Carbon dynamics of boreal mixedwoods in central Canada

Large amount of carbon (C) stored in boreal forest ecosystems plays an important role in the global carbon cycle. Forest ecosystem C is stored in four major pools: live vegetation (above- and belowground), dead woody material (snags, down woody debris, and stumps), organic soil horizons (forest floor), and mineral soil. Carbon cycling among these pools is mainly determined by disturbance. A large portion of previously fire-driven ecosystems in boreal North American forest are now managed. How C cycling of managed stands differs from that of natural stands strongly influences boreal forest carbon balance. In this thesis I first synthesize literature on carbon dynamics in North American boreal forests followed by a study about aboveground biomass C dynamics in boreal mixedwoods after wildfire and clearcut harvest.;Live biomass carbon (C) pool dynamics are central to understanding carbon sequestration of forest ecosystems. Despite its importance, how aboveground live C pools change with stand development of boreal mixedwoods is poorly understood, in particular in relation to different disturbance origins. I quantified above ground live biomass C pools in a postfire chronosequence ranging from 1 to 203 years and a post-logging chronosequence ranging from 1 to 27 years in boreal mixedwood forest in central Canada. Stands were chosen along a successional sequence in boreal mixedwoods. Total live biomass C was significantly affected by time since disturbance and stand origin with time since disturbance explaining most of the variation. The biomass C dynamics was largely determined by overstory tree dynamics as it constituted most of stand biomass. Biomass in postfire stands reached maximum (109.18 Mg C/ha) at the age of 92 years after fire and then declined in older age classes. Post-logged stands had significantly more biomass C compared to postfire stands with the biggest difference at 27 years after disturbance when post-logged stands had 38.5 Mg C/ha more C than postfire stands.;Key Words: Carbon dynamics, carbon pools, live biomass, dead wood, forest floor, mineral soil, boreal forest, mixedwoods, chronosequence, wildfire, clearcut logging.;Carbon pool dynamics in forest ecosystems after stand replacing wildfire are described and compared with carbon dynamics after clearcut harvesting. Following a stand replacing disturbance, 1) live biomass increases rapidly leading to the maximal biomass stage, then stabilizes at stand maturation; 2) dead woody material carbon generally follows a 'U' or'S' shaped pattern during succession; 3) Forest floor carbon increases throughout stand development; 4) mineral soil carbon is the largest terrestrial organic carbon pool and is highly variable, but an overall increase over time has been observed. These pools are connected and throughout stand development C allocation form one pool to another takes place. Wildfire and harvesting differ in many ways, fire being more as a chemical and harvesting as a mechanical disturbance. Fire consumes forest floor and small live vegetation and foliage while during logging much of large size biomass is removed and forest floor is mechanically disturbed. Another major difference is the absence of coarse woody debris after logging. Also, regeneration of the new stand is different after wildfire and logging. Fire often destroys advanced regeneration while it is less affected by logging, resulting different species composition and therefore different stand C dynamics. Overall, the manner in which different types of disturbance effect C dynamics in boreal forest is poorly understood. Further studies following wildfire but especially forest harvesting are needed to improve understanding of the effect of disturbance on C dynamics.