Brief Methods Summary

1. Western US ratings are based on 6699 surveys across the western US west of the Great Plains states, primarily following the US Forest Inventory & Analysis lichen communities indicator protocol. Includes southern AK, WA, OR, CA, ID, NV, AZ, NM, CO, UT, WY, MT.
2. Deposition measure: Community Multi-Scale Air Quality (CMAQ) 5.0.1 modelled estimates for total N and S deposition for the continental US from 2000-2012 at a 12 km grid scale with bi-directional NH3 air-surface exchange using the Massad formulation. Alaska deposition estimates were based on on-site deposition measurements and calibrated lichen element concentrations (Root et al. 2013 Forest Ecol. & Mngmt, 306:1-8).
3. 3-year rolling mean deposition was calculated for each survey site, including survey year and previous 2 years.
4. Survey sites were assigned to deposition increments of 0.5 kg/ha/yr, requiring a minimum of 35 sites per increment. To incorporate the high end of the deposition gradients, with fewer plots, the increment length was increased up to 1.5 kg.
5. Number of detections per deposition increment was plotted for each species occurring in the western US.
6. Splines were fit for each species using the function .smooth.spline. with a flexibility setting (.spar.) equal to 0.5 in the software R (v. 3.1.1 R Development Core Team 2014).
7. The deposition associated with peak detection frequency along the spline was recorded.
8. Sensitivity ratings were assigned as above based on deposition at peak detection frequency after filtering out rare species and species with ambiguous response curves.

Table: Western U.S. Sensitivity Ratings

Taxonomy follows the 19th North American Checklist.