Forest Inventory and Analysis Program, USDA Forest Service, Portland Forestry Sciences Laboratory, 620 SW Main Street, Suite 400, Portland, Oregon 97205, USA.
Ecol Appl. 2012 Oct;22(7):1910-22. doi: 10.1890/11-2075.1.
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 ecosystem "harm" in studies developing empirical critical levels (CLE) for ammonia (NH3) and critical loads (CLO) for N. To be most effective, empirical CLE and/or 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 atmospheric N measurements across 22 long-term monitoring sites in the Los Angeles Basin, California, USA: gas concentrations of NH3, nitric acid (HNO3), nitrogen dioxide, and ozone (n = 10 sites); N deposition in throughfall (n = 8 sites); modeled estimates of eight different forms of N (n = 22 sites); and nitrate deposition accumulated on oak twigs (n = 22 sites). We sampled lichens on black oak (Quercus kelloggii Newb.), and scored plots using two indices of eutroph (N tolerant species) abundance to characterize the community-level response to N. Our results contradict two common assertions about the lichen-N response: (1) that eutrophs respond specifically to NH3 and (2) that the response necessarily depends upon the increased pH of lichen substrates. Eutroph abundance related significantly but weakly to NH3 (r2 = 0.48). Total N deposition as measured in canopy throughfall was by far the best predictor of eutroph abundance (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 mistaken causality in CLE/CLO field studies that lack sufficient calibration data. Our data furthermore suggest that eutroph abundance is primarily driven by N inputs, not substrate pH, at least at the high-pH values found in the basin (4.8-6.1). Eutroph abundance correlated negatively with trunk bark pH (r2 = 0.43), exactly the opposite of virtually all previous studies of eutroph behavior. This correlation probably results because HNO3 dominates N deposition in our study region.
附生地衣群落对过量氮（N）非常敏感，过量氮会导致本地植物群被 N 耐受的“杂草”富营养物种所取代。这种转变通常被用作研究氨（NH3）经验临界水平（CLE）和氮临界负荷（CLO）中生态系统“损害”的指标。为了最有效，经验 CLE 和/或 CLO 必须将地衣的反应与因果污染物紧密联系起来，这在部分情况下是难以实现的，因为 N 测量的高成本限制了它们的使用。在这个案例研究中，我们综合了美国加利福尼亚州洛杉矶盆地 22 个长期监测点的前所未有的大气 N 测量数据：氨气（NH3）、硝酸（HNO3）、二氧化氮和臭氧的气体浓度（n = 10 个地点）；穿透雨（n = 8 个地点）中的 N 沉积；八种不同形式的 N 的模型估计（n = 22 个地点）；以及在栎属树枝上积累的硝酸盐沉积（n = 22 个地点）。我们在黑栎（Quercus kelloggii Newb.）上采样地衣，并使用两个富营养（N 耐受物种）丰度指数对斑块进行评分，以描述群落水平对 N 的反应。我们的结果与关于地衣-N 反应的两个常见说法相矛盾：（1）富营养生物专门对 NH3 做出反应；（2）反应必然取决于地衣基质 pH 的增加。富营养生物丰度与 NH3 呈显著但微弱的相关性（r2 = 0.48）。在冠层穿透雨中测量的总 N 沉积是富营养生物丰度的最佳预测因子（r2 = 0.94），这表明富营养生物对多种形式的 N 做出反应。大多数 N 变量与富营养生物丰度呈显著相关（r2 = 0.36-0.62），彼此之间也呈显著相关（r2 = 0.61-0.98），这表明在缺乏足够校准数据的 CLE/CLO 现场研究中，存在错误因果关系的风险。我们的数据进一步表明，至少在盆地中发现的高 pH 值（4.8-6.1）下，富营养生物丰度主要由 N 输入驱动，而不是基质 pH。富营养生物丰度与树干树皮 pH 呈负相关（r2 = 0.43），与几乎所有先前关于富营养生物行为的研究都恰恰相反。这种相关性可能是由于在我们的研究区域中，HNO3 主导了 N 的沉积。
