Epiphytic macrolichen communities indicate climate and air quality in the U.S. Midwest

Epiphytic lichens directly exposed to atmospheric conditions can help detect how diffuse but pressing global changes may impact regional forest health. For 388 plots in the U.S. Midwest region, we developed indices for climate and air quality based on variation in tree-dwelling macrolichen community compositions (NMS ordination scores), species indicator values, and species environmental optima. Three lichen indices (NMS axis 1 scores, thermophile scores, and climate centroid scores) strongly covaried with thermal and evaporative-demand variables. By contrast, three other indices (NMS axis 2 scores, nitrophile scores and nitrogen centroid scores) were correlated with NHx (reduced N) deposition. Lichen communities had contrasting responses to different forms of N (NOx vs. NHx). Overall, thermal climate variables appeared more influential than air quality in structuring regional communities, based on greater explained variation of community compositions. Richness of species and of potentially adaptive secondary metabolites declined in hot, dry, or NHx-rich sites. With continued monitoring, significant changes in lichen-community based indices could signal directional shifts in forest vegetation. Changes in the thermophile or nitrophile indices more specifically would indicate the agent and rate of change for forest-altering trends.