Slow growth, maintenance of a high leaf:wood ratio and adoption of a clonal growth habit, more than size per se, may increase the life span in trees species. The longevity of black spruce (Picea mariana (Mill.) BSP.) is increased from 200 to 300 years, when it grows as a clonal shrub. We measured the surface area and above- and below-ground biomass of 25 post-fire, stunted black spruce clones identified from RAPDs markers. The maximum age of each clone was deduced from tree-ring dating or by radiocarbon dating of charcoal fragments. The oldest clone was > 1800 years of age. The total surface area of the clones increased with age, ranging from 2.8 to 691.3 m2. The ratio of living aerial parts to the total surface area decreased from 100% to < 50% with post-fire stand age, reflecting fragmentation of clones into many autonomous or potentially autonomous rooted branches (layers). The number of layers increased with age from 12 layers in a 100-year-old individual to more than 80 in a 1800-year-old clone. Biomass allocation and fragmentation can explain the maintenance of a relatively stable leaf : wood ratio of approximately 10% through time in stunted black spruce clones. The fragmentation of layers is the main mechanism ensuring the great longevity of prostrate clones in exposed sites. In the absence of perturbation, stunted black spruce clones may perpetuate for centuries or even millennia.