20th century carbon budget of Canadian forests

Our analysis of the carbon budget of Canada’s forests (1920-1989) indicates that these forest ecosystems have been a C sink of approximately 0.2 Gt C yr^-1. This result challenges the previously-held assumption that forests not directly affected by land use make zero net C contribution to the atmosphere. We attribute our observed C sink to a shift in the forest age-class structure towards a greater average forest age. Forest disturbances, which largely determine Canadian forest dynamics on a time scale of decades, appear to have been less frequent in the period 1920-1970 than in previous decades. They have, however, increased greatly in recent years (1970-1989) and have contributed to a decrease in the C sink. Forests that are subject to large-scale fluctuations in natural disturbance regimes on a time-scale comparable to tree lifetimes do not appear to reach an equilibrium C-exchange with the atmosphere on these time-scales. Assessing C budgets of such forest ecosystems requires an accounting of C dynamics for the entire forest area, not merely for that portion which has recently been affected by anthropogenic disturbances.