Ground-level ozone is one of many air pollutants that can alter a plant's health and ability to reproduce and can make the plant more susceptible to disease, harsh weather, and other environmental stressors. These impacts can lead to changes in the biological community, both in the diversity of species and in the health and growth of individual species. Emissions of NO_x and other ozone forming air pollutants have decreased over the past two decade due to a changing power sector and the implementation of NOX emission reduction programs for the power and other sectors. However, it is unclear whether changes in emissions are continuing to improve tree health and forest communities as changing weather may offset continued declines in ozone concentrations. We evaluated biomass loss for 10 common tree species in the eastern U.S. that have a higher sensitivity to ozone (black cherry, yellow poplar, American sycamore, chestnut oak, quaking aspen, sweet gun, sugar maple, eastern white pine, Virginia pine, and red maple) before and after changes in NO_X emissions and ozone concentrations on reducing biomass loss of these tree species. This analysis reflects new underlying tree abundance data from the U.S. Forest Service, updated tree-biomass functions published by Lee et.al. 2022, which utilize a new approach that better reflects tree abundance and biomass loss estimates due to ozone exposure. Of these 10 tree species examined, four are less sensitive to ozone exposure (sugar maple, Virginia pine, eastern white pine, chestnut oak) and six are more sensitive (black cherry, yellow poplar, American sycamore, quaking aspen, sweet gum, red maple). This analysis focused on modeled relative percent biomass loss of >5 and >10 percent, which are ecologically important indicators for tree and ecosystem health. Tree biomass loss due to ozone exposure declined sharply as air quality improved between 2000-2002 and 2020-2022. The amount of forest area impacted by ozone exposure with >10 percent biomass loss (most impact) decreased by 88 percent while areas with >5 percent biomass loss declined by 40 percent for all tree species across the eastern U.S. Individually each of the 10 tree species showed decreases in total forest area with ecologically important biomass loss of >5 percent between 2000-2002 and 2020-2022 in the eastern U.S. The four least sensitive tree species are not expected to experience biomass loss above 5 percent due to ozone exposure for the current period. Among the six more sensitive trees, black cherry, yellow poplar, and American sycamore are the only tree species expected to experience ecologically important biomass loss of >5 percent for the current ozone exposure with only black cherry having areas with the more severe level of >10 percent biomass loss.

¹ U.S. Environmental Protection Agency, Office of Air & Radiation, Office of Atmospheric Protection, Washington, DC

² U.S. Environmental Protection Agency, RTP, NC

* Corresponding Author: lynch.jason@epa.gov