A bottom up approach to quantify foliar uptake of gaseous elemental mercury by European forests during the 2018 growing season

The deposition of gaseous elemental mercury, Hg(0), from the atmosphere to terrestrial surfaces remains poorly understood mainly due to difficulties in measuring net Hg(0) fluxes on the ecosystem scale. However, there is emerging evidence that vegetation uptake of atmospheric Hg(0) represents a major deposition pathway to terrestrial surfaces. We will present a novel bottom up approach to calculate Hg(0) deposition fluxes to aboveground foliage by combining foliar Hg accumulation rates on the basis of leaf area with species-specific leaf area indices. We analyzed Hg content in 583 foliage samples from major tree species at 10 European forested research sites along a latitudinal gradient from Switzerland to Northern Finland over the course of the 2018 growing season. Foliar Hg concentrations increased over time in all tree species at all sites. We found that foliar Hg accumulation rates normalized to leaf area increased with crown height and decreased with the age of multi-year old needles. We did not detect a clear latitudinal gradient in foliar Hg accumulation rates.

On an ecosystem scale we developed a simple bottom up approach for foliar Hg(0) uptake considering the systematic variations in crown height, needle age and tree species. We calculated species-specific average foliar Hg(0) dry deposition rates for the 2018 growing season of 22 ± 4 µg Hg m^-2 for beech, 16 ± 8 µg Hg m^-2 for oak, 3 ± 0.4 µg Hg m^-2 for birch, 18 ± 10 µg Hg m^-2 for spruce and 8 ± 4 µg Hg m^-2 for pine. For comparison, the average Hg wet deposition flux measured at 4 of our 10 research sites during the same time period was 2.5 ± 0.2 µg Hg m^-2.

Scaling up site-specific deposition rates to the forested area of Europe (EU28) resulted in a total aboveground Hg(0) deposition to foliage of approximately 20 Mg during the 2018 growing season. Our results confirm that vegetation uptake of atmospheric Hg(0) represents a major deposition pathway to terrestrial surfaces. The bottom up approach we used is a promising method to quantify Hg(0) deposition fluxes based on easy-to-do Hg concentration measurements in foliage.