Assessment and quantification of the forest foliar mercury uptake flux across Europe

Mercury (Hg) is a pollutant of great concern for human and ecosystem health. Centuries of anthropogenic Hg emissions to the atmosphere from e.g. gold mines and coal power plants have increased Hg deposition to land and oceans. Human Hg exposure occurs primarily via the consumption of fish and marine mammals due to the large bioaccumulation potential of highly neurotoxic methyl-Hg along marine and freshwater food webs. In the atmosphere, Hg is mainly present as gaseous elemental Hg(0) (> 95%). As Hg(0) is subject to long-range atmospheric transport around the globe, it is essential to identify and constrain major environmental deposition pathways in order to inform Hg mitigation policies and reduce harmful human Hg intake.
Until recent years, it had been unclear, whether forest ecosystems represent a net source or a net sink for atmospheric Hg(0). However, novel studies on stable Hg isotopes have since identified vegetation biomass, including forest foliage, as the major vector for atmospheric Hg deposition to soils. Forest foliage takes up and accumulates atmospheric Hg(0) in foliage tissues throughout the growing season. Atmospheric Hg(0) sequestered by forest foliage is deposited to soils via e.g. litterfall, biomass die-off and throughfall. Soils are a net sink for atmospheric Hg(0), but also re-emit Hg(0) back to the atmosphere. From soils, Hg can be transported to aquatic biota via runoff.
Despite the importance of Hg deposition to forest ecosystems, net foliar Hg(0) fluxes have not been well constrained. Adding to this, the mechanism of foliar Hg(0) uptake is not yet fully understood. The goals of this thesis was to establish a method to measure and investigate net foliar Hg(0) uptake fluxes on a whole forest scale with a focus on i) environmental controls; ii) its magnitude to the European forested area, and iii) projection pathways under climate change.
For measuring the net forest foliar Hg(0) uptake flux, a novel bottom-up method was developed and applied. This method scales measured foliar Hg concentrations per foliage surface area with forest leaf area indices to obtain a foliar Hg flux in units of foliar Hg per unit ground area.
A prerequisite for this bottom-up method was to systematically assess variations in foliar Hg(0) uptake rates within the forest canopy, in order to take variations into account for scaling up foliar Hg(0) uptake rates to the whole forest. At a research site in Switzerland, vertical variations in foliar Hg(0) uptake rates within tree canopies were systematically assessed through seasonal analysis of foliage samples for Hg. Foliar Hg concentrations increased linearly throughout the growing season. Sun-exposed leaves growing at the top of canopies took up more Hg over the same time span than shade leaves of the lower canopy, which can probably ascribed to higher physiological activity of sunleaves compared to shade leaves. Therefore, a correction factor for the bottom-up method was derived in order to use foliage Hg(0) uptake rates by sunleaves for the whole tree canopy. Foliar Hg concentrations increased with age in multi-year old coniferous needles, but relative Hg(0) uptake rates decreased in older needles over two years old. Taking relative biomass proportions of differently aged needles into account, age correction factors of foliar Hg(0) uptake by whole coniferous trees were determined for the bottom-up method.
Using the bottom-up method, foliage Hg(0) uptake fluxes were calculated from foliar Hg(0) uptake rates measured at multiple forest sites in Central and Northern Europe. Foliar Hg(0) uptake fluxes were extrapolated to the forested area of Europe. The total amount of Hg taken up by European forest foliage was calculated to be 20 - 30 Mg Hg growing season-1, representing around a third of the annual anthropogenic Hg emissions of the European Union.
A large European dataset of foliage Hg(0) uptake rates was created by measuring Hg concentrations in > 3500 European foliage samples collected by partners of the ICP Forests biomonitoring network. This dataset allowed for empirical investigations of foliage Hg(0) uptake rates with available foliage- and site-specific meta data. Four major relationships were observed: 1) broadleaves display higher Hg(0) uptake rates per gram dry weight than coniferous needles of the same age; 2) foliage Hg(0) uptake rates correlate with foliage nitrogen concentrations, which is a proxy for foliage physiological activity; 3) foliage Hg(0) uptake rates in pine needles are lower at forest sites, where extended time periods of relatively high ambient vapor pressure deficit prevail during the growing season; and 4) foliage Hg(0) uptake rates in beech, oak and pine are lower at forest sites, where soil water content fell below a soil texture-specific critical soil water content over a relatively long time period during the growing season. The first two relationships suggest, that foliar Hg(0) uptake is positively correlated with the degree of foliage physiological activity, which is larger in broadleaves and in foliage of high nitrogen content compared to coniferous needles and low-nitrogen foliage. The second two relationships indicate, that drought stress owing to dry atmospheric/soil conditions impede foliar Hg(0) uptake in isohydric tree species like pine. All four observed relationships suggest that the foliar Hg(0) uptake occurs via a stomatal pathway and is therefore controlled by stomatal conductance for leaf diffusive gas exchange. Thus, foliar Hg(0) uptake is related to tree metabolic mechanisms of stomatal opening, which is controlled by physiological activity and meteorological parameters.
This stomatal foliar Hg(0) uptake mechanism has implications for foliar Hg(0) uptake flux to European forests in the future. More frequents and severe summer droughts in Central and Southern Europe are projected under climate change. A simulation of the European pine forest foliar Hg(0) uptake flux derived from the water vapor pressure deficit under two different climate change scenarios in 2050 revealed only a slight decrease (3 – 4%) in the total seasonal flux. However, possible future meteorological summer conditions comparable to the European drought year of 2018 would result in a larger decrease of pine forest foliar Hg(0) uptake fluxes, which were calculated as 13% lower in 2018 compared to previous years.
The qualitative assessment and quantification of the forest foliar Hg(0) uptake flux provides new insights into the mechanism and relevance of this flux within the Hg cycle and bears the potential to learn more about stomatal conductance as an important ecophysiological parameter.