National-scale estimates of forest root biomass carbon stocks and associated carbon fluxes in Canada

Canada'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 2.3x10(6) km(2) 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 m(2) yr(-1)) with root growth and replacement of turnover contributing 39.8 % of NPP. Average heterotrophic respiration (R-h) 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.