Nitrogen and sulfur deposition can lead to eutrophication and acidification of soils that change competitive dynamics of herbaceous plant species. This leads to changes in individual plant species presence and shifts in community composition. Previous application of these critical loads has been limited to a small set of points that have limited occurrences in federal lands and a single raster that applies one community to the contiguous United States. This presentation will review two new methods and one new data product to help assign and communicate critical loads for declines in herbaceous species to federal lands.

The first new method uses species location data for the 198 herbaceous species with modeled critical load responses from the iNaturalist database to expand the number of observations used to apply CL to an area of interest. There were 876,873 observations within the contiguous United States, which was filtered to 571,323 observations when removing duplicates and limiting a species distribution to its environmental hull. These additional observations increased the number of NPS units with data from 206 to 261 and the number of USFS units with observations from 149 to 398. Additionally, it allows for a relative ranking of visitor interest in a species based on how observations are uploaded.

With the expansion of critical load data into new areas, it is important for users to understand the implications of species changes. To support this effort, the US Forest Service published a new report that provides information on the (1) distribution and conservation status, (2) species characteristics and habitat, (3) wildlife and livestock uses, (4) other ecosystem services, (5) traditional ecological knowledge, (6) responses to N and S deposition, climate, soil pH, including interactions, and (7) critical loads of each herbaceous species. The data will help users interpret the ecological importance of the species that have critical load exceedances and what other stressors may be impacting them.

The second method takes advantage of a new ecosystem classification product developed by NatureServe to apply regionally-developed critical loads for herbaceous community dynamics. The map displays the distribution of regional ecosystems for North America using the International Vegetation Classification (IVC) Group level. This allows for an accurate extrapolation of critical loads that were developed in a specific ecosystem and integration into our national critical load map. Importantly, this also allows for ecosystems that have shown to be resilient to deposition to be excluded from risk analyses.