The U.S. National Park Service uses critical loads to fulfill its mandate to preserve and protect natural resources, including protecting the integrity of native plant communities against the effects of nitrogen (N) deposition.  Most N critical loads protect ecosystems on broad scales such as ecoregions and species alliances.  However, recent research established critical loads for 205 herbaceous species based on conterminous United States (CONUS) presence-absence data for 348 herbaceous species from field-based studies. This research identified at-risk herbaceous species and applied predictive equations to create N critical loads for herbaceous species in national parks. First, we used N critical loads of probability of occurrence to evaluate potential risk to herbaceous species within 244 national parks in CONUS. Second, we used vegetation plots from nine case-study national parks to evaluate individual herbaceous species’ responses to N deposition and create site-specific N critical loads based on total sulfur deposition as well as edaphic and climatic qualities.

All 348 of the selected herbaceous species were found to be present within the 244 national parks included in this study. Seventeen of these species occurred in over 60% of the national park’s vegetation surveys. Critical loads for individual species ranged from 1.3 kg N ha^-1 yr^-1 to 18.95 kg N ha^-1 yr^-1 across the national parks. We found that an average of 17 species per national park were in exceedance of their CL based on maximum total N deposition. Of the species included in this analysis 64 were expected to respond to N deposition with an increase in probability of occurrence, 41 with a decrease, and 131 with an increase followed by a decrease. We found that 266 of the 348 herbaceous species were present in the nine case study parks, with 13,499 occurrences of one or more species within a vegetation plot overall. Using a subset of species that occurred in multiple plots, we found that 96% of the edaphic, temperature, and precipitation data values for these species fell within the range of data used to set the critical load.  This study provides a framework for creating species- and location-specific critical loads, and results of this research will help park managers identify individual species most at risk to negative impacts of N deposition to prioritize future management.