## 4 Discussions  
### Impact of Defoliation Intensity and Duration  
Defoliated forests are predicted to lose C because photosynthesis is interrupted by the removal of foliage while respiration and decomposition continue [58; 59]. The model simulations showed that more severe defoliation and a longer outbreak duration would result in a higher mortality level, lower recovery capacity, and thus more C loss in balsam fir forest ecosystems. [PERSON] [60] reported that more severe outbreaks and a longer duration would result young and rapid carbon-accumulating trees replacing old and slow carbon-accumulating trees. In our study, annual GPP and NPP recovered significantly only at QC3 in 2015 and 2016 (Figure 4C,F), and this was perhaps due to the fact that QC3 had both the younger age and lower CD than both QC1 and QC2.  
In our sensitivity experiments, we found that when defoliation severity increases, both GPP and Ra decrease. However, we did not find any correlation between GPP or Ra dynamics and the various forest ages investigated under the same defoliation intensity (see Table 4). These results suggest that when defoliation-driven tree mortality is not excessively high, GPP's response to defoliation severity is greater than Ra's in both mature and immature balsam fir forest stands. The possible reason for this difference is that leaves are the only organ of photosynthesis rather than respiration, so defoliation should have a greater and direct impact on GPP than Ra.  
When AD is low, it tends to act as a thinning mechanism, such that NEP exceeds that found in non-defoliated stands. [PERSON] and [PERSON] [61] also found that subsequent forest structure depends on defoliation severity, although SBW outbreaks caused a \"patchier\" response compared to forestry (thinning) operations. In our simulations, C fluxes (i.e., GPP, NPP and NEP) had a rapid decrease when DM occurred in 2013 (at QC1 and QC3) and in 2010 (at QC2). However, the C fluxes were higher under outbreak conditions compared to the no disturbance scenario before DM occurred (Figure 4).  
[PERSON] et al. [24] reported that forest ecosystems respond strongly to C sequestration when defoliation duration varies from 5 to 15 years, but they exhibit a relatively weak response when defoliation duration varies by more than 15 years. In our study, simulated and predicted results also found that defoliation duration influences C losses and recovery in balsam fir forest stands. Long-term defoliation duration results in higher mortality and a lower recovery capacity in forests (Figure 3D-F). However, when the defoliation duration is less than 10 years, tree mortality is low because CD remains below lethal levels with the exception of the most severe defoliation cases.  
Figure 6: Simulated NEP from 2010 to 2016 (on the left side of the vertical dotted line) and predicted NEP under three different defoliation levels (0% AD, 20% AD and 45% AD) from 2017 to 2025 (on the right side of the vertical gray dotted line) for the QC1 (**A**), QC2 (**B**), and QC3 (**C**) sites. Horizontal dotted line represent results for disturbance (control).  
Many studies have shown that insect damage could convert forests from C sinks to C sources [10; 20; 62], and this is consistent with our simulation and sensitivity experimental results. However, we observed a rapid recovery trend in annual NEP if defoliation is reduced (red line after 2015 in Figure 6A-C). In other words, the C source period may not protract following insect outbreaks if defoliation intensity is not excessively high and the duration is relatively short. [PERSON] et al. [23] predicted similar impacts of the hemlock wooly adelgid on eastern United States forests, i.e., an 8% decrease in NEP during the period 2000-2040 and a 12% increase without disturbance during the period 2040-2100.