Potential vulnerability of 348 herbaceous species to atmospheric deposition of nitrogen and sulfur in the United States
Atmospheric nitrogen (N) and sulfur (S) pollution increased greatly across industrialized temperate regions during much of the 20th century. Despite significant declines in recent decades, N and S deposition continue to negatively affect the distribution of many plant species. We used presence/absence data from >14,000 survey sites across the contiguous US to determine how regional variation in N and S deposition affects the distribution of 348 herbaceous species. We found that 53% of species assessed were negatively affected by N and/or S deposition somewhere in the contiguous US. Species-level critical loads of N deposition, above which detection probability declined, varied widely between species (means from 2.9 to 17.6 kg N ha-1 yr-1) and within species (average range of 8 kg N ha-1 yr-1). Wide within species variation urges caution when using any single threshold for a species, and was due to interactions with S deposition and soil pH which influenced local critical loads. Critical loads for 30 (N) and 139 (S) species could not be quantified as they were lower than the minimum deposition experienced. Approximately 17% and 54% of species were experiencing detrimental levels of deposition over more than half their occurrences for N and S, respectively. Although more species increased than decreased with N deposition, increasers were more likely to be introduced species, and high value natives tended to be more vulnerable. We developed predictive equations relating vulnerability from N deposition to simple traits like leaf magnesium and nitrogen concentrations, suggesting that these results may be generalized to species with trait information. These results can inform ecological theory, as well as the protection of natural communities through air quality policies.