Mercury uptake by trees: An observational experiment

We conducted a simple observational experiment to test whether differences in Hg in tissue of red pine (Pinus resinosa Ait.) were related to soil or to atmospheric sources of Hg. We sampled two plantations in each of three areas, and within each plantation sampled two sites with different levels of soil Hg. Woody tissue Hg concentration differed by area, and differences in foliar concentrations, though not statistically significant, were ranked in the same order. Total mass of Hg in forest floor and mineral soil also differed by area, but with ranking opposite that of tissue. On an individual-tree basis, concentrations of Hg in 1994 needles (2-year old) were about twice those in 1995 needles (1-year old) (r = 0.77). Neither woody tissue Hg nor any measure of Hg in soil or forest floor were closely related to foliar levels and some relationships were inverse. We interpret the data to indicate that Hg in plant tissue is derived directly from the atmosphere, not the soil. Tissue concentration by area was closely related to the respective growing season length (1994 needles, r = 0.88; 1995 needles, r = 0.97; wood, r = 0.97), as was total mass of Hg in forest floor and surface mineral soil (r = - 0.80). Other climatic measures, such as growing degree days and actual evapotranspiration, had similar relationships. These relationships imply that both foliar uptake of Hg-0 from the atmosphere and efflux of Hg from the soil system depend on biological activity.