Most critical load research in the United States aims to protect ecosystems from the fertilizing effects of nitrogen deposition or from the acidifying effects of combined nitrogen and sulfur deposition.  An analysis by Horn et al (2018) generated a set of species-specific critical loads to protect common species against growth and survival declines by correlating tree response with total nitrogen and total sulfur deposition.  The sulfur-only critical loads protect 66 tree species against declines in growth or survival.  The National Park Service is applying these sulfur critical loads within national parks and considering how to interpret the impacts of sulfur deposition on trees.  We joined the S critical loads with park species lists along with annual total S deposition rates from Tdep to determine which parks and which tree species received S deposition in exceedance of their critical loads, and what the extent of the predicted declines in growth and survival were based on recent S deposition rates.   

The tree-S critical loads ranged from 0.2 kg to 4.1 kg-S ha^-1 y^-1, while total S deposition from 2016-2018 within national parks ranged from 0.2 to 12.9 kg-S ha^-1 y^-1.   There were 14 tree species with a growth or survival CL less than 1.0 kg-S ha^-1 y^-1; we considered designating these species as ‘highly sensitive species’ and not including their CLs in park CL summaries.  Land managers applying these low CLs have reported healthy stands of trees in areas where deposition rates would predict dramatic declines, and with low S CLs, variation in annual S deposition rates attributable to changes in precipitation rather than changes in S emissions appeared to threaten park species.  Of the 245 national park units with both S deposition data and CLs, 207 park units had at least one highly sensitive species.  There were 61 parks whose only growth S CLs were for highly sensitive species, and 42 parks whose only survival S CLs were for highly sensitive species.  These parks were in the Pacific West and Intermountain West Regions with S deposition rates ranging from 0.2 to 3.2 kg ha^-1 y^-1 in 2016-2018, and ponderosa pine set the lowest S CL for both growth and survival across the majority of these parks.  It may be more appropriate to apply low S CLs regionally rather than exclude them in summarizing park critical loads.