Reduced BVOC emissions from birch forest understory during coupled insect outbreak and microclimatic forcing

Insect outbreaks are intensifying across high-latitude forests, yet their effects on biogenic volatile organic compound (BVOC) emissions from understory vegetation remain poorly resolved under natural outbreak conditions. Here, we examine how a severe geometrid moth outbreak alters understory microclimate and BVOC emissions in a subarctic mountain birch forest, using field measurements from two growing seasons spanning increasing herbivory to the peak outbreak year. We find that understory BVOC emissions decline markedly during the outbreak (38-70%), despite more emission-favourable abiotic conditions. We suggest that this decline results from the combined effects of direct herbivory on susceptible understory species and canopy-mediated feedbacks following mountain birch defoliation. These feedbacks likely intensify radiative, thermal, and drought stress in the understory, leading to widespread loss of BVOC-emitting green vegetation. Our findings demonstrate that severe geometrid outbreaks can suppress, rather than enhance, BVOC emissions by shifting emission control from abiotic stimulation to green biomass limitation, while highlighting that BVOC responses to herbivory are context-dependent and species-specific.