A continental analysis of ecosystem vulnerability to atmospheric nitrogen deposition

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 for15,136 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

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

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

Sulfur And Wetland Plant Diversity: Calcareous Rich Fens As Model Systems

Plant diversity in groundwater-fed wetlands is typically extraordinarily high, yet the biogeochemical controls of this diversity are still incompletely understood. I hypothesized that plant community composition could be related to a combination of fine-scale and broad-scale variation in sulfide via direct phytotoxicity and indirect mediation of phosphorus release from iron, coupled with gradients in other chemical constituents such as calcium. I measured porewater chemistry and associated plant species composition at nine groundwater-fed wetlands (rich fens), including one rich fen in which I intensively sampled 400 locations to capture finescale heterogeneity. Porewater sulfate and calcium concentrations were higher at the intensively sampled fen overlying gypsum geology than at other rich fens. Sulfide was highly variable within and across fens, ranging over two orders of magnitude in many fens. Inversely related concentrations of sulfide and ferrous iron in porewater were consistent with tight chemical coupling but were not readily traceable to phosphorus availability. Spatial patterns of sulfide and ferrous iron were conserved across seasons, with sulfide peaking with temperature in summer and ferrous iron peaking in fall at intermediate temperature. I used the corrected Akaike information criterion (AICc) to select among competing models of toxin, nutrient, and mixedchemistry influences on vegetation. In the intensively sampled fen, models with a negative sulfide parameter provided the best explanation of total plant cover, cover of the three most frequently occurring species, dicot species density, and plant height. Calcium and phosphorus combined with sulfide to explain some plant responses, but phosphorus alone did not explain any plant responses at the fine scale. Sulfide had a limited relationship with vegetation at the regional scale, only secondarily explaining total plant cover after first accounting for site-to-site variability. Gamma diversity values for individual sites were a negative power function of within-site sulfide variability values, with average alpha diversity for each site dominating. Overall, results from this work confirmed the relationship of rich fen vegetation to calcium and suggested that direct sulfide toxicity was a persistent but more moderate than expected stress to rich fen plants, while indirect sulfide mobilization of phosphorus was less important to plants than sulfide toxicity

Simkin_et_al_2016_data_from_PNAS_Diversity_and_N_deposition

Each row in the "Simkin_et_al_2016_data_from_PNAS_Div_and_N_dep.csv" dataset represents one of 15,136 sites, and each of the 16 columns is a separate variable. The "README.csv" file contains descriptions corresponding to the 16 column header names, including the site coordinates (latitude and longitude), the response variable (species richness), the environmental variables (niitrogen deposition, precipitation, temperature, and pH), the output variables (critical loads of nitrogen with 95% confidence interval, as well as corresponding exceedance values), and assorted vegetation classification and data source variables. The "README.csv" also has further details about the multiple vegetation data sources corresponding to the abbreviated values in the "proj_orig" variable. The description of vegetation data sources is a text file adaptation of Table S1 in the Supplemental Material of Simkin et al. 2016 in the PNAS paper

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

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

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

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