Epiphytic lichen communities are highly sensitive to excess nitrogen (N), which causes the replacement of native floras by N-tolerant, “weedy” eutrophic species. This shift is commonly used as the indicator of “harm” in studies developing empirical critical levels (CLE) for ammonia (NH3) and critical loads (CLO) for N. To be most effective, empirical CLE/CLO must firmly link lichen response to causal pollutant(s), which is difficult to accomplish in field studies in part because the high cost of N measurements limits their use. For this case study we synthesized an unprecedented array of N measurements across 22 long-term monitoring sites in the Los Angeles Basin, California: gas concentrations of NH3, nitric acid (HNO3), nitrogen dioxide, and ozone (n = 10); N in throughfall (n = 8); modeled estimates of eight different forms of N (n = 22); and nitrate accumulated on oak twigs (n = 22). We sampled lichens on black oak (Quercus kelloggii) and scored plots using two indices of eutroph abundance to characterize the community-level response to N.
Results/Conclusions
Our results contradict two common misconceptions about the lichen-N response: 1) that eutrophs respond specifically to NH3, and 2) that that response is necessarily dependent upon the increased pH of lichen substrates. Eutroph abundance related significantly but weakly to NH3 (r2 = 0.48). Nitrogen deposition as measured in canopy throughfall was by far the best predictor (r2 = 0.94), indicating that eutrophs respond to multiple forms of N. Most N variables had significant correlations to eutroph abundance (r2 = 0.36 – 0.62) as well as to each other (r2 = 0.61 – 0.98), demonstrating the risk of mistaking correlation for causality in CLE/CLO field studies that lack sufficient calibration data. Our data furthermore suggest eutroph abundance is primarily driven by N inputs, not pH-- at least at the high pH values found in the basin (4.8 - 6.1). Eutrophs correlated negatively with trunk pH (r2 = 0.43), exactly the opposite of results from virtually all previous studies of eutroph behavior. This correlation is probably spurious and results because HNO3 dominates N deposition in our study region.