Global warming alters mercury accumulation in trees

Abstract. Stomatal uptake of mercury (Hg) by trees is the major pathway of atmospheric Hg to the terrestrial environment and most studies have suggested that Hg concentrations in tree rings are determined primarily by gaseous elemental mercury (GEM) concentrations in the atmosphere. Most studies on tree-ring Hg records were aiming to reconstruct historical atmospheric Hg emissions at polluted sites. However, the role of changing climate such as rising temperature and frequent drought events on tree-ring Hg concentrations has been rarely addressed. We show that the overall Hg load in tree rings in oaks and Douglas fir from contaminated and uncontaminated sites in Germany has been strongly determined by local atmospheric Hg emissions, but its long-term evolution has been largely determined by hydroclimate and did not follow the trend of GEM emissions in Europe. Due to different physiological heat adaption strategies Central European oaks show continuously increasing tree-ring Hg concentration with rising temperature and precipitation rates during the past century, whereas coniferous trees show a strongly declining trend in Hg concentration in the same period, which was also observed at other sites within the Northern Hemisphere. Our findings indicate that besides atmospheric Hg concentrations, climate change alters the Hg accumulation in different forest types and most likely the related transfer of atmospheric Hg to the soil.