Lichen responses to different forms of nitrogen in the Los Angeles Basin: Implications for critical levels and loads

Background/Question/Methods

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.