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

Tropospheric ozone (O₃), 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 O₃ damage, a comprehensive understanding of vegetation responses to O₃ 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 O₃ remain unclear. Using continuous and simultaneous observations of O₃, SIF, and carbon fluxes in a subtropical evergreen forest, we investigated the response of canopy SIF to ambient O₃ exposure and explored the underlying mechanisms of vegetation’s response to O₃. Our findings reveal that high ambient O₃ alters both the canopy SIF and its relationship with GPP in the subtropical evergreen forest. Specifically, canopy SIF decreases when O₃ 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 O₃ uptake by leaf. Furthermore, extremely high O₃ concentrations decouple the linear SIF-GPP relationship (especially in wet season), indicating that high O₃ 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 O₃ damage. This results in high O₃ 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 O₃ stress detection and the different responses of structure and physiology under varying water conditions, which could effectively advance our understanding of plant O₃ stress mechanisms across different climates.