Forest Harvesting Does Not Lead to Hotter Methylmercury “Hot Spots” at the Forested Upland-Peatland Ecotone

Carl P J Mitchell, University of Toronto Scarborough, Physical and Environmental Sciences, Toronto, ON, Canada, Kristine Haynes, University of Toronto, Department of Geography, Toronto, ON, Canada, Susan Eggert, USDA Forest Service, Northern Research Station, Grand Rapids, United States, Randy Kolka, USDA Forest Service, Northern Research Station, Grand Rapids, MN, United States and Stephen D Sebestyen, USDA Forest Service, Northern Research Station, Vallejo, CA, United States

Contact First Author: Carl P J Mitchell; carl.mitchell@utoronto.ca

Previously Published Material: Some parts were previously presented at last year's CGU meeting, but not submitted yet to a journal.

Abstract ID#: 34277

 

English Abstract:
In small bogs, hot spots of methylmercury production and/or accumulation often occur near their margins because these areas receive larger inputs, via connections to upland runoff, of limiting nutrients and minerals required by methylating bacteria. Hypothetically, upland disturbances such as forest harvesting, which tend to increase runoff, may exacerbate these hot spots. This hypothesis was tested in a field experiment in northern Minnesota wherein hillslope interflow runoff and chemical fluxes were measured using runoff trenches across three forested experimental plots. Following more than a year of pre-treatment data, two of the experimental plots were harvested; one was clearcut with most of the branch debris left on the forest floor and the second was clearcut with ~85% of harvesting debris removed, simulating a biomass harvest (called “clearcut+biomass harvest”). The third plot was left undisturbed as a control. Prior to forest harvesting, peat methylmercury concentrations at the upland-peatland ecotone were spatially and seasonally variable (mean ± standard deviation: 6.4 ± 2.7 ng/g). As expected in the control plot, neither hillslope runoff nor total mercury fluxes changed significantly across the pre-/post-impact period. Both forest harvests had a significant impact on runoff (40-50% increase) and mercury fluxes (60-70% increase). Contrary to predictions, methylmercury concentrations in peat at the ecotone to these forest plots decreased significantly in the year following forest harvesting. We also observed considerably less spatial and temporal variability within the upland-peatland ecotone, post-harvest. Most likely, the increased runoff and chemical fluxes following forest harvesting led to more equally widespread promotion of demethylation. At least in the short-term (1-2 years), forest harvesting does not appear to exacerbate the formation of methylmercury in hydrologically connected peatlands in this region nor does biomass harvesting significantly worsen any potential impacts from traditional clearcutting alone.
Back to: Mercury Biogeochemistry and Environmental Change III Posters