BVOCs and CO₂ fluxes under an herbivory outbreak in a Subarctic Birch Forest

Subarctic forests act as major emitters of biogenic VOCs and important carbon sinks whose balance can be rapidly altered by biotic and abiotic stressors. We analyzed the diel cycles of major VOCs during the growing seasons of 2021, 2022, and 2023 in a subarctic mountain birch forest (Betula pubescens) near Abisko, northern Sweden. Herbivory stress caused by caterpillar outbreaks was associated with changes in VOC emissions, particularly during the complete defoliation event in 2023, highlighting the importance of incorporating VOC dynamics into assessments of ecosystem responses to disturbance and climate change.
Complementing VOC observations, we examined CO₂ fluxes using four years (2021-2024) of eddy covariance data. The 2023 outbreak transformed the forest from a near-neutral carbon balance to a strong source (annual NEE = +150.147 gC m⁻²), compared to modeled undisturbed conditions predicted by RandomForest (+0.195 gC m⁻²) and LPJ-GUESS (−102.637 gC m⁻²). In the recovery year (2024), the observed balance returned to +5.168 gC m⁻². These findings demonstrate that insect outbreaks simultaneously disrupt carbon dynamics and biogenic VOC emissions, reinforcing the need to integrate both processes into models of northern ecosystems under climate change.