Critical loads of nitrogen and critical loads of sulfur for herbaceous species in national parks

Russell, Nicholas; Bell, Michael; Felker-Quinn, Emmi; Clemens, Cody

doi:10.36967/2294131

Herbaceous species comprise large portions of plant biodiversity and play a pivotal role in maintaining ecosystem health and function. Excessive deposition of atmospheric pollutants, such as nitrogen (N) and sulfur (S), can have harmful effects on herbaceous species leading to changes in community composition and disruptions in ecosystem services. To monitor the effects of deposition, critical loads—deposition thresholds below which harmful ecosystem effects do not occur—have been established for herbaceous species for ecoregions and open and closed canopy environments. While useful, these broad categorizations cannot capture the variation of individual species responses to deposition. Clark et al. (2019) developed critical loads of N and of S for a decline in probability of occurrence of 348 individual herbaceous species using species locations and local pH, temperature, and precipitation data. Species specific critical loads allow land managers to assess overall risk to herbaceous species communities as well as monitor individual species that may be sensitive to N or S deposition. This report applies the critical loads developed by Clark et al. (2019) to assess risk to herbaceous species in national parks. It uses the 198 focal herbaceous species from the original 348 herbaceous species, that were shown to have a strong and statistically significant responses to nitrogen and sulfur deposition. Species presence-absence data was used to identify the park occurrences of the 198 focal herbaceous species from Clark et al. The critical loads for these species were then compared to long-term N and S deposition from 1985–2011 and short-term deposition from 2016–2018 summarized to national parks. Critical load percentiles at the minimum, 10th percentile, median, and maximum levels were also calculated for each park. Tenth percentile critical loads were emphasized in this study as they provide a level of deposition at which 90% of species in a park are protected and highlights the species in a park that are most sensitive to deposition. Plant species responded to the over fertilization of N deposition or the soil acidification of S deposition. Greater atmospheric concentrations of N and S were experienced in the Midwestern and Eastern parks exposing more plant species in those parks to higher N and S deposition resulting in more Clark et al. species reached threshold deposition in parks in those areas. The highest number of species with N deposition in exceedance of their critical load of N were in national parks in the eastern United States and the Midwest. The greatest decline in the number of N critical load exceedances between the two deposition datasets were in parks in the eastern United States while the greatest increase in N critical load exceedances were in midwestern parks. Exceedances of S critical loads declined in all parks across the United States, with the largest declines in eastern parks. Of the 198 species in this analysis, 135 had their critical load of N exceeded in at least one national park and 95 had their critical load of S exceeded in at least one national park. Although there has been widespread improvement in deposition, there are still parks with large numbers of herbaceous species N and S critical load exceedances that may be likely to experience shifts in plant species composition. Direct intervention and management are needed to prevent detrimental effects to ecosystem services and visual resources in parks. Identifying species sensitive to N and S within parks allows park resource managers to make informed management decisions and assists in restoration efforts and curbing invasive species. This study also contributes a dataset comprised of the critical load values and percentiles which park resource managers can use to monitor species and develop focused intervention strategies.

Critical loads of nitrogen and critical loads of sulfur for herbaceous species in national parks

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