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 values for RTBB (2.75), RTAB (5.70), and RTB (3.31), where 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 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.