Sensitive tree species remain at risk despite improved air quality benefits to US forests
Atmospheric nitrogen (N) and sulfur (S) deposition can significantly affect forest biodiversity and production by altering the growth and survival of trees. Three decades of air quality regulations in the United States have led to large reductions in oxides of N (44–81%) and S (50–99%) emissions and associated deposition. Here we evaluated the magnitude and extent of effects over 20 years from atmospheric N and S deposition on the growth and survival of 94 tree species—representing 96.4 billion trees and an average of 88% of forest basal area across the contiguous United States (CONUS). Overall, species’ growth and survival rates have responded positively to declining deposition, but we find that decreases of at least 2.5 kg ha−1 yr−1 N are needed across 19.8% (growth) and 59.5% (survival) of the CONUS to prevent detrimental effects to sensitive species. Reduced forms of N (NHx = NH3 + NH4+) are now the dominant form of N deposition in 45.4% of the CONUS—notably in agricultural regions—and exclusively need to be reduced by ≥5.0 kg ha−1 yr−1 N in some areas. Further S deposition decreases of ≥1.0 kg ha−1 yr−1 S are needed in 50.4% (growth) and 56.2% (survival) of the CONUS to protect sensitive species and, notably, evergreen trees. Total basal area is increasing in much of the country (85.2%) because of N fertilizing effects, but these growth increases could result in biodiversity loss. Our findings can be used to evaluate past successes of air quality policies and the future benefits of air pollution reductions to terrestrial ecosystems.