Mercury accumulation in leaves of different plant types – the significance of tissue age and specific leaf area

Mercury, Hg, is one of the most problematic metals from an environmental
perspective. To assess the problems caused by Hg in the environment it is
crucial to understand the processes of Hg biogeochemistry, but the
exchange of Hg between the atmosphere and vegetation is not sufficiently
well characterised. We explored the mercury concentration, [Hg], in
foliage from a diverse set of plant types, locations and sampling periods
to study whether there is a continuous accumulation of Hg in
leaves/needles over time. Measurements of [Hg] were made in deciduous and
conifer trees in Gothenburg, Sweden (Botanical Garden and city area) as
well as of evergreen trees in Rwanda. In addition, data for wheat from an
ozone experiment conducted at Östad, Sweden, were included. Conifer data
were quantitatively compared with literature data. In every case where
older foliage was directly compared with younger, [Hg] was higher in older
tissue. Covering the range of current year up to four-year old needles,
there was no sign of Hg saturation in conifer needles with age. Thus, over
time scales of approximately one month to several years, the Hg uptake in
foliage from the atmosphere always dominated over Hg evasion. Rwandan
broadleaved trees had generally older leaves due to lack of seasonal
abscission and higher [Hg] than Swedish broadleaved trees. The
significance of atmospheric Hg uptake in plants was shown in a wheat
experiment where charcoal filtrated air lead to significantly lower leaf
[Hg]. To search for general patterns, the accumulation rates of Hg in the
diverse set of tree species in the Gothenburg area were related to the
specific leaf area (SLA). Leaf area based [Hg] was strongly negatively and
non-linearly correlated with SLA, while mass-based [Hg] had a somewhat
weaker positive relationship with SLA (both relationships with p <
0.001). An elaborated understanding of the relationship behind [Hg] and
SLA would support large-scale modelling of Hg uptake by vegetation and Hg
circulation in general.