Opportunities to monitor long term ecosystem changes are rare, especially in the case of lichen communities. In the late 1970’s Lorene Sigal surveyed fifty sites in the montane regions around the Los Angeles (LA) air basin for lichens, and found that ~50% of the montane species listed in a 1913 Lichen Flora of Southern California were extirpated, and many other species were found in low abundance. Sigal concluded that observed patterns of species decline were correlated with ozone/oxidant pollution gradients in the basin. More recent evidence of chronic nitrogen (N) deposition led researchers to question this conclusion, as N deposition and ozone concentration gradients often overlay each other. In the summer of 2008, we resurveyed twenty three of Sigal’s sites, using her methodology (transects measuring lichen abundance and species diversity), and USDA Forest Service Forest Inventory and Analysis (FIA) Lichen Indicator methods (gross estimates of relative cover for lichen species in a standardized area).Integrating the two methods allowed us to compare temporal trends the LA air basin, and geographic trends by tying in to FIA lichen survey data from sites throughout the United States. We also measured bark and twig pH on Quercus kelloggii, and nitrate deposition to twigs.
Results/Conclusions
Since many of our sites were co-located with USDA FS air quality monitoring sites, we were able to compare our twig and bark pH and nitrate data with air quality values. We found significant correlations between twig nitrate values, N concentrations, and lichen community composition, suggesting that twigs are a good tool for surface deposition measurements in arid climates. Nitrogen tolerant lichens were more abundant at sites with higher ambient concentrations of both ammonia and gaseous nitric acid. Areas with chronically high levels of N deposition showed either abundant pollution tolerant lichen communities, or significantly lower lichen cover altogether.
Our results to date suggest that the chronic nature of pollution in the LA basin is threatening even species considered pollution tolerant thirty years ago. While air quality in the LA basin is markedly improved in the last twenty years, it remains some of the worst in the country. This is reflected in the depauperate lichen communities and shifts over time from sensitive to tolerant lichen species. This project shows the continued usefulness on both lichens as biomonitors of air quality, and of long term monitoring.
