Anthropogenic nitrogen and sulfur deposition can have negative impacts on forested ecosystems. Negative responses begin to occur when deposition exceeds a threshold referred to as a critical load. Lichen are generally more sensitive to increased levels of nitrogen and sulfur deposition than other communities within the forest which makes them effective bioindicators of forest health. This study uses critical loads associated with a decrease in lichen species richness to determine the effects of sulfur and nitrogen deposition on ecosystems within national parks. The goal of this study was to identify how many national parks are in exceedance of lichen critical loads and how this varies spatially within a park’s boundary. Additionally, the change in the risk to lichen species richness was compared between 2000 and 2017 to identify areas of improvement, degradation, and continued high risk. We evaluated the critical load exceedances using Total N and Total S estimates from the NADP Total Deposition Model (2000-2002 and 2015-2017). Forested national park units were characterized using the National Land Cover Database.

Our analysis determined that 229 National Park Service units had forested ecosystems, of which 209 were identified as having exceeded the lichen species richness critical load for nitrogen and 8 for sulfur deposition. For nitrogen, we determined that between 2000 and 2017, 158 National Park Service sites exhibited an improvement in conditions for lichen communities, while 29 sites degraded based on current deposition levels. For sulfur, 213 sites exhibited an improvement in conditions for lichen communities, while 11 sites degraded. For example, between 2000 and 2017 the area in exceedance in Acadia National Park decreased from 12,757 acres to 22 acres and the maximum decrease in richness due to N deposition went from 44% to 27%. Our research will be influential for researchers determining high risk areas within National Park Service forested communities as well as identifying areas of improvement that can be used to conduct targeted recovery surveys. Results identifying degraded areas will be useful for specific parks to locate regions of decreased forest health.