Atmospheric nitrogen and sulfur pollution increased over much of the United States during the twentieth century from fossil fuel combustion and industrial agriculture. Despite recent declines, nitrogen and sulfur deposition continue to affect many plant communities in the United States, although which species are at risk remains uncertain. We used species composition data from >14,000 survey sites across the contiguous United States to evaluate the association between nitrogen and sulfur deposition and the probability of occurrence for 348 herbaceous species. We found that the probability of occurrence for 70% of species was negatively associated with nitrogen or sulfur deposition somewhere in the contiguous United States (56% for N, 51% for S). Of the species, 15% and 51% potentially decreased at all nitrogen and sulfur deposition rates, respectively, suggesting thresholds below the minimum deposition they receive. Although more species potentially increased than decreased with nitrogen deposition, increasers tended to be introduced and decreasers tended to be higher-value native species. More vulnerable species tended to be shorter with lower tissue nitrogen and magnesium. These relationships constitute predictive equations to estimate critical loads. These results demonstrate that many herbaceous species may be at risk from atmospheric deposition and can inform improvements to air quality policies in the United States and globally.
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The datasets generated during and/or analysed during the current study are available in the EPA Environmental Dataset Gateway repository (https://edg.epa.gov/metadata/catalog/main/home.page) at https://doi.org/10.23719/1500914.
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We thank J. T. Smith, S. Leduc and J. Compton for commenting on earlier versions of this manuscript, as well as the NADP Critical Loads of Atmospheric Deposition (CLAD) Working Group that has provided helpful discussions. We also thank B. Cade at the USGS for his statistical advice throughout the project, and R. Payne, K. Wilkins and C. Stevens for sharing their TITAN data for cross comparisons. The original work underpinning this project (Simkin et al.22) was supported under the USGS Powell Center Working Group ‘Evidence for shifts in plant species diversity along N deposition gradients: a first synthesis for the United States’, EPA Contract no. EP-12-H-000491, National Park Service grant no. P13AC00407 and USGS grant no. G14AC00028. Continuation of this research represented here was supported under the EPA Office of Research and Development Safe and Healthy Communities (no. SHC 4.61.4). US-NSF Dimensions of Biodiversity award DEB-1046355 (DMW) supported collecting the functional trait data. J.B. was supported by the USGS Ecosystems and Land Change Science programs. The USGS supports the conclusions of research conducted by their employees and peer reviews and approves all of their products consistent with USGS Fundamental Science Practices. The views expressed in this manuscript are those of the authors and do not necessarily reflect the views or policies of the US EPA or the USDA Forest Service. Mention of trade names or commercial products is for descriptive purposes only and does not constitute endorsement or recommendation for use by the United States Government.
The authors declare no competing interests.
Peer review information: Nature Plants thanks Nancy Dise, Han van Dobben and Stuart Weiss for their contribution to the peer review of this work.
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Supplementary Methods, Supplementary Table 1, Supplementary Tables 3–6, Supplementary Fig. 1 legend and Supplementary Figs. 2–4.
Full spreadsheet of analytical results for 348 spp. assessed (with metadata included).
GLM graphical results for all 198 species with robust results.
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Nature Plants (2019)