## 4 Discussions
### Model Performance, Limitations, and Future Research  
To improve the ability of TRIPLEX to model impacts of insect disturbances, we developed the new TRIPLEX-Insect model which is able to simulate and predict C dynamics and tree mortality caused by defoliation. This ability to test scenarios will be useful in planning SBW management. The TRIPLEX-Insect model is inherited from the original model (TRIPLEX 1.0) and expands its capabilities to simulate key variables related to SBW disturbances, such as changes in mean DBH, mean height, total stand volume, biomass, and C dynamics. The extensive application of this model, however, will first require further research on the five following aspects:  
(1) To accurately model the influence of defoliation on regional forest C dynamics, a comprehensive understanding of the spatial and temporal development of insect outbreaks is required [50; 63]. In our study, we only calibrated and validated the model at the stand level, yet spatial processes that drive outbreaks to cross thresholds and become catastrophic disturbances have not been completely simulated and are difficult to predict using the current version of the TRIPLEX-Insect model. In the future, we intend to link our model to remote sensing analyses to scale up the model from a stand level to a landscape level for regional-level applications.  
(2) Another key element that will need to be included in our model is the influence of species composition. In the current application, we tested the model in balsam fir stands whereas the SBW has been observed to have a reduced effect in mixed-host hardwood stands [64; 65; 66]. To more fully understand forest level C losses, we will also need to include other host species, such as white and black spruce. Even if these species are less vulnerable to mortality caused by the SBW [16], it has recently been shown that associational susceptibility occurs when spruce trees are mixed with fir trees, and as outbreak severity and duration length increase [37].  
(3) Climate can also influence SBW outbreak characteristics, such as duration and intensity. For instance, seasonal temperatures and precipitation impact SBW life cycles (e.g., feeding and survival rates [49; 67; 68]) and population dynamics (e.g., dispersal area and develop rate [49; 69; 70]). To more accurately predict ecosystem response to SBW disturbances in the future, forthcoming modeling research should investigate the effects of future climate change on defoliation dynamics.  
(4) Although our model is not capable of considering all SBW dynamics which impact ecosystem C budgets (e.g., insect respiration processes and decomposition of dead larvae), our method of simulating defoliation directly as an input parameter will be easily applicable to similar pest species from which defoliation data are regularly collected. Furthermore, since the TRIPLEX-Insect model can run with only an AD input, it should be possible to modify weights of monthly defoliation distribution and foliage waste for other defolator species.  
(5) TRIPLEX did not initially consider the concurrent simulation of tree mortality and regeneration; thus, the accuracy of the long-term simulations on C dynamics is limited. In this study, we only predicted short-term C dynamics (from 2017 to 2020) at sites in the North Shore region of Quebec, Canada. This approach of only modeling C dynamics during the outbreak when establishment of new balsam fir was chosen so that we could avoid uncertainty, linked to the post-disturbance establishment of new trees, when testing our model. This could also cause some uncertainty related to model simulations due to simple assumptions concerning C pool totals and regeneration deficiencies.