New study helps quantify logging as part of the forest carbon cycle
By Bob Berwyn
FRISCO — Scientists and resource managers have long been trying to solve the carbon-balance equation from using wood for energy, but they may have to add a new factor to their calculations.
A research team led by scientists with Dartmouth College found that logging releases large amounts of carbon from the soil. The processes by which that happens aren’t completely understood, but the findings suggest that calls for an increased reliance on forest biomass should be re-evaluated.
Forest carbon analyses are incomplete unless they include deep soil, which stores more than 50 percent of the carbon in forest soils, said Dartmouth Professor Andrew Friedland, one of the co-authors of the paper recently published in the journal Global Change Biology.
Friedland emphasized that the study doesn’t call for an end to logging.
“We need to keep cutting forests, all sorts of reasons, but we need to know what the consequences are,” he said.
Most global atmospheric studies often don’t consider carbon in deep (or mineral) soil because it is thought to be stable and unaffected by timber harvesting. But careful measurements of forest plots show deep soil can play an important role in carbon emissions in clear-cutting and other intensive forest management practices.
To measure the carbon, the scientists laid out two-foot by two-foot frames in different types of forests, some un-logged, some logged very recently, and others logged more than 10 or 15 years ago. They carefully excavated the soil 10 centimeters at a time and sampled for carbon, as deep as two to three feet.
“We know exactly how much volume we’re removing, and we can extrapolate that to the entire forest,” he said, adding that forest soils go back to a “normal” background level of carbon about 75 to 100 years after they’re logged.
“Our paper suggests the carbon in the mineral soil may change more rapidly, and result in increases in atmospheric carbon dioxide, as a result of disturbances such as logging … increased reliance on wood may have the unintended effect of increasing the transfer of carbon from the mineral soil to the atmosphere. So the intended goal of reducing carbon in the atmosphere may not be met,” he said.
Mineral soil carbon responses can vary highly depending on harvesting intensity, surface disturbance and soil type.
“Analysis of forest carbon cycles is central to understanding and mitigating climate change, and understanding forest carbon cycles requires an in-depth analysis of the storage in and fluxes among different forest carbon pools, which include above-ground live and dead biomass, as well as the below-ground organic soil horizon, mineral soil horizon and roots,” Friedland said.
Breaking up the soil at the surface may have a “priming effect” on deeper soil layers, as nutrients and enzymes trickle down and stimulate carbon dioxide production, he said, explaining one of the hypotheses for the findings.