Although coarse root (diameter > 1 cm) biomass can account for approximately 30% of total biomass in forest ecosystems, coarse root biomass models are rarely available, especially for larger trees. In this study, we used a back-hoe excavator and pressurized water to excavate the coarse root systems of large size trees of Abies balsamea L., Picea mariana (Mill.) BSP, Pinus banksiana Lamb., and Populus tremuloides Michx. in the boreal mixedwood forest of central Canada in order to develop allometric equations that link stem diameter at breast-height (DBH) and height to coarse root biomass. All regressions for coarse root biomass using DBH or height alone, or both DBH and height as predictors were significant (R-2 >= 0.796, P < 0.05). The DBH - coarse root biomass models had higher R-2 values than the height - coarse root biomass models for all four species, indicating that DBH was a better predictor of coarse root biomass than height. In addition, the DBH-height - coarse root biomass models did not have higher R-2 values than the DBH - coarse root biomass models. All but one DBH - coarse root biomass model from the published literature with similar DBH range underestimated or overestimated coarse root biomass using the data from this study. It would appear that coarse root biomass allometric equations are probably site-specific because above- and below-ground biomass allocation differs with site condition. However, differences among studies may also be attributable to sampling methodologies. (C) 2011 Elsevier Ltd. All rights reserved.