Ozone alters heat-driven Biogenic VOC responses: evidence from forest and urban tree species under sequential stress

Predicting plant responses to changing climate, particularly to heat extremes and elevated near-ground ozone, is a key obstacle in robustly quantifying future biogenic volatile organic emissions (BVOCs) and understanding the climate feedback loop. Scaling BVOC emissions from leaf to landscape level and identifying stress-specific emission fingerprints require controlled-chamber experiments with sequential stress exposure that realistically mimic natural events.

In a climate-controlled chamber, periodic stress exposures were applied to forest (Fagus sylvatica L., Quercus robur L.) and urban tree species (Castanea sativa Mill., Tilia cordata Mill.) during summer 2024 and 2025. The forest species were exposed to heat (~40°C) and nocturnal ozone stress (100-120 ppb), while urban species experienced heat stress (~40°C) and a 72h simultaneous ozone exposure (100-120 ppb). BVOC emission fluxes were measured using proton-transfer reaction time-of-flight mass spectrometry and compared across pre-stress, heat, and combined heat–ozone conditions.

Heat stress strongly increased BVOC emissions, with urban tree species showing 2–8-fold increases in isoprene and ~3-fold increases in monoterpenes, along with elevated sesquiterpenes and green leaf volatiles. Forest species showed more selective heat-induced emissions, primarily in monoterpenes and green leaf volatiles. In contrast, combined ozone–heat stress following ozone exposure suppressed most BVOC emissions by 30–60%, largely independent of species, despite differing ozone treatments. The concurrent increase of methyl salicylate, a stress-alarm compound, emissions under combined stress compared to heat alone showed a non-additive physiological response. Heat stress consistently yielded the highest OH reactivity of BVOCs across all species and decreased by 10–30% following ozone-mixed heat exposure. A cross-investigation using machine learning and positive matrix factorization identified stress- and species-specific VOC fingerprints, with a good agreement.

These multi-stress experiments provide mechanistic insight into stress-induced BVOC emissions and could improve parameterizations of BVOC emissions in Earth system and air-quality models under increasing pollution and heat events.