Forests play a major role in drawing-down and storing atmospheric carbon dioxide. Currently, an estimated three-quarters of the carbon stored on land is locked up in forest ecosystems. But when a tree experiences internal decay, it starts to release carbon back into the atmosphere. Because this decay is often not visible from the outside, it’s not clear how extensive the phenomenon is. Might we have overestimated the amount of carbon that our forests can store?

Now scientists have developed a non-destructive way to measure the amount of decay inside a tree. The results show that internal decay needs to be incorporated into our carbon accounting models.

Robert Marra from The Connecticut Agricultural Experiment Station and Nicholas Brazee from the University of Massachusetts, both in the US, carried out tomographic scans on trees, to see if they measured internal decay reliably. Deep inside the Great Mountain Forest in Connecticut, the scientists selected 72 of the three most common hardwood tree species — American beech, sugar maple and yellow birch.

Using novel lightweight portable tomographic equipment, Marra and Brazee generated sonic tomographs of every tree, building up a picture of the interior. Thirty-nine of the trees were then felled, and the scientists compared the tomographic images with the relevant cross-section of tree trunk, to test the accuracy of the scanning technique in identifying internal rot.

The internal state of the felled trees verified that the tomographic scans were a reliable measure of internal decay, with errors of no more than 2%.

“In some cases, the results were quite a surprise, with decay occurring in a number of trees that showed no sign of decay externally,” says Marra, who published the findings in Environmental Research Letters (ERL).

Based on the tomography alone, Marra and his colleagues were able to identify decay in the interiors of 47 of the 72 trees. The amount of decay ranged from 0.13% to 36.7%

Current carbon sequestration models don’t directly account for internal decay. Marra and his colleagues hope that their non-invasive tomographic scanning methodology can now be applied to larger-scale forest studies to develop understanding of the extent of internal decay. Ultimately the aim is to feed this information into carbon sequestration models, so that we have a clearer picture of how much carbon our forests can store.