Leaf Age Affects Mercury Accumulation in Evergreen Plants

We investigated the mercury (Hg) concentration of the full range of needle age classes (NACs) in two conifers, nine NACs in Picea abies and fourteen in Abies pinsapo var. marocana, as well as three leaf age classes (LACs) in two broadleaved evergreen species, Trochodendron aralioides and Rhododendron catawbiense. Additionally, the Hg concentration of the wooden branch segments to which the NACs were attached in the two conifers was studied. Picea abies showed a continued Hg accumulation over all NACs, but with an age-dependent decline in the accumulation rate. In Abies pinsapo var. marocana, maximum needle concentrations of Hg were reached after eight years. The concentration remained constant for NACs 9–14, indicating that needles had become saturated with Hg. The Hg concentrations of the branch segments were much lower than those of the needles in the older NACs. Over the three LACs of Trochodendron aralioides and Rhododendron catawbiense there was a steady increase in concentration with a weak indication of a declining Hg uptake rate in older leaves. The average needle/leaf lifetime Hg uptake rate per year was only half that of broadleaved species across all NACs and LACs. We conclude that in conifers maintaining a larger number of NACs there is a decline of the Hg accumulation rate in older NACs. In future biogeochemical research (empirical and modelling) and biomonitoring studies, the age of sampled leaves needs to be considered to account for the age dependence of leaf Hg concentration and accumulation rate.