A Chilling Solution: Measuring Below-ground Carbon Without Destroying Trees

The photosynthetic process of plants has been estimated to account for almost half of the carbon circulating in the Earth’s systems. Reliable data has been developed on carbon cycling in the above-ground processes of trees, but how much carbon is actually moved and stored below the ground has still not been determined. Most methods to study below-ground processes involve destroying the roots as well as the mycorrhizal communities that live symbiotically with root systems

“Below-ground carbon allocation is one of the least understood processes in tree physiology,” says Johnsen. “Being able to accurately measure it is essential for modeling forest and ecosystem productivity and carbon sequestration, but most methods disturb the root-mycorrhizal continuum that plays an essential role in nutrient transport.”

One method of estimating below-ground carbon allocation involves girdling the tree, cutting through the phloem to stop the movement of carbon into roots. This method leaves the root-mycorrhizal continuum intact, but still destroys the tree. Johnsen and his fellow researchers decided to try chilling the phloem to temporarily interrupt carbon movement and leave the tree alive. Though the technique has been used on herbaceous plants in controlled environments, Johnsen’s experiment represents the first test of the method on trees and, in particular, on large trees in the field.

The researchers chilled the phloem of 10 loblolly pine trees in a stand that receives annual fertilization, comparing responses with those from physically girdled trees in both fertilized and unfertilized stands to determine whether the technique would give accurate results. They wrapped each tree in 30 coils of copper tubing, then circulated anti-freeze cooled to less than 35 degrees Fahrenheit through the tubing, measuring carbon dioxide efflux from the soil to determine if carbon movement was reduced The researchers hypothesized that carbon movement in trees would differ at varying points in the year; this was confirmed in their study.

“There was no response to either chilling or physical girdling in the experiments we did in the spring,” says Johnsen. “We think this is because above-ground growth was so rapid and below-ground processes were getting carbon from starch reserves.”

Fall experiments, however, showed that both chilling and girdling rapidly reduced soil carbon dioxide efflux, showing that both techniques stop or reduce the movement below ground of carbon recently produced by photosynthesis. The difference is that once the chilling was stopped, the effect was rapidly reversed, while the physically girdled trees died.

“This phloem-chilling method can be applied to the same trees at various times of the year and under a variety of environmental conditions, giving us the means to generate robust estimates of carbon allocation needed to construct more realistic and reliable carbon cycle models,” says Johnsen.