Ageing forests and carbon storage: a case study in boreal balsam fir stands

The pattern of change in carbon (C) accumulation with forest ageing can vary greatly amongst different forest types. Documenting how C accumulates in various forest ecosystems in the absence of logging makes it possible to predict what would be the outcome of extending forest rotations or in dedicating more land to conservation on C storage. This study was conducted in boreal balsam fir (Abies balsamea (L.) Mill.) forests of Quebec, in eastern Canada. We compared carbon stocks in forest pools (aboveground (live) biomass, deadwood, FH horizon and mineral soil) of mature (70 years after harvest) vs old-growth stands (stands with no signs or history of human disturbance). Total ecosystem C stocks were not significantly different between mature and old-growth stands. However, as mature stands transition to old-growth stage, there appears to be a shift of C from live biomass pools towards deadwood and soil FH horizons. Coarse woody debris in old-growth stands were also found to be at more advanced stages of decay. The variability of C stocks was also high amongst old-growth stands; however, there was no obvious difference in structural diversity between mature and old-growth stands. Results suggest that ageing balsam fir stands through the lengthening of forest rotations (e.g. past the maturity age of 70 years) or by placing them under conservation, while not creating an important C sink, still contributes to maintain large forest C stocks across landscapes. Preserving or increasing the presence of old-growth forests is an important aspect of ecosystem-based forest management; our study concludes that it could also be compatible with sustainable forest carbon management.