Insect outbreak progression over three years shapes biogenic volatile emissions in a mountain birch forest

Geometrid moth outbreaks are important natural disturbances in mountain birch (Betula pubescens ssp. czerepanovii) forests of northern Fennoscandia and are expected to intensify with climate change. Impacts of these outbreaks and moth herbivory on biogenic volatile organic compound (VOC) emissions of birch remain unquantified despite the importance of VOCs in plant-herbivore interactions and atmospheric chemistry. Here, we quantify in situ VOC emission responses of mountain birch (n  = 256) to naturally increasing geometrid herbivory across three consecutive growing seasons. We show that VOC emissions increased with rising herbivore pressure across the years, culminating in the outbreak, during which early-season VOC emissions were higher than at peak herbivory. At peak herbivory, birch emitted more complex VOC blends, indicating seasonal shifts in composition. Following the outbreak, a second leaf flush sustained emissions of some VOCs. Our findings demonstrate that VOC emission rates and compositions are dynamically linked to leaf phenology and herbivory intensity across multiple years.