Effects of Clearcutting and Residual Biomass Harvesting on Hillslope Mercury Mobilization and Downgradient Mercury Accumulation

Mercury (Hg) in forest runoff varies geographically and in relation to silvicultural practices. There is also considerable uncertainty about how forest management practices, such as residual biomass removal, affect Hg mobilization, and downgradient cumulative effects. In this study, total Hg, dissolved organic carbon (DOC), and sulfate (SO42-) mobilization in runoff were compared among unharvested and harvested hillslopes with and without residual biomass removal, and peat soil and invertebrate methylmercury (MeHg) concentrations were assessed in a down-gradient peatland. Using a before-after-control-impact design, runoff from three adjacent hillslopes was monitored pre-harvest (2010-2011) and post-harvest (2012-2013) from snowmelt to freeze-up at the USDA Forest Service's Marcell Experimental Forest in northern Minnesota. Dilution, due to increased available hillslope soil water, led to decreased THg and DOC concentrations after harvest at both harvested hillslopes. Sulfate concentrations did not statistically change following harvest. Compared to its removal, leaving residual biomass significantly increased yields of all solutes but only DOC yields significantly increased when the residual biomass was removed. In the adjacent down-gradient peatland, peat MeHg concentrations decreased, whereas no change was observed in peat below the unharvested hillslope. There was no discernible change post-harvest in MeHg levels among several peatland macroinvertebrate taxa. This study provides a much-needed hillslope-scale understanding of Hg response to forest management practices, highlighting that increased solute yields from hillslopes do not necessarily stimulate Hg methylation or invertebrate bioaccumulation in down-gradient peatland systems.

Automated summary

This study compared the effects of different forest management practices on the migration of mercury and related substances in runoff, and assessed the mercury concentrations in downstream peat soil. The study found that the removal of residual biomass significantly increased solute yields, but did not necessarily stimulate mercury methylation or accumulation in invertebrates.