Tree Biomass Sensitivity to Ozone Exposure: Insights From a Decade of Free-Air Experiments

Tropospheric ozone (O₃) is a pervasive stressor that impairs forest biomass and alters carbon allocation strategies. This study assessed biomass responses across 17 woody taxa under free-air controlled exposure (FACE), integrating a decade of experiments conducted with an analogous exposure regime applied to deciduous and evergreen species. The analysis provided a comparative evaluation of existing flux-based metrics. Statistical analyses revealed consistent reductions in relative total (RTB), aboveground (RTAB), and belowground (RTBB) biomass with increasing O₃ uptake in terms of phytotoxic ozone dose (POD₁ mmol m^-2). Deciduous species reached the 4% biomass reduction threshold (CL₄) at lower POD₁ levels for RTBB (10.21), RTAB (13.16), and RTB (10.77) and displayed relatively small ${△}_{{\mathrm{POD}}_1}$ values for RTBB (2.75), RTAB (5.70), and RTB (3.31), where ${△}_{{\mathrm{POD}}_1}$ represents the increment in O₃ uptake required to reach the CL₄ threshold. In contrast, evergreen species showed higher CL₄ for RTBB (11.48), RTAB (15.40), and RTB (13.86) and larger ${△}_{{\mathrm{POD}}_1}$ values for RTBB (8.40), RTAB (12.32), and RTB (10.78), reflecting a slower biomass decline. Contrasting relationships suggest that leaf habit-specific patterns are associated with divergent carbon allocation strategies under O₃ stress. In deciduous species, POD₁ and Leaf Index Flux (LIF) were negatively correlated with shoot-to-root ratio (S/R), whereas in evergreen species, both indices were positively correlated with leaf area ratio (LAR) and S/R. In conclusion, flux-based metrics provided a biologically robust framework for quantifying O₃-induced biomass losses, revealing higher sensitivity in deciduous species than in evergreens and highlighting the root as the most vulnerable compartment under O₃ exposure. The findings should be interpreted considering the spatial and temporal constraints of a single-site FACE experiment and the focus on O₃ as a stand-alone stressor without interaction effects. Future research should combine O₃ uptake with multi-stressor frameworks to better predict biomass and carbon responses in complex field conditions.