SYMP 17

Synthesizing Ecosystem Responses to Nitrogen Deposition in the U.S. in a Changing Climate: Are There Useful Indicators Across Systems and Scales?

Nitrogen (N) deposition continues to present a significant threat to ecosystem health in the U.S., however the complexity of N cycling in terrestrial ecosystems can make it difficult to identify response patterns and thresholds. Better characterization of responses to N deposition under context of varying environmental conditions is essential to predicting the health of ecosystems in the context of climate change. Large-scale datasets have allowed, for the first time, analyses of responses of key receptors such as lichens, herbaceous plants, and trees to N deposition at the national scale. These empirical analyses have the advantage of integrating existing N deposition gradients with many other factors influencing biotic responses and allowing national-scale patterns to be described. This work helps us both understand the current condition of these three key receptors and develop deposition-response relationships to evaluate potential responses to future N deposition in terms of species-specific growth, plant diversity, or shifts in community composition. This symposium will also explore how, by incorporating climate and integrating analyses across receptors, we can improve our understanding of ecosystem response to N deposition and our ability to choose the appropriate indicators of ecosystem susceptibility. While the comparative analysis is most robust in the Northeast, where empirical data for each receptor were collected from the same plots, assessment across the US is valuable, providing insights into the most sensitive ecosystems that may be used to set ecosystem critical loads for N deposition. The critical load--the deposition below which no harmful effect occurs over the long term--provides an ecosystem assessment tool with great potential to simplify complex scientific information and communicate effectively with the policy community and the public. The findings of these analyses are highly relevant to policymakers in assessing likely impacts of N deposition and considering how reductions in air pollution might affect terrestrial ecosystems; they are also useful to resource managers in identifying sensitive species and possible management strategies. Incorporating climate factors into these empirical critical loads, which have a much improved scientific basis because of the scope of data analyzed, represents a tremendous advance in their reliability and utility for assessing the impacts of multiple stressors. Speaker will be asked to (1) interpret their data in the context of the responses of other receptors, and (2) consider the best approaches to use for setting critical loads and identifying indicators in the context of a changing climate.