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

Abstract

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.

Boundary‐Crossing Field Research Marks the Way to Evidence‐Based Management of Mercury in Forest Landscapes

“…Highlighted in yellow are the environmental system boundaries crossed in a 4 year long forest harvest Before‐After‐Control‐Impact experiment conducted at the Marcel Experimental Forest (North‐central USA) that are reported by McCarter et al. (2022) and companion studies. Two more boundaries that warrant consideration are highlighted in blue.…”

“…Scientific studies that cross these boundaries improve the understanding of forest harvest effects on the mobilization and subsequent bioaccumulation of mercury in these landscapes. Highlighted in yellow are the environmental system boundaries crossed in a 4 year long forest harvest Before-After-Control-Impact experiment conducted at the Marcel Experimental Forest (North-central USA) that are reported by McCarter et al (2022) and companion studies. Two more boundaries that warrant consideration are highlighted in blue.…”

“…McCarter et al. (2022) is thus a welcome addition to the literature with its thorough analysis of results from an exceptionally comprehensive experiment on responses to forest harvest using before‐after‐control‐impact (BACI) design. Two years of pre‐treatment data were collected from three instrumented hillslopes and the wetland which those hillslopes drained into.…”

“…Unfortunately, in the competition for funds with other environmental threats, Hg research does not have the privilege of being counted among substances of "emerging" concern that have dedicated funding streams. Nonetheless, despite the challenges, McCarter et al (2022), were able to show that it is possible to cross multiple system boundaries with a study of a management intervention (forest harvest) that spans 4 years (2 years of both pre-and post-treatment). The research infrastructure of the Marcell Experimental Forest was undoubtedly a key ingredient in making this possible, which is also important to bear in mind as one prioritizes between research infrastructure investment and funding for research projects that require research infrastructures.…”

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McCarter et al (2022) have provided a carefully planned and executed study of how forest harvest influences the mobility and bioaccumulation of mercury. This is a valuable addition to the knowledge of how one can adapt forestry to reduce the risks of releasing more mercury (Hg) into the environment.

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