The epiphytic corticolous moss Dicranum montanum, a common inhabitant of upland Quercus rubra (northern red oak) trunks in southwestern Pennsylvania, is a useful biomonitor to evaluate levels of total mercury (Hg) in stemflow within mixed-oak forests. Moss samples were collected annually during October–November of 2000–2005 at 11 permanent research plots on a ridgetop and analyzed for total-Hg concentrations. The mean total-Hg concentration in moss tissue across all 11 plots for all 6 years (n = 66) was 424 ng/g dry wt. Total Hg for 9 of the 11 plots exhibited a downward time trend during the 6-year period; the trend line for the remaining two plots was nearly horizontal. When all data were combined, the linear regression based on mean Hg values for all plots was significant at p = 0.028 over the 6-year study period, with an adjusted R-square of 67.6%. This downward trend likely reflects reduction of airborne Hg emissions in the region due to several factors, including closure of steel mills and coke ovens in the Johnstown–Pittsburgh area, co-benefit of sulfur dioxide emissions controls, and reduction of Hg emissions from municipal and hospital incinerators.