Spruce Budworm Defoliation Influences Greenhouse Gas Concentrations in Streams

Abstract One of the most significant natural disturbances in the Canadian boreal and hemiboreal forest is tree defoliation by eastern spruce budworm (SBW), which may increase nutrient and carbon inputs to catchment soils, and in runoff. Closely connected to catchments, headwater streams are known to be significant emitters of greenhouse gases (GHGs); however, how insect defoliation influences stream GHG concentrations remains unknown. To fill this knowledge gap, we measured the carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) concentrations in 12 headwater streams along a gradient of ongoing defoliation in the Gaspé Peninsula, Québec. Streams were sampled 11 times between June and October in 2023 and 2024, and we assessed the role of cumulative defoliation and tree mortality, as well as physical‐chemical, and climatic drivers in explaining GHG variability. We found that CH 4 and N 2 O concentrations increased in response to elevated defoliation and tree mortality across watersheds and years. Elevated gas concentrations also coincided with higher overall total dissolved nitrogen concentrations in all streams, and a reduction in discharge in 2024. In contrast, CO 2 concentrations were influenced by stream slope, potentially through enhanced gas exchange, with limited effects from defoliation. Our study suggests a more indirect response of stream GHG concentrations to SBW disturbance, where a time lag resulting in higher nitrogen concentrations combined with lower discharge potentially increased within stream CH 4 and N 2 O production. This work demonstrates the broader ecosystem impacts of insect disturbance, anticipated to increase with climate change, that should be considered in forest management.