Quantification of Ozone Exposure Impacts and Their Uncertainties on Growth and Survival of 88 Tree Species Across the United States

Exposure to ambient ozone (O3) impacts vegetation through cascading effects. Numerous tree species experience biomass loss and increased mortality following O3 exposure. Quantifying levels of O3 at which deleterious impacts occur to individual trees under in situ conditions is essential for the effective management of air and forest resources. Previously, O3 critical levels (CLs) were established based on controlled seedling experiments. However, seedling experiments may fail to reflect O3 impacts on mature trees. In this study, we report the first results for empirically derived, species-specific O3 CLs for tree growth and survival using a database of ∼1.5 million trees observed over time across the conterminous United States. Using a machine learning approach, we calculated O3 CLs for 88 tree species (growth: 72 species; survival: 51 species). To our knowledge, most tree species we assessed have no prior determination of O3 CLs. Estimated O3 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 O3 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 O3 CLs for nearly all tree species for both growth and survival. Nationally, O3 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.

Impact/Purpose

Exposure to ozone pollution can harm plants, including trees, and understanding the amount of ozone that leads to tree damage is important for managing natural resources. The amount of ozone that harms tree seedlings for a limited number of species has been determined in prior research, but less is known about the impacts of ozone on mature trees. There is also little or no information about the impacts to most tree species in the United States. In this research, we used a large dataset of trees that were monitored over time, combined with information about historical ozone concentrations, to quantify the effects of ozone on mature trees from 88 species. Our results show that, nationally in recent years, ozone may have reduced the chance of survival for some tree species, but little impact on tree growth was found. In the western United States, however, both the growth and survival of tree species was reduced by ozone exposure. Our results provide a new resource to understand how ozone may impact trees as well as the uncertainty of those impacts. These results can help improve management of natural resources.

Citation

Pavlovic, N., S. Chang, K. Craig, C. Scarborough, Justin G. Coughlin, Jeffrey D. Herrick, AND C. Driscoll. Quantification of Ozone Exposure Impacts and Their Uncertainties on Growth and Survival of 88 Tree Species Across the United States. American Geophysical Union, Washington, DC, 130(12):e2024JD042063, (2025). [DOI: 10.1029/2024JD042063]

DOI: Quantification of Ozone Exposure Impacts and Their Uncertainties on Growth and Survival of 88 Tree Species Across the United States