Coupled insect-outbreak and microclimatic forcing suppresses BVOC emissions in subarctic understory plants

Insect herbivory is a major disturbance in subarctic ecosystems, yet its impact on biogenic volatile organic compound (BVOC) emissions from understory vegetation remains poorly understood. Here, we use a severe, natural geometrid moth outbreak in a mountain birch forest in northern Fennoscandia to quantify i) outbreak-associated changes in understory vegetation and ii) indirect effects of mountain birch canopy loss on understory microclimate and BVOC emissions. Across two growing seasons (moderate herbivory vs. peak outbreak), we combined enclosure BVOC measurements (n = 131) from three dominant understory plant species with near-surface spectral proxies of vegetation greenness and microclimatic data. The geometrid outbreak caused widespread mountain birch canopy defoliation and reduced understory greenness. Moreover, mountain birch canopy loss increased incident solar radiation in the understory, raised understory canopy surface temperature, and reduced soil moisture. Despite these warmer and brighter conditions, which typically promote BVOC release, understory BVOC emissions declined in the outbreak year, indicating that loss of photosynthetic tissue constrained emission capacity. Empetrum nigrum showed the strongest reduction in total BVOC emissions (72%) despite a pronounced shift toward stress-induced blends during the outbreak year. Total emissions of Vaccinium myrtillus were reduced by 55% and showed modest compositional change including late-season increases in isoprene under warm, high-PAR conditions in the outbreak year. Graminoids were comparatively resilient, showing limited compositional shifts and only minor reductions in total BVOC emissions. Together, these results indicate that BVOC emissions from subarctic understories are jointly controlled by direct herbivory and canopy-mediated microclimatic feedbacks, and that large-scale insect outbreaks can directly or indirectly suppress, rather than enhance, BVOC emissions when green leaf loss outweighs biochemical induction. Accounting for these coupled pathways is essential for predicting biosphere–atmosphere interactions in a warming Arctic where insect outbreaks are expected to intensify.