Growth of temperate forests is impacted by atmospheric deposition of sulfur (S) and nitrogen (N). Both S and N deposition are the primary constituents of acid rain; however, N deposition also has eutrophying effects and contributes to other mechanisms that impact tree growth and survival. Often, the impacts of S and N deposition are concurrent. Additionally, individual tree responses to S and N deposition are species-specific and depend on site conditions in which the species occur. Understanding the separate and concerted impacts of S and N deposition on tree growth and survival will help identify how tree demographics of whole forests are changing and be invaluable in setting limits for S and N deposition to protect forests.
We used United States Forest Service Forest Inventory and Analysis (FIA) data with modeled S and N deposition rates and climate data to identify species-specific responses of trees associated with S and N deposition across the conterminous United States. Growth and survival estimates were made from repeated measurements of more than 1.1 million individual trees measured between 2000 and 2016. The shape of growth and survival responses to N deposition were allowed to be increasing, decreasing, increasing then decreasing, or flat; while responses to S deposition were assumed to be decreasing or flat.
Preliminary growth and survival results indicate that 74 and 66 tree species were associated with S and/or N deposition respectively, and only 5 species of the 94 species examined indicated no relationship with S or N deposition in either growth or survival. Of the 94 tree species, 30 increased in growth with N deposition, 6 decreased, and 19 increased then decreased; and for survival, 5 increased, 6 decreased, and 26 increased then decreased across N deposition gradients. In the analysis, we only allowed non-positive responses to S deposition, for which 41 and 51 of the 94 species showed negative growth and survival responses respectively.
Without S deposition in the models, growth and survival responses to N deposition decreased significantly for several species regardless of the correlation between S and N deposition. Nonetheless, forest sensitivities to S and N deposition were species-specific. Atmospheric S and N deposition has likely and will continue to differentially affect forest composition through these individual species responses, and species-dependent ecosystem services will also shift with changes in deposition rates.