Forests are generally recognized as carbon sinks, meaning they absorb more carbon from the atmosphere than they release, and soils store between 25% and 45% of the carbon in the ecosystem. Forests around the world are changing, however: As living ecosystems, they are undergoing alterations due to land use changes, warming temperatures, fire, and insect and disease infestations. As these forces reshape landscapes, they also alter the role of forests in atmospheric carbon dioxide concentrations.
However, mixed results in past research have caused disagreement in the scientific community over exactly how forest carbon cycles respond to widespread tree mortality.
In a new study, Fraterrigo et al. examined how insect infestations, a significant driver of tree mortality, shaped the storage and transformation of soil carbon in a southern Appalachian forest. The researchers framed the study around an outbreak of hemlock woolly adelgid at the Coweeta Hydrologic Laboratory, a U.S. Forest Service experimental forest in the Nantahala Mountain Range in western North Carolina. The insects attacked eastern hemlocks, a codominant species in riparian forests. To mimic the disturbance, the researchers girdled hemlocks at separate study plots to accelerate tree mortality. They visited the plots annually from 2004 to 2014.
After a decade of monitoring, the team found that the soil carbon storage capacity was not reduced by the disturbance. Although tree mortality did not trigger a release of stored carbon to the atmosphere, it did affect how and where it was stored.
The results indicate that soil carbon pools near the surface moved deeper underground in response to the hemlock mortality. As soil microbe organic matter consumption increased, the carbon pools bound to small soil minerals increased as the larger organic matter particles decomposed. These changes to the carbon cycle happened more quickly in the girdled plots and offered a preview of what insect-infested sites will look like in the future.
These conclusions differ from those of similar studies that found that increased tree mortality led to the release of soil carbon to the atmosphere, which may relate to differences in how undisturbed vegetation picks up the slack in the carbon cycle across ecosystems. Deeper, mineral-bound carbon is believed to be more persistent in the ecosystem, but it remains unclear what spurred the transition of carbon from the surface to subsurface pools. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2018JG004431, 2018)
—Aaron Sidder, Freelance Writer