SYMP 17-6
Synthesis of critical loads research for the U.S.: State-of-the-science under current and changing climate and a roadmap forward

Thursday, August 13, 2015: 10:40 AM
308, Baltimore Convention Center
Christopher M. Clark, National Center for Environmental Assessment, US Environmental Protection Agency, Washington, DC

Nitrogen and sulfur deposition are two of the major stressors to terrestrial and aquatic ecosystems nationally and internationally. Despite major advances in the past few decades to reduce emissions of this stressor in the eastern U.S., it remains uncertain whether these reductions are sufficient to protect sensitive ecological end points, and whether these trends will spread to other regions nationally. In addition, climate and climate change are likely to affect ecological vulnerability to this stressor through multiple processes linked to stressor inputs and complex internal feedbacks. Here we synthesize information from this symposium and from other sources to provide an overview of current knowledge of critical loads in the U.S. and how these may be affected by climate change.


There have been several recent advances in the field of critical loads research in the U.S. over the past few years. Researchers have updated estimates of sensitivity for epiphytic lichen nationally, reporting new areas of concern and clarifying key mechanistic processes. A large national-scale effort for understory herbs demonstrates that diversity does not decrease everywhere with deposition, but that there are large areas of the country where edaphic and climatic conditions enhance vulnerability to species losses even at low levels of deposition. Furthermore, species level analysis identifies key invasive species that increase with deposition, and ecologically valuable herbaceous species that decrease with deposition. A parallel effort on forest trees nationally covering 114 species identifies which species increase, decrease, and are unaffected by current deposition, and has developed an extensive database for linking changes in soil chemistry with tree physiological response.

We synthesize these efforts to develop a coherent picture for the state-of-knowledge for critical loads research in the U.S. We discuss varying sensitivities across ecological end points, how these individual end points can be combined to assess ecosystem sensitivity, and how the sensitivity of these end points are affected by climate and climate change. We discuss key knowledge gaps, and start to link these effects on ecological receptors to changes in ecological function and ecosystem services. Finally, we discuss various approaches for quantifying uncertainty, and present a suggested roadmap for future research.