Detecting the nitrogen critical loads on European forests by means of epiphytic lichens. A signal-to-noise evaluation

• Lichens support the quantification of empirical critical loads for ecosystems at large scales.
• It was possible to determine a nitrogen critical load of 2.4 kg ha−1 yr−1.
• The correlations between lichens and pollutants were higher in nemoral-boreal conifer forests.

Lichens are considered to be among the most sensitive organisms for several types of pollutants. In this work, we analyzed a dataset of 286 epiphytic lichen species observed on 1155 trees at 83 ForestBIOTA plots, which is a subsample of approx. 500 plots of the European ICP Forests Level II network. We aimed at examining the amount of nitrogen deposition for which a significant variation of the relative diversity of morpho-functional groups of epiphytic lichens in the sampled plots is expected. Moreover, the study aimed at determining how much variance of these diversity variables could be explained by nitrogen depositions only. We used correlation and multiple regression models as well as hierarchical partitioning to evaluate the relative importance of environmental predictors in explaining variation in lichen diversity descriptors. The analysis splits the variation explained by each variable into a joint effect together with the other explanatory variables, and into an independent effect not shared with any other variable. The percentage of macrolichens in the plots was shown to be the most important indicator, since 56.7% of its variation could be explained by deposition, particularly by nitrogen compounds. It was shown that approx. 75% of the ForestBIOTA plots are affected by an unsustainably high throughfall nitrogen deposition. Based on these outcomes, it was possible to determine a nitrogen critical load of 2.4 kg ha−1 yr−1.