AbstractCanada's forests play an important role in the global carbon cycle through carbon (C) storage and C exchange with the atmosphere. While estimates of aboveground biomass have been improving, little is known about belowground C storage in root biomass. Here we estimated the contribution of roots to the C budget of Canada's 2.3 × 106 km2 managed forests from 1990 to 2008 using the empirical modeling approach of the Carbon Budget Model of the Canadian Forest Sector (CBM‐CFS3) driven by detailed forestry data sets from the National Forest C Monitoring, Accounting and Reporting System. The estimated average net primary production (NPP) during this period was 809 Tg C yr−1 (352 g C m2 yr−1) with root growth and replacement of turnover contributing 39.8 % of NPP. Average heterotrophic respiration (Rh) was 738 Tg C yr−1 (321 g C m−2 yr−1), which resulted in a net ecosystem production (NEP) value of 31 g C m−2 yr−1(71 Tg C yr−1), and on average only 8.7% of NPP remained in the system as NEP. Estimated average root C stocks were 2.38 Pg (1235 g C m−2), mostly in coarse roots (≥ 5 mm diameter), and had an average root to shoot percentage (belowground to aboveground biomass) of 25.6%. Detailed monitoring of C exchange between forests and the atmosphere and an improved understanding of the belowground processes and their response to environmental changes are needed to improve our understanding of the terrestrial C budget.