Abstract

Exposure to ambient ozone (O-3) impacts vegetation through cascading effects. Numerous tree species experience biomass loss and increased mortality following O-3 exposure. Quantifying levels of O-3 at which deleterious impacts occur to individual trees under in situ conditions is essential for the effective management of air and forest resources. Previously, O-3 critical levels (CLs) were established based on controlled seedling experiments. However, seedling experiments may fail to reflect O-3 impacts on mature trees. In this study, we report the first results for empirically derived, species-specific O-3 CLs for tree growth and survival using a database of similar to 1.5 million trees observed over time across the conterminous United States. Using a machine learning approach, we calculated O-3 CLs for 88 tree species (growth: 72 species; survival: 51 species). To our knowledge, most tree species we assessed have no prior determination of O-3 CLs. Estimated O-3 CLs (W126) for 5% decline in the tree growth rate ranged from 1.9 to 37.6 ppm-hr (mean: 10.5 ppm-hr), and estimated O-3 CLs for 1% tree survival probability decline ranged from 1.9 to 37.9 ppm-hr (mean: 9.3 ppm-hr) across species. Recently (2016-2018), portions of the western United States exceeded O-3 CLs for nearly all tree species for both growth and survival. Nationally, O-3 exposure levels were below the growth CLs for most species, while levels may have exceeded the survival CLs for some species. Our results provide new evidence of the range and uncertainty of impacts of ozone exposure on trees across the United States.