Differential vulnerability of 348 herbaceous species to atmospheric deposition of nitrogen and sulfur across the contiguous U.S.

Christopher Clark1, Sam Simkin2, Edith Allen3, William Bowman4, Jayne Belnap5, Matthew Brooks6, Scott Collins7, Linda Geiser8, Frank Gilliam9, Sarah Jovan10, Linda Pardo11, Bethany K. Schulz12, Carly J. Stevens13, Katharine N. Suding14, Heather L. Throop15 and Donald M. Waller16

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.

 

1US EPA, Clark.Christopher@epa.gov
2National Ecological Observatory Network, samuel.simkin@colorado.edu
3University of California, eallen@ucr.edu
4University of Colorado, William.Bowman@colorado.edu
5US Geological Survey, jayne_belnap@usgs.gov
6US Geological Survey, mlbrooks@usgs.gov
7University of New Mexico, scollins@unm.edu
8USDA Forest Service, lgeiser@fs.fed.us
9Marshall University, gilliam@marshall.edu
10USDA Forest Service, sjovan@fs.fed.us
11USDA Forest Service, lpardo@fs.fed.us
12USDA Forest Service,
13Lancaster Environment Centre,
14University of Colorado,
15New Mexico State University,
16University of Wisconsin,