Mercury in coniferous and deciduous upland forests in northern New England, USA: implications of climate change

Abstract. Climatic changes in the northeastern US are expected to cause coniferous stands to transition to deciduous stands over the next hundred years. Mercury (Hg) sequestration in forest soils may change as a result. In order to understand potential effects of such a transition, we studied aboveground vegetation and soils at paired coniferous and deciduous stands on eight mountains in Vermont and New Hampshire, USA. Organic horizons at coniferous stands accumulated more total Hg (THg; 42 ± 6 g ha⁻¹) than deciduous stands (30 ± 4 g ha⁻¹). Total Hg pools in the mineral horizons were similar for coniferous (46 ± 8 g ha⁻¹) and deciduous stands (45 ± 7 g ha⁻¹). Soil properties (C, % clay, and pH) explained 56 % of the variation in mineral soil Hg concentration when multiply regressed. Foliar and bole wood Hg concentrations were generally greater for coniferous species than deciduous species. Using allometric equations, we estimated that aboveground accumulation of Hg in foliage and woody biomass was similar between vegetation types but that coniferous stands have significantly smaller annual litterfall fluxes (0.03 g ha⁻¹ yr⁻¹) than deciduous stands (0.24 g ha⁻¹ yr⁻¹). We conclude that organic horizon Hg accumulation is influenced by vegetation type but mineral horizon Hg accumulation is primarily controlled by soil properties. Further investigations into the effect of vegetation type on volatilization, atmospheric deposition, and leaching rates are needed to constrain regional Hg cycling rates.