Critical loads of nitrogen deposition and exceedance of critical loads are being used increasingly by resource managers and policy makers to assess the risk of detrimental effects to forests. A recent analysis has allowed us to calculate critical loads for tree growth and survival in a systematic way for over fifty species across the continental US.

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.

We used these data to set critical loads for growth (for 32 species) and survival (for 54 species) for individual tree species and for level 3 ecoregions across the continental US. Using FIA expansion factors we were able to assess the extent of forest area at risk from nitrogen deposition. These species-specific responses will allow resource managers to understand the likely impacts from atmospheric nitrogen deposition and to manager their forests accordingly. The critical loads and exceedance maps will allow policy makers to assess the likely impacts of current and future deposition scenarios.