Table S2.

Summary of predictor variable sources

VariableSource
Wet N depositionNADP (2013) Annual National Trends Network (NTN) Maps by Analyte. Available at nadp.sws.uiuc.edu/ntn/maps.aspx (National Atmospheric Deposition Program, Illinois State Water Survey).
Dry N depositionCMAQ (Community Multiscale Air Quality) model. Provided by R. L. Dennis in 2014 (39).
Precipitation, mmPRISM (2013) 30-year normals. Parameter-elevation relationships on independent slopes model. Available at prism.nacse.org/normals/ (Northwest Alliance for Computational Science & Engineering at Oregon State University) (1981–2010 normals) (40).
Temperature, °CPRISM (2013) 30-year normals. Parameter-elevation relationships on independent slopes model. Available at prism.nacse.org/normals/ (Northwest Alliance for Computational Science & Engineering at Oregon State University) (1981–2010 normals) (40).
Soil pHProvided with vegetation data; otherwise from USDA SSURGO soil dataset. SSURGO (2014) Geospatial Data Gateway. Available at https://gdg.sc.egov.usda.gov/GDGOrder.aspx?order=QuickState (USDA Natural Resources Conservation Service).