Physiological and climate controls on foliar mercury uptake by European tree species

Abstract

Abstract. Despite the importance of vegetation uptake of atmospheric gaseous elemental mercury (Hg(0)) within the global Hg cycle, little knowledge exists on the physiological, climatic, and geographic factors controlling stomatal uptake of atmospheric Hg(0) by tree foliage. We investigate controls on foliar stomatal Hg(0) uptake by combining Hg measurements of 3569 foliage samples across Europe with data on tree species' traits and environmental conditions. To account for foliar Hg accumulation over time, we normalized foliar Hg concentration over the foliar life period from the simulated start of the growing season to sample harvest. The most relevant parameter impacting daily foliar stomatal Hg uptake was tree functional group (deciduous versus coniferous trees). On average, we measured 3.2 times higher daily foliar stomatal Hg uptake rates in deciduous leaves than in coniferous needles of the same age. Across tree species, for foliage of beech and fir, and at two out of three forest plots with more than 20 samples, we found a significant (p<0.001) increase in foliar Hg values with respective leaf nitrogen concentrations. We therefore suggest that foliar stomatal Hg uptake is controlled by tree functional traits with uptake rates increasing from low to high nutrient content representing low to high physiological activity. For pine and spruce needles, we detected a significant linear decrease in daily foliar stomatal Hg uptake with the proportion of time during which water vapor pressure deficit (VPD) exceeded the species-specific threshold values of 1.2 and 3 kPa, respectively. The proportion of time within the growing season during which surface soil water content (ERA5-Land) in the region of forest plots was low correlated negatively with foliar Hg uptake rates of beech and pine. These findings suggest that stomatal uptake of atmospheric Hg(0) is inhibited under high VPD conditions and/or low soil water content due to the regulation of stomatal conductance to reduce water loss under dry conditions. Other parameters associated with forest sampling sites (latitude and altitude), sampled trees (average age and diameter at breast height), or regional satellite-observation-based transpiration product (Global Land Evaporation Amsterdam Model: GLEAM) did not significantly correlate with daily foliar Hg uptake rates. We conclude that tree physiological activity and stomatal response to VPD and soil water content should be implemented in a stomatal Hg model to assess future Hg cycling under different anthropogenic emission scenarios and global warming.

Remarkable Variation in the Process of Hg Accumulation in Timberline Forests Indicates an Aggravated Hg Burden in Alpine Forests Under Climate Warming

“…However, our results show a much higher Hg concentration in tissues of AF compared to RS, especially a two times higher Hg concentration in needles of AF. This is because needles of AF have a much longer lifespan (up to 8 years in alpine regions) than foliage of RS (several months), and the longer lifespan for atmospheric Hg 0 exposure can set off the lower instantaneous uptake of Hg rate of AF (Wang, Yuan, Lin, & Feng, 2021; Wang, Yuan, et al., 2020; Wohlgemuth et al., 2022), thus promoting a higher Hg concentration in needles.…”

Deforestation as an anthropogenic driver of mercury pollution

“…Similar to CO2, the efficacy of reforestation for Hg mitigation depends on whether the storage of Hg in soils is over a longterm period. Potential benefits of enhanced Hg and CO2 uptake on land can be reversed by human or natural disturbances, e.g., climate change increasing the frequency of wildfireswhich re-emit Hg and carbon from terrestrial ecosystems -and droughts -which reduce Hg and CO2 uptake by plants 30,50 . Thus, mitigation of Hg pollution by conserving and increasing forest area can only be realized with concurrent efforts to sustainably manage land areas and preventing severe climate change.…”

Global change effects on biogeochemical mercury cycling

“…In an illustrative worst-case scenario where the Amazon is completely converted to savannah, the GEOS-Chem model predicted that 400 Mg of Hg 0 would not be sequestered in Amazonian soils annually, but mostly deposited to oceans. The physiological, climate and geographic factors controlling stomatal Hg 0 uptake by foliage were recently investigated in a database of > 3000 European foliage samples (Wohlgemuth et al 2022 ). Foliar stomatal Hg 0 uptake varied with tree functional group, with deciduous leaves showing 3.2 times higher uptake than coniferous leaves, and foliar Hg 0 uptake generally increased with nutrient (leaf nitrogen) and moisture availability.…”