###### Abstract  
The Spruce budworm (SBW) defoliates and kills confer trees, consequently affecting carbon (C) exchanges between the land and atmosphere. Here, we developed a new TRIPLEX-Insect sub-model to quantify the impacts of insect outbreaks on forest C fluxes. We modeled annual defoliation (AD), cumulative defoliation (CD), and tree mortality. The model was validated against observed and published data at the stand level in the North Shore region of Quebec and Cape Breton Island in Nova Scotia, Canada. The results suggest that TRIPLEX-Insect performs very well in capturing tree mortality following SBW outbreaks and slightly underestimates current annual volume increment (CAI). In both mature and immature forests, the simulation model suggests a larger reduction in gross primary productivity (GPP) than in autotrophic respiration (Ra) at the same defoliation level when tree, especially at moderate or low defoliation levels (i.e., when not all foliage produced within a given year is consumed) [13]. The last SBW outbreak started in the 1970s and 1980s in eastern North America, continued for almost two decades and was considered extremely severe (e.g., 80%-100% annual defoliation) [14]. Tree defoliation and ultimately mortality vary greatly between stands during an outbreak as well as between outbreaks themselves [15]. Older stands are more vulnerable than younger, more vigorous stands [16]. Generally, C dynamics and insect disturbances lead to temporary changes in environmental conditions. However, insect outbreaks can cause long-term changes to ecosystem structure and function [17; 18; 19], which can result in long-term impacts in C dynamics. We therefore need to better understand how insect outbreaks interact with stand structure (e.g., how defoliation severity and duration interact with stand age) to improve our estimates of forest C losses and our understanding of regional and global C cycles. Modeling is a complementary tool to traditional observation and experimental approaches that can provide long-term and large spatial-scale perspectives on the effects of forest insect epidemics as well as evaluate interactions among multiple factors [20]. In recent years, there are some studies that have been conducted on C dynamics following insect outbreaks [21; 22]. For example, [PERSON] et al. [23] used an ecosystem demography model in conjunction with a stochastic model to predict the impact of hemlock woolly adelgid (_Ad edge t s t sugae_, Annand) on C dynamics in the eastern United States. [PERSON] et al. [24] also simulated a linear decrease in net ecosystem productivity (NEP) with increasing defoliation intensity by the gyspsy moth (_Lymantria dispar_ L.). In addition, [PERSON] et al. [25] simulated coupled C and nitrogen (N) dynamics following mountain pine beetle (_Dendroctons ponderosae_ ([PERSON]), Coleoptera: Curculionidae, Scolytinae) outbreaks in the western United States of America. They found that the severity of the outbreak itself was an important factor which led to the initial decline in NEP, and required 80 to 100 years or more following the disturbance to recover. However, because CLM4 does not explicitly contain an age-class distribution model for trees, the impact of tree age on C dynamics following insect outbreaks is not known. In Canada, the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) has been used to simulate and forecast ecosystem C dynamics impacted by SBW disturbances [10; 26; 27]. Such studies found that a decrease in NEP may convert boreal forests from C sinks into C sources after SBW outbreaks. However, CBM-CFS3 is an empirically driven model that is not designed to estimate gross primary productivity (GPP) and respiration in forest ecosystems. It is therefore unable to explain in detail the flow of C dynamics under such conditions. TRIPLEX, a process-based hybrid model [28; 29], integrates the advantages of both empirical and mechanistic components.