Multi-stress interaction effects on BVOC emission fingerprints from Oak and Beech: A cross-investigation using Machine Learning and Positive Matrix Factorization

Abstract. Forest ecosystems are increasingly stressed through heatwaves, drought periods, and other factors such as ozone pollution or insect infestations. These stressors have a profound impact on the emissions of biogenic volatile organic compounds (BVOC) from trees, which in turn influence aerosol formation and atmospheric oxidation cycles and thus feedback on the atmospheric cleansing capacity and climate change itself. While previous studies have investigated the impacts of specific stressors on BVOC emissions, analyses of combined stress effects are rare, even though the stressors seldomly occur in isolation. This study investigates the impact of heat and (nighttime) ozone stress, both individually and in combination, on BVOC emissions from two ecologically significant temperate tree species: European beech (Fagus sylvatica L.) and English oak (Quercus robur L.). In a climate-controlled chamber, both tree species were subjected to heat stress (38 ± 3.3 °C) and ozone stress (∼ 120 ppb), separately and in combination. BVOC emission rates were measured using proton transfer reaction time-of-flight mass spectrometry, and the results were compared across pre-stress, heat, ozone, and combined heat-ozone conditions. Heat stress elicited the strongest emission increases of isoprene, monoterpene, and green leaf volatiles in both species, while ozone suppressed the emissions of most BVOCs. Combined stress led to non-additive responses different from those in single-stress scenarios. Both machine learning and positive matrix factorization analyses were performed to identify key VOC fingerprint markers that may be applied to identify stress-impacted emissions from field data, and both methods showed good agreement. The OH reactivity of the emissions, which serves as a measure for their atmospheric chemistry and ozone formation impacts, was consistently highest under heat stress for both species. However, nighttime ozone stress led to reduced OH reactivity of emissions (by 10 %–18 %). Our results underscore that the study of realistic combinations of stressors is crucial to understand future BVOC emissions and indicate that BVOC emissions could alter atmospheric chemistry and feedback with air quality and climate as heatwaves and pollutant-induced stress become more frequent due to climate change.