Limited effect of ozone reductions on the 20-year photosynthesis trend at Harvard forest

Ozone (O₃ ) damage to leaves can reduce plant photosynthesis, which suggests that declines in ambient O₃ concentrations ([O₃ ]) in the United States may have helped increase gross primary production (GPP) in recent decades. Here, we assess the effect of long-term changes in ambient [O₃ ] using 20 years of observations at Harvard forest. Using artificial neural networks, we found that the effect of the inclusion of [O₃ ] as a predictor was slight, and independent of O₃ concentrations, which suggests limited high-frequency O₃ inhibition of GPP at this site. Simulations with a terrestrial biosphere model, however, suggest an average long-term O₃ inhibition of 10.4% for 1992-2011. A decline of [O₃ ] over the measurement period resulted in moderate predicted GPP trends of 0.02-0.04 μmol C m^-2  s^-1  yr^-1 , which is negligible relative to the total observed GPP trend of 0.41 μmol C m^-2  s^-1  yr^-1 . A similar conclusion is achieved with the widely used AOT40 metric. Combined, our results suggest that ozone reductions at Harvard forest are unlikely to have had a large impact on the photosynthesis trend over the past 20 years. Such limited effects are mainly related to the slow responses of photosynthesis to changes in [O₃ ]. Furthermore, we estimate that 40% of photosynthesis happens in the shade, where stomatal conductance and thus [O₃ ] deposition is lower than for sunlit leaves. This portion of GPP remains unaffected by [O₃ ], thus helping to buffer the changes of total photosynthesis due to varied [O₃ ]. Our analyses suggest that current ozone reductions, although significant, cannot substantially alleviate the damages to forest ecosystems.