The allometry and partitioning of above- and belowground tree biomass was studied in an age-sequence of four eastern white pine (Pinus strobus L.) forests (2-, 15-, 30-, and 65-year-old) in southern Ontario, Canada. Biomass in each tree component, i.e. foliage, branch (live and dead), bark, stem and root was quantified by destructive tree harvesting. Allometric biomass equations were developed for each tree component within each stand (age class) and across the entire age-sequence. Mean above- and belowground tree biomass was 0.3, 54, 105, and 529 kg tree−1 and 0.1, 13, 17, and 99 kg tree−1 in the 2-, 15-, 30-, and 65-year-old stands, respectively. The relative portion of stem biomass to total tree biomass increased from 25% for the 2-year-old stand to 69% for the 65-year-old stand, whereas the relative contribution of canopy biomass components (foliage and branches) decreased with stand age. Root to shoot biomass ratio decreased from 0.32 in the 2-year-old stand to 0.24, 0.16, and 0.22 in the 15-, 30-, and 65-year-old stands, respectively, indicating a decrease during the first few decades after stand establishment. Allometric relationships between biomass of individual aboveground tree components and tree diameter changed with stand age and thus resulted in age-specific allometric equations. In contrast, a single allometric equation could predict total aboveground and belowground, and total tree biomass from tree diameter only across the entire age-sequence. The relationship between tree component biomass and stem volume, expressed as biomass expansion factor (BEF), decreased for all above- and belowground tree components and total tree biomass with increasing stand age. Changes in tree biomass allocation and allometry throughout different stages of forest stand development need to be considered in order to improve forest biomass and carbon sequestration accounting on regional and national scale.