Many ecosystems are now dominated by introduced species, and because dominant species drive ecosystem properties, these changes lead to increased uncertainty in estimates of carbon storage and cycling. We examined aboveground biomass in forests dominated by the introduced tree Rhamnus cathartica (common buckthorn) relative to forests dominated by native species, and measured aboveground biomass increment over a three-year period (2005–2008). Three of the four lowest biomass levels occurred in R. cathartica-dominated forests, and biomass in these forest types was stored primarily in trees 10–20cm DBH. By contrast, forests dominated by native trees (including those with R. cathartica understories) had the six highest biomass levels, and biomass was stored primarily in trees >50cm DBH. On average, forests dominated by R. cathartica stored half as much aboveground biomass (14.6±3.3kg/m 2 ) as forests dominated by native tree species (28.9±8.3kg/m 2 ). R. cathartica-dominated forests also had half the aboveground biomass increment of native-dominated forests (0.28 vs. 0.60kg/m 2 /year). Although known anecdotally as a fast-growing species, R. cathartica growth rates declined with increasing size. Between 2005 and 2008, R. cathartica individuals <10cm DBH grew faster than native species; however, R. cathartica individuals >10cm DBH grew consistently slower than native species. Overall, our findings indicate that intrinsic size limitations on R. cathartica will lead to lower biomass stocks in forests where it acts as a canopy dominant relative to forests dominated by native tree species.