Print PageHuman activity in the last century has led to an exponential increase in nitrogen (N) emissions and deposition. This N deposition has reached a level that has caused or is likely to cause alterations and damage in many ecosystems across the United States. The critical load approach is an ecosystem assessment tool with great potential to simplify and effectively communicate complex scientific information to policy makers and the public. A critical load is defined as the level of a pollutant below which no detrimental ecological effect occurs over the long term according to present knowledge. Empirical critical loads are determined from observations of detrimental responses of an ecosystem to a given, observed N deposition input. This level of N deposition is set as the critical load and extrapolated to other similar ecosystems. Empirical critical loads for N are based on measurements from gradient studies, field experiments, or observations. Empirical critical loads are especially useful in a policy context because they are directly linked to observations of damage to ecosystems in the field.
Empirical critical loads for nutrient N in U.S. ecoregions, inland surface waters, and wetlands range from 1 to 39 kg N ha-1 y-1. This broad range spans the range of N deposition observed over most of the country. The empirical critical loads for N tend to increase in the following sequence for different life forms: diatoms, lichens and bryophytes, mycorrhizal fungi, herbaceous plants and shrubs, trees.
The objective of this on-going analysis is to refine existing critical loads for nutrient N based on finer scale resolution of the biotic and abiotic factors that influence the critical load. The primary factors included in this analysis were land cover and precipitation volume. We used land cover information to constrain the range of critical loads presented by ecoregions for public lands. We also used a simple empirical model to estimate critical loads for lichens using precipitation and ecoregions-specific lichen response thresholds as the main input variables.
Results/Conclusions
We found increased variation within ecoregions for critical loads when we refined our broad analysis using both qualitative and quantitative approaches. Using recent research findings to refine critical loads estimates is important for effective, science-based resource management. Empirical models, such as this lichen model, represent an important next step, because they provide a systematic way of estimating critical loads across large areas and because they pave the way for dynamic model development.
