title: Conditional vulnerability of plant diversity to atmospheric nitrogen deposition across the United States.
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.
authors.affiliation: Institute of Arctic and Alpine Research and Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO 80309; samuel.simkin@colorado.edu.
authors.forename: Samuel M
authors.initials: SM
authors.lastname: Simkin
authors.affiliation: Department of Botany and Plant Sciences, University of California, Riverside, CA 92521; Center for Conservation Biology, University of California, Riverside, CA 92521;
authors.forename: Edith B
authors.initials: EB
authors.lastname: Allen
authors.affiliation: Institute of Arctic and Alpine Research and Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO 80309;
authors.forename: William D
authors.initials: WD
authors.lastname: Bowman
authors.affiliation: National Center for Environmental Assessment, United States Environmental Protection Agency, Washington, DC 20460;
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Clark
authors.affiliation: Southwest Biological Science Center, United States Geological Survey, Moab, UT 84532;
authors.forename: Jayne
authors.initials: J
authors.lastname: Belnap
authors.affiliation: Western Ecological Research Center, United States Geological Survey, Oakhurst, CA 93644;
authors.forename: Matthew L
authors.initials: ML
authors.lastname: Brooks
authors.affiliation: Fort Collins Science Center, United States Geological Survey, Fort Collins, CO 80226;
authors.forename: Brian S
authors.initials: BS
authors.lastname: Cade
authors.affiliation: Department of Biology, University of New Mexico, Albuquerque, NM 87131;
authors.forename: Scott L
authors.identifier: http://orcid.org/0000-0002-0193-2892
authors.initials: SL
authors.lastname: Collins
authors.affiliation: Pacific Northwest Region Air Resource Management Program, United States Department of Agriculture Forest Service, Corvallis, OR 97339;
authors.forename: Linda H
authors.initials: LH
authors.lastname: Geiser
authors.affiliation: Department of Biological Sciences, Marshall University, Huntington, WV 25755;
authors.forename: Frank S
authors.initials: FS
authors.lastname: Gilliam
authors.affiliation: Forest Inventory and Analysis Program, United States Department of Agriculture Forest Service, Portland, OR 97339;
authors.forename: Sarah E
authors.initials: SE
authors.lastname: Jovan
authors.affiliation: Northern Research Station, United States Department of Agriculture Forest Service, Burlington, VT 05405;
authors.forename: Linda H
authors.initials: LH
authors.lastname: Pardo
authors.affiliation: Forest Inventory and Analysis Program, United States Department of Agriculture Forest Service, Anchorage, AK 99501;
authors.forename: Bethany K
authors.initials: BK
authors.lastname: Schulz
authors.affiliation: Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, United Kingdom;
authors.forename: Carly J
authors.initials: CJ
authors.lastname: Stevens
authors.affiliation: Institute of Arctic and Alpine Research and Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO 80309;
authors.forename: Katharine N
authors.initials: KN
authors.lastname: Suding
authors.affiliation: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287; School of Life Sciences, Arizona State University, Tempe, AZ 85287;
authors.forename: Heather L
authors.initials: HL
authors.lastname: Throop
authors.affiliation: Department of Botany, University of Wisconsin, Madison, WI 53706.
authors.forename: Donald M
authors.initials: DM
authors.lastname: Waller
mesh_terms: D001272:Atmosphere; D044822:Biodiversity; D009584:Nitrogen / Q000032:analysis*; D010944:Plants / Q000145:classification*; D014481:United States
chemical_list: D009584:Nitrogen
keywords: climate; diversity; nitrogen deposition; plant species richness; soil pH