Ecological thresholds under atmospheric nitrogen deposition for 1200 herbaceous species and 24 communities across the United States
Nitrogen (N) emission and atmospheric deposition has increased significantly during the last century and has become a stressor for many N-sensitive plant species. Understanding individual and community herbaceous plant species thresholds to atmospheric N deposition can inform emissions reduction policy. Here we present results using Threshold Indicator Taxa Analysis (TITAN) applied to more than 1200 unique plant species and 24 vegetation communities (i.e., alliances) across the United States (U.S.), to assess vulnerability to N deposition. Alliance-level thresholds (change points) for species decreasing in abundance along the gradient ranged from 1.8¿14.3 kg N ha¿1 yr¿1 and tended to be lower in the west than the east, which suggests that eastern communities, where N deposition has been historically higher, may have already lost many sensitive species. For the species that were present in more than one alliance, over half had a variable response to the N deposition gradient, suggesting local factors affect vulnerability. Significant progress has been made during the past 30 years to reduce N emissions, which has reduced the percentage of plots at risk to N deposition from 72% to 35%. Nevertheless, over a third of plots remain at risk, and an average reduction of N deposition of 20.2% would protect half of the plots from exceedances.