Low Biomass, High Impact: Mosses and Lichens Dominate Mercury Sequestration in the Alpine Forest

Mosses and lichens are commonly employed as bioindicators of atmospheric mercury (Hg) pollution. In this study, we conducted an integrated assessment of Hg cycling in mosses and lichens in an alpine coniferous forest. Results show that these nonvascular plants constitute a significant and previously overlooked sink for atmospheric Hg in alpine forest ecosystems. Our findings reveal a marked increase in Hg concentration in mosses and lichens with height along the forest vertical canopy profile, accompanied by a progressive negative shift in Δ¹⁹⁹Hg values. These trends indicate that microhabitat conditions strongly regulate Hg accumulation and potential re-emission in epiphytic communities. Strikingly, although mosses and lichens represent only 1.4% of total forest biomass, they account for 40% of the vegetation Hg pool. Moreover, the contributions from mosses and lichens in litterfall and throughfall deposition together have already exceeded the contribution from leaf litterfall and direct atmospheric rainfall Hg deposition. This study necessitates the need to integrate mosses and lichens into global assessments of atmospheric Hg, as their omission leads to substantial underestimation of forest Hg sinks.