Decline in Net Ecosystem Productivity Following Canopy Transition to Late-Succession Forests

Boreal forests are critical to the global carbon (C) cycle. Despite recent advances in our understanding of boreal C budgets, C dynamics during compositional transition to late-succession forests remain unclear. Using a carefully replicated 203-year chronosequence, we examined long-term patterns of forest C stocks and net ecosystem productivity (NEP) following stand-replacing fire in the boreal forest of central Canada. We measured all C pools, including understorey vegetation, belowground biomass, and soil C, which are often missing from C budgets. We found a slight decrease in total ecosystem C stocks during early stand initiation, between 1 and 8 years after fire, at −0.90 Mg C ha⁻¹ y⁻¹. As stands regenerated, live vegetation biomass increased rapidly, with total ecosystem C stocks reaching a maximum of 287.72 Mg C ha⁻¹ 92 years after fire. Total ecosystem C mass then decreased in the 140- and 203-year-old stands, losing between −0.50 and −0.74 Mg C ha⁻¹ y⁻¹, contrasting with views that old-growth forests continue to maintain a positive C balance. The C decline corresponded with canopy transition from dominance of Populus tremuloides, Pinus banksiana, and Picea mariana in the 92-year-old stands to Betula papyrifera, Picea glauca, and Abies balsamea in the 203-year-old stands. Results from this study highlight the role of succession in long-term forest C dynamics and its importance when modeling terrestrial C flux.