Carbon, Energy And Water Fluxes At Mature And Disturbed Forest Sites, Saskatchewan, Canada

Fire and harvesting are important disturbances in the boreal forest, driving net biome production. Measurements of net ecosystem production (NEP) over mature forest stands have been made from flux towers for about a decade at the Boreal Ecosystem Research and Monitoring Sites (BERMS) in central Saskatchewan, Canada. Over the last few years, the network of towers has been expanded to include stands that were recently disturbed by fire or harvesting. These new towers are part of Fluxnet-Canada, a national network that ties into the international Fluxnet network. Data from 2001 and 2002 show that forests burned in 1998 (F98) or harvested in 1994 (HJP94) were net carbon sources to the atmosphere with annual NEP values in the range of −50 to −130 g C m⁻² y⁻¹. A site burned in 1989 (F89) was a carbon sink of about 68 g C m⁻² y⁻¹ in 2002. NEP at a mature jack pine site (SOJP) ranged from 41 to −23 g C m⁻² y⁻¹ in this period, whereas, an old black spruce site (SOBS) ranged from 68 to 21 g C m⁻² y⁻¹. A mature aspen site (SOA) had the greatest NEP with 361 g C m⁻² y⁻¹ in 2001 and 139 g C m⁻² y⁻¹ in 2002, the contrast between years attributed to differences in spring temperature and leaf emergence, as well as a long-term drought at the site. Evapotranspiration (ET) followed similar patterns although the F89 site had the highest ET. The deciduous tree components of SOA and F89 are likely responsible for lower summer Bowen ratios with greater energy going into ET, compared to coniferous sites. These data indicate that there is still a broad gap in our knowledge of carbon fluxes to forest stands between about 10 and 50 years of age. This is especially true in understanding the dynamics of the growth of successional vegetation along with heterotrophic respiration, highlighted by predictions of more boreal fire disturbance in the future.