A novel sentinel of atmospheric pollution in forests

Air pollution poses a threat to forest integrity. Lichens and mosses are commonly used to monitor air quality in forests, but they may fall short in capturing spatiotemporal pollution dynamics or in revealing community-level ecotoxicological effects. Here, we introduce a novel ecosystem model complementing existing approaches. Dendrotelms are water-filled tree cavities hosting aquatic biota sustained by allochthonous organic detritus. At the canopy–atmosphere interface, tree crowns capture airborne pollutants, which then runoff via stemflow and are ultimately trapped in the dendrotelm, where they accumulate in sediment layers. We measured concerning levels of eight potentially toxic elements (Zn, Cu, Pb, As, Cd, Ni, V, Cr) in the dendrotelm sediments of three old-growth European Beech forests, but remained low in the Amazonian rainforest location (Mato Grosso, Brazil). Interestingly, the observed among-site contamination levels were positively related to historical NOx emissions in each site (β = 0.9). In the Massane forest, we hypothesised that the hump-shaped relationships between elemental concentrations and the size of the trees hosting the dendrotelms were underlain by a function of tree age integrating over changes in multi-decadal atmospheric deposition. We used this rationale and parameters from the relationships between elemental concentrations and tree sizes at a given point in time to reconstruct historical pollution trends spanning almost a century. These projected temporal reconstructions matched direct observations from temporal contamination profiles obtained with short-lived radionuclide dating (Pb-210ex and Cs-137). Most elements increased from the 1950s, peaked in the 1980s, and decreased until now, thanks to the implementation of air quality policies. Enrichment factors were high in the Massane forest (10–35), suggesting an anthropogenic origin which has to be confirmed by isotope tracing. Lead concentrations in the dendrotelm sediments, probably one of the most harmful metals at these concentrations, reduced invertebrate densities in the European forests we investigated, but not in the less contaminated Amazonian site, highlighting the ecological legacy of airborne pollution in European forests. Our approach fills important qualities required to improve our appraisal of atmospheric pollution in forests, that is, interrogating spatial, temporal, and ecotoxicological scales with a comprehensive approach that can be operated globally.