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To understand its source,distribution,storage,and translocation in the subtropical forest ecosystems,mercury(Hg)concentrations and stable isotopes in forest biomass tissues(foliage,branch,bark,and trunk)were investigated at Ailao Mountain National Nature Reserve,Southwest China.The total Hg(THg)concentrations in the samples show the following trend:mature foliage(57 ± 19 ng g-1)＞ bark(11 ± 4.0 ng g-1)＞ branch(5.4 ± 2.5 ng g-1)＞ trunk(1.6 ± 0.7 ng g-1).Using the measured THg concentrations and the quantity of respec-tive biomasses,the Hg pools in the forest are:wood(60 ± 26 μg m-2)＞ bark(51 ± 18 μg m-2)＞ foliage(41 ± 11 μg m-2)＞ branch(26 ± 8.3 μg m-2).The tree biomasses displayed negative δ202Hg(-1.83％o to-3.84‰)and △199Hg(-0.18‰ to-0.62‰).The observed △200Hg(-0.08‰ to 0.04‰)is not significantly from zero.A △199Hg/△201Hg ratio of 1.05 was found in tree biomasses,suggesting that mercury has undergone Hg(Ⅱ)photoreduction processes.A Hg-isotope based binary mixing model suggests that Hg in the tree biomasses mainly originated from foliage uptake of atmospheric Hg0,constituting 67％,80％,and 77％ of Hg in wood,branch,and bark,respectively.Our study sheds new light on the transportation and sources of Hg in the subtropical forest ecosystems.