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.