Conntribution of northern forests to the global C cycle: Canada as a case study

Abstract

Boreal forests are exposed to periodic stand-replacing disturbances such as wildfire. Unchanging disturbance regimes in unmanaged forests result in an age-class structure in which the proportion of forest area is largest in the youngest age class and decreases exponentially in older age classes. The current (ca. 1970) age-class structure of Canadian forests contains a much smaller proportion of the forest area in each of the two youngest 20-yr age classes than in each of the next three age classes (i.e., the 40 to 99-yr age-classes). We hypothesize that more intensive disturbance regimes in the late 19th and early 20th centuries, compared to disturbances in the period 1920 to 1969, have resulted in this unusual age-class structure. The reduction in disturbance regimes has resulted in an increase of the average forest age and therefore an increase in total forest biomass carbon (C). This C sink is obtained without altering age-dependent growth or decomposition rates. If the average forest age of Canadian forests continues to increase, additional C sequestration of forests, (i.e., the C sink strength) will diminish. This result of a C sink in Canadian forest ecosystems is supported by more detailed C budget calculations for the year 1986.