Quantifying the role of ozone-caused damage to vegetation in the Earth system: A new parameterization scheme for photosynthetic and stomatal responses

Abstract. Surface ozone (O₃) is the primary air pollutant threatening global vegetation. It typically reduces photosynthetic rate and stomatal conductance, leading to changes in carbon, water, and energy cycles, vegetation structure and composition, and climate. Several parameterization schemes have been developed to integrate the photosynthetic and stomatal responses to O₃ exposure in regional and global process-based models to simulate time- and space-varying O₃ plant damage and its cascading dynamic influence. However, these schemes are calibrated based on limited observations and often fail to reproduce the response relationships in observations, impeding accurate assessments of the role of O₃ plant damage in the Earth system. This study proposes a new parameterization scheme to utilize the extensive observations from O₃ fumigation experiments to inform large-scale modeling. It is built on 4210 paired data points of photosynthetic and stomatal responses compiled from peer-reviewed literature, over six times larger than those employed in earlier schemes. Functions of phytotoxic O₃ dose (POD) are found to accurately reproduce the statistically significant linear or nonlinear relationships observed between POD and either relative leaf photosynthetic rate or relative stomatal conductance for needleleaf trees, broadleaf trees, shrubs, grasses, and crops. These eliminate the practice in earlier schemes of setting response functions as constants and applying the response function from one vegetation type to another. It outperforms the old scheme in the Community Land Model (CLM) which skillfully reproduces the observed response for crop photosynthetic rate only. The nonlinear response functions we developed depict decreasing plant sensitivity with increases in POD, enabling models to implicitly capture the variability in plant ozone tolerance and the shift among plant species for both intra- and inter-PFT within a vegetation type observed in the real world. Then, the new scheme is incorporated into the Community Earth System Model version 2.2 (CESM2.2), specifically its land component CLM5, to quantify the global impacts of present-day O₃ plant damage by comparing the simulations with and without O₃ plant damage. Results show that O₃ exposure reduces the global leaf photosynthetic rate by 8.5 % and stomatal conductance by 7.4 %, around half the estimates using the old scheme. Furthermore, the new scheme improves global GPP simulations, decreasing RMSE by 11.1 % relative to simulations without O₃ plant damage and by 11.7 % compared against the old scheme. These results underscore the importance of including O₃ plant damage in large-scale process-based models and the effectiveness of the new scheme in global assessing and projecting the role of O₃ plant damage in the Earth system.