Tree Rings Mercury\nControlled by Atmospheric Gaseous\nElemental Mercury and Tree Physiology

Tree rings are an emerging atmospheric mercury (Hg) archive.\nQuestions\nhave arisen, though, regarding their mechanistic controls and reliability.\nHere, we report contrasting tree-ring Hg records in three collocated\nconifer species: Norway spruce (Picea abies), Scots pine (Pinus sylvestris),\nand European larch (Larix decidua),\nwhich are from a remote boreal forest. Centennial atmospheric Hg trends\nat the site, derived from varved lake sediments, peats, and atmospheric\nmonitoring, indicated a steady rise from the 1800s, peaking in the\n1970s, and then declining. Prior to ca. 2005, larch and spruce tree\nrings reproduced the peak in the atmospheric Hg trend, while pine\ntree rings peaked in the 1930s, likely due to the prolonged sapwood\nperiod and ambiguity in the heartwood–sapwood boundary of pine.\nSince ca. 2005, tree rings from all species showed increasing Hg concentrations\nin the physiologically active outer rings despite declining atmospheric\nHg concentrations. The good agreement between Hg and nitrogen concentrations\nin active tree-ring cells indicates a similar transport mechanism\nand cautions against their applicability as atmospheric Hg archives.\nOur results suggest that tree-ring Hg records are controlled by atmospheric\nHg and tree physiology. We provide recommendations for using tree-ring\nHg archives that take tree physiology into account.