Results of a controlled field experiment to assess the use of tree tissue concentrations as bioindicators of air Hg

Abstract

The utility of trees as bioindicators of atmospheric mercury (Hg) depends upon how accurately tree tissue concentrations reflect air-Hg concentrations at a given location and time. The relationship between air-Hg, and Hg concentration in tree tissues was investigated using potted Pinus nigra (Austrian pine) saplings 6 to 7 years in age moved from a tree farm in Oregon, USA, to three locations with different weather, two with different air total Hg (THg) concentrations, all with different predominant gaseous oxidized Hg chemistry, and one being impacted by the marine boundary layer. An aqueous Hg bromide root spike showed no significant effect on above ground tissue concentration. Over two growing seasons, needles, bark, and tree rings were sampled and analyzed for THg concentrations to compare with those collected when the trees originally arrived in Reno, NV (control) versus that measured in the new locations. Overall, foliar Hg concentrations increased significantly at all new locations relative to the control. Spring sample concentrations were lower than fall for foliage at two locations, indicating resorption of Hg along with nutrients. All trees had higher mean Hg concentrations in the outermost tree rings relative to the control, consistent with increased air total Hg concentrations and changing air chemistry. Higher concentrations were measured in the outer bark of trees where ambient GOM concentrations were highest. Higher inner bark concentrations were correlated with the more humid location where high methylmercury concentrations have been measured in fog. Results demonstrated that Pinus nigra tissues are effective biomonitors for air Hg exposures and that translocation from leaves to rings via the phloem is an important pathway for sequestration in plant tissue. We also show that plants are active assimilators of atmospheric Hg, and as such, local environmental conditions influence ring, bark, and foliage concentrations. The latter has important implications for understanding tissue Hg concentrations, and supports the use of tree ring Hg concentrations as archives.