We have developed a GIS tool (N-CLAS) that allows federal land managers, policy makers, and other interested parties to calculate site-based critical loads of nitrogen (N) and exceedance for the northeastern United States.  N-CLAS is based on species-specific tables that set optimal growth thresholds for multiple soil, site, and climate parameters.  N-CLAS critical loads can be calculated for all species at a site or a subset of species.  Calculations determine whether growth conditions are optimal or sub-optimal. When conditions are sub-optimal, the site will be more susceptible to detrimental impacts of N deposition, and the adjusted critical load will be in the lower half of the critical load range. Changing site conditions will result in altered critical loads.  Climate change, harvesting, insect pests, fungal pathogens, and natural disasters can all affect growth conditions, site N-cycling, and critical loads.  Here we use maps and tables generated with N-CLAS to compare critical loads of N and exceedance for Class I areas in the Northeastern United States under current conditions and with the Hadley CM3 high (no change in current emissions) climate change scenarios.