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Issue 10
A Chilling Solution
Kurt Johnsen, team leader of the SRS Southern Institute of Forest Ecosystems Biology Team based in Research Triangle Park, NC, is always looking for new ways to measure carbon storage in trees without destroying them. Late last year, along with fellow SRS team members Chris Maier, Felipe Sanchez, Peter Anderson, John Butnor, and Richard Waring from Oregon State University, Johnsen published the first proof of concept for a reversible, nondestructive chilling method to study belowground carbon.
It’s been estimated that half the carbon cycling through the Earth’s systems is tied up in the photosynthetic process of plants. Though reliable data have been developed on the carbon cycling that takes place aboveground in trees, carbon processes belowground have yet to be accurately quantified. Understanding belowground carbon allocation is further stymied by study methods that destroy the tree as well as the soil fungi (mycorrhizae) that live symbiotically among root systems.
“What happens to carbon belowground is somewhat of a black box,” says Johnsen. “It’s certainly one of the least understood parts of tree physiology. Accurate measurements of belowground carbon allocation are essential for modeling forest productivity and carbon sequestration.”
One method that’s been used to estimate belowground carbon allocation involves girdling the tree by cutting through the phloem, the thin layer beneath the bark that transports carbohydrates and other products of photosynthesis down towards the roots. Girdling stops the movement of carbon, but does not physically disturb the roots, allowing researchers to study how root-mycorrhizae interactions affect carbon allocation. The problem is that this method kills the tree.
Johnsen and his fellow researchers decided to try chilling the phloem layer to temporarily interrupt the movement of carbohydrates towards the roots. This would allow scientists to study the same trees over time under different conditions. Although the chilling method had been used before on herbaceous plants, Johnsen’s experiment was the first test of the method on large trees in the field.
For the experiment, researchers selected 10 loblolly pine trees in a stand that’s fertilized annually. They wrapped the stems of the trees in coils of copper tubing, then circulated antifreeze through the tubing to cool it to less than 35 degrees Fahrenheit. They measured the CO2 released from the soil to determine if carbon movement through the phloem had been reduced, testing the accuracy of their results by comparing them with those from physically girdled trees in both fertilized and unfertilized stands.
“We found that both chilling and girdling quickly reduce the movement of carbon belowground in trees,” says Johnsen. “The difference is that once we stopped the chilling, the trees recovered within 12 hours, while the girdled trees died. Using the chilling method, we can study the same trees at various times of the year under a variety of conditions, which allows us to begin to understand belowground processes—and carbon sequestration—more accurately.”
