Background/Question/Methods Species richness of epiphytic macrolichens from timed samples (Lichen S) has potential as an indicator of forest ecosystem response to environment. Lichens data (currently available from most forested areas of continental USA) are from surveys on permanent forested plots as part of the USDA Forest Service Forest Inventory and Analysis Program (FIA). We explored, with Spearman correlations and linear regression models, relationships of Lichen S to variables representing climate, geography, and air quality, the latter also surrogates for general human impact. We had 1399 plots for western USA region and 1087 plots for eastern USA region (each with ecoregion subdivisions), including 135 plots in 5 western Great Lakes states. In 2 ecoregions of each region Lichen S alone lacks potential as an indicator (correlation rho² <.20, regression r² < .30).
Results/Conclusions Eastern and western USA regions show contrasting patterns. In western USA Lichen S is most strongly correlated with geographic variables such as elevation that integrate climate patterns, showing potential to indicate response to climate. Regression models (r² = .30 to .59) include different ‘predictors’ for different ecoregions; climate values vary widely and often independently. Air pollution is relatively low and localized, but modeled NH₄⁺ deposition often has negative correlations with Lichen S, suggesting some potential for indicating response. In contrast, in eastern USA with air pollution relatively high across large areas, modeled NO_3^- deposition is the strongest variable in models (regression r² = .30 to .53) ‘predicting’ Lichen S in the east region and most ecoregions. Climate varies from warm to cool with moderate moisture range, but climate and geographic variables are often weakly correlated with Lichen S. Where they are important, environment is also strongly correlated with air pollution. In the east Lichen S alone has little potential to indicate response to climate, but strong potential to indicate response to pollution/human impact. Results show a pattern of lower Lichen S around 41 degrees north latitude corresponding to the Ohio Valley and other east urban/industrial corridors, with higher Lichen S especially north and at higher elevations. It appears that lower lichen diversity in warmer and/or lower areas (natural eastern pattern) has been further reduced by air pollution and other human impact in many of those same areas. The western Great Lakes states show similar patterns; Lichen S is higher to the northwest (cooler and less NO₃^-) and lower to the southeast (warmer and more NO_3^-; regression r² = .43, .46).