, more sophisticated data collection projects have measured carbon fluxes in forests around the world. In particular, data has been shared between memebers of FLUXNET, a global network of observatory towers that measure the exchange of carbon dioxide, water vapour and energy between ecosystems and the atmosphere.

Now Sebastiaan Luyssaert of the University of Antwerp, Belgium, and his colleagues have taken advantage of all this new data to produce a meta-analysis of studies that monitored 519 plots of temperate and boreal forest between 15 and 800 years of age. Their conclusion, published in Nature this week, is that old-growth forests are, in general, still absorbing carbon1. Primary boreal and temperate forests, which make up 15% of global forests, sequester about 1.3 gigatonnes of carbon a year, give or take half a gigatonne. That amounts to about 10% of the global net ecosystem productivity, which was previously accounted for elsewhere].

Dying dogma

The conclusion makes sense, according to Susan Ustin, a plant ecologist at the University of California, Davis. When determining the age of a tree, one counts its rings. Each of those rings represents the transformation of atmospheric carbon into the living tissue of the tree. In any one year, the death and decomposition of roots or branches may outweigh the carbon sequestered in the trunk - but over time, any significant growth must involve net carbon uptake. "If they are carbon neutral at 400 years old, how are they going to make it to 1,000?" she asks. "If it was really carbon neutral, the trees would die."

Overturning the old idea that mature forests are carbon neutral may be the work of more than one paper, and this certainly isn't the first to propose that they continue to absorb the greenhouse gas. But Luyssaert hopes this analysis will help tip the scales. "Just challenging the dogma isn't new, but the data that has been used to challenge it was a lot more limited in the past," he says.

The implications are many. Scientists who were assuming that old-growth was carbon neutral may have consequently been overestimating sequestration in other ecosystems. Climate models may have to be re-examined. And policies that give credits to governments or companies for sequestering carbon may want to incorporate the protection of old-growth forests into their menu of options.

Indeed, the heartwarmingly green action of planting a tree may actually be second-best to keeping an old tree from the axe: "probably for a couple hundred years, until the young one got big enough to have the same amount of carbon as one of these old trees," estimates Ustin.

Tim Griffis, a University of Minnesota researcher who mans one of FLUXNET's observation towers, adds that the work "shows the power of the FLUXNET network". But that network is getting harder to operate, as it segues from being cutting-edge research into part of a longer-term dataset. "Many in the community are already finding it difficult to keep their sites funded," says Griffis. "I think there does need to be a serious conversation about how we are going to keep this record going."

Credit: Pavel Filatov / Alamy