Mercury concentration records in tree rings of sessile oak and Douglas fir – The role of pollution and climate

Abstract. Stomatal uptake of mercury (Hg) by trees and litterfall are the major pathways of atmospheric Hg to the terrestrial environment. However, underlying processes and controlling factors of Hg accumulation in tree rings which have been used in numerous studies to reconstruct historical atmospheric Hg concentrations at polluted sites are poorly understood. Especially the role of changing climatic conditions such as rising temperature and frequent drought events compared to local Hg contamination levels have been widely ignored. We investigated tree rings of oaks at a former Hg mining site and an unpolluted reference site and compared the oak tree-ring records with tree rings of Douglas fir from unpolluted sites, all located in western Germany. Our data indicates that the overall Hg accumulation in trees is determined by local Hg loads in soil, but did not follow the historical record of atmospheric Hg pollution. In contrast, the long-term evolution of tree-ring Hg concentrations can statistically be attributed to changing hydroclimatic conditions. We suggest that different physiological heat adaption strategies of the investigated oaks are likely to lead to continuously increasing tree-ring Hg concentration with rising temperature and precipitation rates during the past century, whereas the investigated Douglas fir trees show a strongly declining trend in Hg load in the same period. Our findings indicate that climate conditions appear to overwrite changes in atmospheric Hg and may alter the Hg accumulation in different forest types and likely the related transfer of atmospheric Hg to the soil through litterfall.