Air-quality bioindication in the greater Central Valley of California, with epiphytic macrolichen communities

Air-quality monitoring in the United States is typically focused on urban areas even though the detrimental effects of pollution often extend into surrounding ecosystems. The purpose of this study was to construct a model, based upon epiphytic macrolichen community data, to indicate air-quality and climate in forested areas throughout the greater Central Valley of California (USA). The structure of epiphytic lichen communities is widely recognized as an effective biological indicator of air-quality as sensitivities to common anthropogenic pollutants vary by species. We used nonmetric multidimensional-scaling ordination to analyze lichen community data from 98 plots. To calibrate the model, a subset of plots was co-located with air-quality monitors that measured ambient levels of ozone, sulfur dioxide, and nitrogen dioxide. Two estimates of ammonia deposition, which is not regularly monitored by any state or federal agency in California, were approximated for all plots using land-use maps and emissions estimates derived from the California Gridded Ammonia Inventory Modeling System. Two prominent gradients in community composition were found. One ordination axis corresponded with an air-quality gradient relating to ammonia deposition. Ammonia deposition estimates (r = -0.63 and -0.51), percentage nitrophilous lichen richness (r = -0.76), and percentage nitrophile abundance (r = -0.78) were correlated with the air-quality axis. Plots from large cities and small, highly agricultural towns had relatively poor air-quality scores, indicating similar levels of ammonia deposition between urban and agrarian land uses. The second axis was correlated with humidity (r = -0.58), distance from the coast (r = 0.62), kriged estimates of cumulative ozone exposure (r = 0.57), maximum one-hour measurements of ozone (r = 0.58), and annual means of nitrogen dioxide (r = 0.63). Compared to ammonia, ozone and nitrogen dioxide impacts on lichen communities are poorly known, making it difficult to determine whether the second axis represents a response to climate, pollution, or both. Additionally, nitric acid may be influencing lichen communities although the lack of deposition data and research describing indicator species prevented us from evaluating potential impacts.