Incorporating Insect Disturbances into Terrestrial Biosphere Model: Impacts and Challenges

Insect disturbances significantly impact multiple functions of forest ecosystems, yet their representation in terrestrial biosphere models remains limited. To address this gap, we developed an insect impacts module in the terrestrial biosphere model QUINCY (QUantifying Interactions between terrestrial Nutrient CYcles and the climate system model). The new module represents bark-beetle and defoliator impacts by introducing standing dead biomass pools and insect-mediated nutrient cycling pathways, and effectively capturing key processes such as snag decay, larvae pool dynamics, compensatory leaf growth, and carbon starvation due to over-defoliation. Model validation against multiple forest sites registering insect disturbances demonstrated good agreement with observed trends in forest dynamics, carbon fluxes, water and energy exchanges, and nutrient transformations during insect disturbances. Long-term simulations revealed that severe insect outbreaks can reduce ecosystem carbon storage by up to 6% for a horizon of 50 years, primarily due to accelerated nutrient leaching through litter decomposition. These results emphasize the critical role of insect disturbances in shaping vegetation carbon dynamics and highlight the importance of integrating these processes into global vegetation models. Our results further underscore the need for observational datasets, including field and satellite-based measurements, to constrain and improve model representations of insect disturbances. By advancing understanding of insect impacts and their interactions with climate, our study contributes to reducing uncertainties in projections of vegetation dynamics and the terrestrial carbon sink under future climate change.