Recent advances for critical loads of herbaceous species and influences of climate change

Christopher Clark1 and Samuel Simkin2

Nitrogen deposition is known to be a major stressor to herbaceous species across most industrialized areas owing to the potential for both acidification and eutrophication to influence community dynamics. Previous work (Simkin et al. PNAS 2016) quantified the critical loads for changes in total herbaceous species richness across the contiguous U.S., using data from over 15,000 plots nationwide. Here we expand that research to examine which species are positively, negatively, and unaffected by N deposition to disentangle which species are responsible for the aggregate patterns reported earlier. We found that across the 188 species with adequate data and robust statistical results, there were many species relationships with N deposition, including increasers (11%), decreasers (13%), non-responders (21%), and hump-shaped responders (55%). Critical loads varied widely across species, from <2 kg N ha-1 yr-1 to almost 18 kg N ha-1 yr-1). When examined geographically across the country, it was clear that there were “winners and losers” co-occurring almost everywhere in the country, with the highest numbers of losers in the upper Midwest and parts of the east. This work will help identify areas and species of particular concern from impacts due to N deposition, and help inform policy decisions on national air quality under the NAAQS.


1National Center for Environmental Assessment, United States Environmental Protection Agency,
2University of Colorado Boulder,