Can saxicolous lichens be used to monitor atmospheric deposition? Disentangling the influence of bedrock and canopy cover on biodiversity and elemental content of lichens.

Saxicolous lichens are known to be affected by air pollutants, but are often overlooked as potential biomonitors, even though they may be the only viable option for data collection in certain environments. In this study, we investigated the influence of bedrock and forest canopy on the biodiversity and the elemental content of native epiphytic and saxicolous lichens in metal-rich environments (serpentinite and granite outcrops in the Slavkov Forest, Czech Republic). We compared the accumulation of potentially toxic elements in the saxicolous lichen Parmelia saxatilis (L.) Ach. with that in the epiphytic species Hypogymnia physodes (L.) Nyl. and H. tubulosa (Schaer.) Hav. Generalized linear models (GLMs) revealed similar accumulation patterns for Fe, As, Cr, and Ni, for which both substrate type (tree vs. rock) and bedrock composition (granite vs. serpentinite) were significant factors. Thalli collected from serpentinite rocks exhibited significantly higher concentrations of Fe, Cr, Ni, and As, regardless of canopy cover, while Al was more abundant in saxicolous lichens on granite. Zn and Cd accumulated more in epiphytic lichens, whereas Pb was higher in saxicolous ones. Secondary metabolites as well as anatomical and morphological differences, rather than substrate, may possibly explain such divergences in Pb, Zn, and Cd contents. Nevertheless, the concentrations of Cu, along with those of Zn, Cd, and Pb, were of the same order of magnitude across both lichen types and not affected by bedrock. These findings suggest that saxicolous lichens can serve as effective biomonitors in environments where epiphytic lichens are not available.