• Chen, Cen
  University of Maine, School of Forest Resources
  
• Wei, Xinyaun
  University of Maine, School of Forest Resources
  
• Weiskittel, Aaron
  University of Maine, School of Forest Resources
  
• Hayes, Daniel J.
  University of Maine, School of Forest Resources
  

Disturbances are a primary force regulating the successional development and associated carbon cycling in forest ecosystems, but our understanding of these dynamics is based on studies mostly limited to relatively homogeneous and high intensity forest disturbances such as harvesting and wildfire. Contrarily, extensive insect defoliations such as that caused by spruce budworm (SBW; Choristoneura fumiferana (Clem.)) often are species specific and vary greatly in their intensities and consequent influences on forest carbon dynamics over large spatial and temporal extents. In this study, we applied an individual tree growth model across a large forested area in the state of Maine, USA to simulate the impact of SBW outbreaks on above-ground carbon stock in merchantable trees, which is the primary carbon pool of interest in forest management and its associated downstream industries. The model, the Acadian Variant of the Forest Vegetation Simulator (FVS-ACD), provides the necessary resolution needed to capture the spatio-temporal variability in SBW defoliation and its impacts on the mixed species, structurally complex spruce-fir (Picea-Abies) forests of Maine. Our evaluation of two 40-year SBW outbreak cycles (1975–2015 and 2017–2057) indicates that, due primarily to changing species composition (particularly significant increases of hardwood species), this carbon stock of spruce-fir forests was not reduced and not significantly different than what it would be without an outbreak at the end of either cycle. However, there were temporary decreases in carbon growth immediately following these outbreaks. These decreases were at smaller scales in the 40-year cycle starting 2017 where significant increases of hardwood species along with decreases of SBW's primary host species were observed. As this trend of changing species composition continues in this region, a future SBW outbreak would cause less reduction in carbon growth and less likely to result in a decrease of carbon stock in spruce-fir forests. Overall, the utilized tree-level modeling framework may prove useful for assessing the influence of complex disturbances like insect defoliation in other regions.