Site-specific 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:10.36967/2294529

Herbaceous species (vascular plants with non-woody stems such as grasses, flowering plants, and ferns) in national parks play an integral role in maintaining biodiversity and ecosystem function. The diversity and distribution of herbaceous species are dependent on local climate and soil conditions, some of which are altered by deposition of anthropogenic air pollutants nitrogen (N) and sulfur (S). Clark et al. (2019) developed critical loads of N (CLN-HS) and critical loads of S (CLS-HS) for 198 herbaceous species which represent the maximum annual deposition that an herbaceous species can tolerate with no decrease in the probability of occurrence. Here we apply critical loads for the 198 species to location data from the NPS Vegetation Mapping Inventory (VMI) for 209 park units and assess the risk of decline based on current deposition. This report is a companion to a report which used park species lists from NPSpecies and park-wide estimates of deposition to assess risk from deposition to park herbaceous communities (Russell et al. 2022). By contrast, this report uses site-specific species presence and deposition levels based on the plot location. For the 106 herbaceous species with a threshold response to N deposition, soil pH, temperature, and precipitation data specific to VMI plot locations were used to calculate site-specific CLN-HS, which reflect the influence of other environmental factors on plant sensitivity to deposition. Species with an increasing or decreasing response to N or S deposition have static CLs that do not incorporate other environmental factors into their CL calculations. Deposition exceedances of plot level critical loads of N and of S were assessed against total N and S deposition from 2016–2018 to determine expected decline in probability of occurrence. The plots used to determine the critical loads are mostly distributed in the Midwest and eastern United States which limits the number of species within the study for western parks; especially in the desert southwest. Across all parks, there was an average of 24 species with a CLN-HS and 20 species with a CLS-HS. Parks to the east of the Rocky Mountains had an average of 45 species with a CLN-HS and 48 species with a CLS-HS, while parks to the west of the Rocky Mountains had an average of 11 species with a CLN-HS and 12 species with a CLS-HS. The difference in these numbers are due to the plots within the original dataset from which the CLs were calculated being weighted towards the east coasts and thus the western species may not be represented in the dataset. Nine case study parks were selected to evaluate regional differences in CLN-HS and CLS-HS exceedances and provide an example of how the data can be used assess the spatial distribution of species at risk to negative impacts from N and S deposition. From the case study parks, midwestern and eastern parks had over 10% of their flora represented by the species in the analysis, while parks west of the Rocky Mountains all had under 10% of their flora represented. Along with establishing local responses to deposition, park and resource managers can use site-specific critical loads to develop targeted management plans for species or sensitive areas at risk within their parks. Each site has a species list of occurrences and risk of response to current deposition that can assist in active management of biodiversity and ecosystem health. Additionally, the NPS VMI survey plots can be used to expand the number of species that meet minimum model requirements and increase the number of species for which we can model critical load responses.

Site-specific critical loads of nitrogen and critical loads of sulfur for herbaceous species in national parks

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