Tropospheric ozone alters solar-induced chlorophyll fluorescence and its relationship with gross primary production in a subtropical evergreen forest

• Canopy SIF of the subtropical evergreen forest decreases once exceeds O 3 threshold. • The threshold is higher in dry season than in wet season due to reduced stomatal conductance. • Elevated O 3 concentrations cause plant physiology damage first, then structure. • SIF and GPP gradually decouple with the elevated O 3 concentrations. • SIF-GPP relationship at high O 3 is affected more by structure in dry season but physiology in wet season. Tropospheric ozone (O 3 ), as a harmful air pollutant and greenhouse gas, is globally increasing in concentration, raising concerns about its detrimental effects on ecosystems. To develop scientific strategies for protecting vegetation from O 3 damage, a comprehensive understanding of vegetation responses to O 3 is essential. Solar-induced chlorophyll fluorescence (SIF) offers a novel approach to studying how plants respond to stress, yet the extent and mechanism by which SIF reflects vegetation changes under varying ambient O 3 remain unclear. Using continuous and simultaneous observations of O 3 , SIF, and carbon fluxes in a subtropical evergreen forest, we investigated the response of canopy SIF to ambient O 3 exposure and explored the underlying mechanisms of vegetation’s response to O 3 . Our findings reveal that high ambient O 3 alters both the canopy SIF and its relationship with GPP in the subtropical evergreen forest. Specifically, canopy SIF decreases when O 3 concentrations exceed 60 ppb, but the threshold for a decrease in canopy SIF differs between the dry season (75 ppb) and the wet season (45 ppb), probably due to reduced stomatal conductance in the dry season, which limits O 3 uptake by leaf. Furthermore, extremely high O 3 concentrations decouple the linear SIF-GPP relationship (especially in wet season), indicating that high O 3 exposure more strongly affects SIF and thus weakens the ability of SIF to track GPP dynamics. Our analysis of the decomposed radiative, structural, and physiological components shows that plant physiology is more vulnerable to O 3 damage. This results in high O 3 concentrations influencing the SIF-GPP relationship primarily through structure in the dry season and through plant physiology in the wet season. These findings highlight the ability of SIF for plant O 3 stress detection and the different responses of structure and physiology under varying water conditions, which could effectively advance our understanding of plant O 3 stress mechanisms across different climates.