Assessing critical loads nationwide and in relationship to Native American tribal lands

Jason Lynch1 and Clara Funk2

Anthropogenic atmospheric nitrogen (N) and sulfur (S) deposition from air pollution have impacted our lakes and streams, forests, grasslands, and lichen communities across the U.S.  The good news is that emission reduction programs for sulfur dioxide (SO2) and nitrogen oxide (NOx), such as the Acid Rain Program and Cross-State Air Pollution Rule, have reduced N and S deposition mostly in the eastern U.S., allowing for some ecosystems to begin to recover from air pollution impacts.  In some areas of the U.S., the reductions in N and S deposition is now protective of some ecosystem elements. However, N and S deposition for some regions remains high or is increasing, making it hard to know whether current levels of deposition are protective of ecosystems.  Here we completed a national assessment using six different critical load endpoints and there exceedances to assess the impact of N and S deposition on our natural ecosystems.  Given that Native American tribal lands contain many natural areas, we also compared critical loads and their exceedances on tribal lands to estimated values of the surrounding region and nationwide.  Contrasting these results will help to understand whether natural ecosystems on tribal lands are impacted at a higher level than the nationwide average.  The critical load endpoints assessed included: forest ecosystems, herbaceous plants, lichens, nitrate leaching, forest soil and surface water acidity.  Forest ecosystems, herbaceous plants, lichens, nitrate leaching for N are empirical critical loads based on Pardo et al. 2011.  The forest soil and surface water critical loads for acidity are steady-state mass balance critical loads based on multiple approaches and methods.  All critical loads are part of the National Critical Load Database (NCLD) developed by the Critical Load of Atmospheric Deposition (CLAD) science committee as part of the National Atmospheric Deposition Program (NADP).  Ecological impacts of a given endpoint were assumed to occur when deposition is greater or exceeds the critical load.  Critical load exceedances for N and N+S were based on annual values of Total Deposition (Tdep) for the period from 2001-2014.   Urban and agriculture lands were excluded from this analysis using National Land Cover Data (NLCD).