Storage and Export of Atmospheric Hg, Pb, Al, and Fine Particulate Matter (PM2.5) from Forest Trees

A mass balance of whole-tree inventories and fluxes in throughfall and litterfall reveals that mature forest trees strongly retain atmospheric deposition of fine particulate matter (PM2.5) and metals, including Hg, Pb, and Al, and store this material over decadal time scales. Wet and dry deposition are retained with similar efficiencies (56 and 52% of totals, respectively). Accumulation of Hg by direct absorption of gaseous elemental mercury (GEM) contrasts with crustal metals (e.g., Al) that are likely to be resuspended from the surrounding ecosystem. Due to long storage times and nonsteady state with respect to declining industrial emissions, long-lived nonfoliar tissues of the canopy, including lichen, moss, mold, bark, and arboreal soil (hereafter phyllosphere), may store 10-100 times the contemporary fluxes of Hg and Pb as a legacy of past emissions. Release of stored legacy Hg and Pb is mediated by dissolved organic carbon (DOC) and fine particulate matter (FPOM) even as new deposition continues to be absorbed in complex exchange within the phyllosphere. These results advance a new understanding of metal biogeochemical cycling in forest canopies that includes substantial storage, exchange, and release over decadal time scales.