Have improvements in ozone air quality reduced ozone uptake into plants?

Peak levels of ozone (O-3)-quantified by concentration metrics such as accumulated O-3 exposure over a threshold of 40 ppb (AOT40) and the sigmoidal-weighted cumulative exposure (W126)-have decreased over large parts of the United States and Europe in the last several decades. Past studies have suggested that these improvements in AOT40 and W126 indicate reductions in plant injury, even though it is widely recognized that O-3 flux into leaves, not ambient O-3 concentration, is the cause of plant damage. Using a new dataset of O-3 uptake into plants derived from eddy covariance flux towers, we test whether AOT40, W126, or summer mean O-3 are useful indicators of trends in the cumulative uptake of O-3 into leaves, which is the phytotoxic O-3 dose (POD or PODy, where y is a detoxification threshold). At 32 sites in the United States and Europe, we find that the AOT40 and W126 concentration metrics decreased over 2005-2014 at most sites: 25 and 28 sites, respectively. POD0, however, increased at a majority (18) of the sites. Multiple statistical tests demonstrate that none of the concentration metrics-AOT40, W126, and mean O-3-are good predictors of POD0 temporal trends or variability (R-2 <= 0.15). These results are insensitive to using a detoxification threshold (POD). The divergent trends for O-3 concentration and plant uptake are due to stomatal control of flux, which is shaped by environmental variability and plant factors. As a result, there has been no widespread, clear improvement in POD over 2005-2014 at the sites we can assess. Decreases in concentration metrics, therefore, give an overly optimistic and incomplete picture of the direction and magnitude of O-3 impacts on vegetation. Because of this lack of relation between O-3 flux and concentration, flux metrics should be preferred over concentration metrics in assessments of plant injury from O-3.