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Conditional vulnerability of plant diversity to atmospheric nitrogen deposition across the United States

  1. Donald M. Wallerq
  1. aInstitute of Arctic and Alpine Research and Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO 80309;
  2. bDepartment of Botany and Plant Sciences, University of California, Riverside, CA 92521;
  3. cCenter for Conservation Biology, University of California, Riverside, CA 92521;
  4. dNational Center for Environmental Assessment, United States Environmental Protection Agency, Washington, DC 20460;
  5. eSouthwest Biological Science Center, United States Geological Survey, Moab, UT 84532;
  6. fWestern Ecological Research Center, United States Geological Survey, Oakhurst, CA 93644;
  7. gFort Collins Science Center, United States Geological Survey, Fort Collins, CO 80226;
  8. hDepartment of Biology, University of New Mexico, Albuquerque, NM 87131;
  9. iPacific Northwest Region Air Resource Management Program, United States Department of Agriculture Forest Service, Corvallis, OR 97339;
  10. jDepartment of Biological Sciences, Marshall University, Huntington, WV 25755;
  11. kForest Inventory and Analysis Program, United States Department of Agriculture Forest Service, Portland, OR 97339;
  12. lNorthern Research Station, United States Department of Agriculture Forest Service, Burlington, VT 05405;
  13. mForest Inventory and Analysis Program, United States Department of Agriculture Forest Service, Anchorage, AK 99501;
  14. nLancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, United Kingdom;
  15. oSchool of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287;
  16. pSchool of Life Sciences, Arizona State University, Tempe, AZ 85287;
  17. qDepartment of Botany, University of Wisconsin, Madison, WI 53706

  1. Edited by Sarah E. Hobbie, University of Minnesota, Saint Paul, Saint Paul, MN, and approved February 23, 2016 (received for review August 4, 2015)

Significance

Human activities have elevated nitrogen (N) deposition and there is evidence that deposition impacts species diversity, but spatially extensive and context-specific estimates of N loads at which species losses begin remain elusive. Across a wide range of climates, soil conditions, and vegetation types in the United States, we found that 24% of >15,000 sites were susceptible to N deposition-induced species loss. Grasslands, shrublands, and woodlands were susceptible to species losses at lower loads of N deposition than forests, and susceptibility to species losses increased in acidic soils. These findings are pertinent to the protection of biodiversity and human welfare and should be considered when establishing air quality standards.

Abstract

Atmospheric nitrogen (N) deposition has been shown to decrease plant species richness along regional deposition gradients in Europe and in experimental manipulations. However, the general response of species richness to N deposition across different vegetation types, soil conditions, and climates remains largely unknown even though responses may be contingent on these environmental factors. We assessed the effect of N deposition on herbaceous richness for 15,136 forest, woodland, shrubland, and grassland sites across the continental United States, to address how edaphic and climatic conditions altered vulnerability to this stressor. In our dataset, with N deposition ranging from 1 to 19 kg N⋅ha−1⋅y−1, we found a unimodal relationship; richness increased at low deposition levels and decreased above 8.7 and 13.4 kg N⋅ha−1⋅y−1 in open and closed-canopy vegetation, respectively. N deposition exceeded critical loads for loss of plant species richness in 24% of 15,136 sites examined nationwide. There were negative relationships between species richness and N deposition in 36% of 44 community gradients. Vulnerability to N deposition was consistently higher in more acidic soils whereas the moderating roles of temperature and precipitation varied across scales. We demonstrate here that negative relationships between N deposition and species richness are common, albeit not universal, and that fine-scale processes can moderate vegetation responses to N deposition. Our results highlight the importance of contingent factors when estimating ecosystem vulnerability to N deposition and suggest that N deposition is affecting species richness in forested and nonforested systems across much of the continental United States.

Footnotes

  • ↵1To whom correspondence should be addressed. Email: samuel.simkin{at}colorado.edu.

  • Author contributions: S.M.S., E.B.A., W.D.B., C.M.C., J.B., and M.L.B. designed research; S.M.S., E.B.A., W.D.B., and C.M.C. performed research; S.M.S. and C.M.C. analyzed data; and S.M.S., E.B.A., W.D.B., C.M.C., J.B., M.L.B., B.S.C., S.L.C., L.H.G., F.S.G., S.E.J., L.H.P., B.K.S., C.J.S., K.N.S., H.L.T., and D.M.W. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • Data deposition: The data reported in this article have been deposited in the Dryad Digital Repository, datadryad.org (doi: 10.5061/dryad.7kn53).

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1515241113/-/DCSupplemental.

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