Mercury Pools in Forest Soils Along Three Chronosequences in the Northeastern US.

Understanding how trace metals in forests responds to clear-cutting on short-term (< 50 yrs) and longer timescales is important for considering impacts on downslope aquatic ecosystems. Mercury has been widely dispersed across the northeastern US from regional and global emissions. The objective of this study was to determine if there are long-term changes to forest soil Hg following clear-cutting and if this is found across multiple study areas. In 2011 through 2013, we sampled the forest floor and mineral soil along three clear-cut chronosequences at three forests. The study areas were Adirondack Ecological Center (AEC) NY, Bartlett Experimental Forest (BEF) NH, and Harvard Forest (HF) MA. In each chronosequence, 4 to 5 forest stands with known ages and similar site characteristics were sampled. Soil properties, such as % clay and soil pH, were similar among forest stands at each study area but varied among study areas. Forest floor and mineral soil Hg concentrations ranged from 0.11 – 0.42 and 0.03 – 0.31 mg kg^-1, respectively. Mercury pools varied significantly among study areas, likely due to different parent materials, deposition rates and soil properties. Stands were grouped as regenerating stands (harvested < 50 yrs ago) and older stands (harvested > 50 yrs ago. Two-Way ANOVA with post hoc Tukey HSD found forest floor and mineral soil Hg pools were also different among regenerating and maturing stands. Changes in weighted Hg pools with stand age were regressed with a third order polynomial because of non-linear functions used in studies on soil carbon. Forest floor and mineral soil pools reached a minimum at 27-yrs of age but recovered after 50-yrs of age. We conclude that clear-cutting does change Hg pools, possibly from enhanced leaching or volatilization.