Net ecosystem productivity following fire in the Canadian boreal forest

In Canada, about 2 million ha of forests burn annually and about 8 million ha are lost during extreme fire years. In addition to the carbon directly released to the atmosphere, there is a need to assess the carbon balance of the vegetation being established after a fire. A fire chronosequence was selected to represent the post-fire natural regeneration of the boreal forest. The monitoring sites, located in the boreal plains of Central Saskatchewan, are forests naturally burned in 1998, 1989 and 1977. Dead tree boles and varying amounts of woody debris are present in all three sites, representing the decay through a 25-year period. The vegetation chronosequence is characterized by trembling aspen suckers, being replaced by the regenerating jack pine and black spruce. Net ecosystem productivity (NEP) measurements using eddy covariance since 2001 show that very young forests (less than 5 years) are net carbon sources, whereas a 13-year-old forest is a moderate carbon sink, on an annual basis. However, even a 4-year-old fire can be a carbon sink during the summer. In late summer, respiration processes often dominate photosynthesis, even in a 26-year-old forest. Interannual variability can be substantial, depending on vegetation succession and climate. Here we present daily NEP for these sites over a three-year period.