Comparison of carbon dynamics following fire and harvesting in Canadian boreal forests

Mature boreal forests are very important sinks for atmospheric carbon dioxide (CO₂), the major greenhouse gas (GHG) implicated in global warming. However, following disturbance (e.g. fire, harvesting, wind-throw, insects and diseases), boreal forests may become a CO₂ source for several years. Fire and harvesting are recognised as major forest renewal processes in many forests, especially the Canadian boreal forest. It is estimated that on average 2-3 million ha of forest are burned by fire each year while 1 million ha are harvested. Both fire and harvesting impact the age and species composition and alter the surface characteristics, thus impacting the carbon and energy dynamics of the forest. Although a few studies have evaluated carbon dynamics of boreal forests following fire or harvest, none have directly compared the two forest renewal mechanisms. The major objective of this study was to compare carbon dynamics following fire and harvesting. The study used net ecosystem CO₂ exchange data collected from three post-fire sites (F77; burned in 1977, F89; burned in 1989, F98 burned in 1998), three post-harvest sites (HJP75; harvested in 1975, HJP94; harvested in 1994, HJP02; harvested in 2002) and one mature site (OJP; last burned in 1929). The sites are located in central Saskatchewan, Canada and the data were collected continuously throughout the year using eddy covariance techniques. The sites are within 100 km of each other and they experience similar climate. Data collected in 2005 showed that total annual net ecosystem productivity (NEP) was -78, 53, -43, 79, -31, -123 and 36 g C m^-2 y^-1 for F77, F89, F98, HJP75, HJP94, HJP02 and OJP, respectively. This indicates that regardless of disturbance method, young boreal forests are carbon (C) sources, but as they mature they become C sinks. Surprisingly, F77 continued to be a C source even though F89 and HJP75 (closer in age) were significant C sinks. In general, the burned sites had higher gross ecosystem productivity (GEP) and ecosystem respiration (ER) than the harvest sites, with F77 having the highest ER (984 g C m^-2 y^-1) and HJP02 the lowest (222 g C m^-2 y^-1). The ratio of annual ER to annual GEP was 1.09, 0.94, 1.09, 0.87, 1.06, 2.24 and 0.94 for F77, F89, F98, HJP75, HJP94, HJP02 and OJP, respectively. At all sites except HJP02, ER increased exponentially with soil temperature at the 2 cm depth with the two fire sites (F77 and F89) having the highest response. At HJP02 the response was linear. The response of GEP to photosynthetic active radiation (PAR) was greatest at F89 and least at HJP02. It is likely that most of the differences among the young sites are caused by variation in the development of vegetation following disturbance.