Evidence for accelerated carbon cycling following disturbance from an insect outbreak in southern Appalachian forests
Understanding how disturbance affects the long-term trajectory of soil carbon cycling in forest ecosystems is increasingly important given the rapid pace at which human activities are altering the landscape. In the southern Appalachians, infestation by the hemlock woolly adelgid (HWA; Adelges tsugae) has resulted in the mortality of hemlock trees (Tsuga canadensis, T. caroliniana) and shifted formerly mixed coniferous-deciduous forests to primarily deciduous forests. While the effects of large-scale, forest clearing disturbances have been extensively studied, the impacts of less catastrophic disturbances on ecosystem carbon cycling remain unclear. We predicted that loss of hemlock from this system would result in accelerated soil carbon cycling as a result of large inputs of organic matter from dying hemlocks and a shift towards a community with more labile litter inputs. To assess changes in soil carbon cycling following hemlock mortality, we compared soil carbon pools at two depths (0-10 cm, 10-30 cm) immediately following and ten years post-disturbance by HWA. Soil organic matter was physically fractionated into the fast-cycling particulate organic matter and slow-cycling mineral-associated organic matter pools. We also examined in situ net soil respiration prior to observation of HWA, one, two, and ten years post-disturbance to evaluate changes in carbon outputs.
In the ten years following hemlock mortality, concentrations of fast-cycling carbon increased by 207% (±86 SE) and slow-cycling carbon increased by 84% (±42) in the top 10 cm of soil. In deeper soils, fast-cycling carbon decreased by 24% (±14) whereas slow-cycling carbon increased by 163% (±65). Soil CO2 efflux increased by 30% between 2004 and 2014. Soils in this system are, on average, storing more carbon ten years post-disturbance while also losing more via soil CO2 efflux. Increases in soil organic carbon pool sizes are likely a result of increased inputs of leaf, root, and branch litter from dying hemlocks and the subsequent decomposition of these materials. Moreover, previous studies have shown increased growth of co-occurring hardwood trees and shrubs following hemlock mortality in response to increased light, space, and nutrients. Increased growth of remaining trees and thus increased root growth may explain the observed overall increase in soil respiration. These changes suggest that hemlock loss as a result of an introduced pest has significantly altered the carbon cycle in this system, and point to the importance of understanding the effects of disturbance in mixed forest ecosystems.