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, 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.nitrogen deposition | plant species richness | diversity | soil pH | climate

Growth and survival relationships of 71 tree species with nitrogen and sulfur deposition across the conterminous U.S.

“…Unresolved complexity in geographic, biotic, and other factors have led to non-uniform responses of individual tree species to N deposition, as seen in both gradient studies [4–8] and ecosystem-scale N addition experiments [2, 9–11]. Evaluating how tree species differ in their response to N deposition is critical for assessing the resilience of forests to chronic N deposition and the resulting impacts on forest composition [3, 12]. …”

Human Alteration of the Nitrogen Cycle and Its Impact on the Environment

“…Nitrogen is the major component of DNA, RNA, and organic molecules like proteins. [1] Although the Earth's surface contains 78 percent nitrogen gas, most organisms cannot use it directly, as they do with oxygen and carbon dioxide. This is because the triple bonds between nitrogen atoms in a N2 molecule make it unreactive, while living organisms utilize relatively reactive nitrogen.…”

How climatic conditions, site, and soil characteristics affect tree growth and critical loads of nitrogen for northeastern tree species

“…Recent work has improved our understanding of plant response to N deposition (Bobbink and Hettelingh 2011, Pardo et al 2011c, Simkin et al 2016) and climate change (Campbell et al 2009, Groffman et al 2012, Iverson et al 2008, Ollinger et al 2008. However, less is known about the impact of interactions between N deposition and climate change on plant communities (Porter et al 2013).…”