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Abstract: Forest harvest and subsequent stand development can have major effects on the carbon cycle of boreal stands. Carbon dioxide (CO2) fluxes of a three-point black spruce harvest chronosequence located in the boreal forest of eastern North America were measured over a one-year period at the ecosystem scale with the eddy covariance technique and CO2 efflux from soils was measured with a portable infrared gas analyzer. The three sites (pre-harvest, recently harvested, and juvenile) were 105-, 8- and 33-years old, respectively. On an annual basis, the pre-harvest site (EOBS) was a weak carbon sink (6±4gCm−2 yr−1), the recently harvested site (HBS00) a source (−87±3gCm−2 yr−1) and the juvenile site (HBS75) a moderate to strong sink (143±35gCm−2 yr−1). Annual gross ecosystem production (GEP) at the pre-harvest site was only 28% greater than at the recently harvested site (646±6 versus 504±5gCm−2 yr−1), while GEP at the juvenile site (1107±32gCm−2 yr−1) was more than double that at the recently harvested site, suggesting significant physiological constraints to photosynthesis at the pre-harvest site. Annual ecosystem respiration (R e) followed the same pattern, but intersite differences were somewhat less (640±8 to 591±6 to 964±50gCm−2 yr−1). Annual soil respiration (R s) decreased following harvest from 593 to 500gCm−2 yr−1 and increased with further stand development to 644gCm−2 yr−1, although the changes were less than for GEP and R e. Q 10 and R 10 of R s for the snow-free period varied between sites, were lowest for the recently harvested site, and appeared to be related to GEP via substrate supply. The annual ratio of R s to R e was lower for the juvenile site (67%) than for the pre-harvest and recently harvested sites (93 and 85%, respectively). These results quantify how some of the major physiological processes that influence the carbon cycle of boreal black spruce stands evolve following harvest and should be useful for better incorporating stand-age effects into regional and global scale models. [Copyright &y& Elsevier]

Process-based models are effective tools to synthesize and/or extrapolate measured carbon (C) exchanges from individual sites to large scales. In this study, we used a C- and nitrogen (N)-cycle coupled ecosystem model named CN-CLASS (Carbon Nitrogen-Canadian Land Surface Scheme) to study the role of primary climatic controls and site-specific C stocks on the net ecosystem productivity (NEP) of seven intermediate-aged to mature coniferous forest sites across an east–west continental transect in Canada. The model was parameterized using a common set of parameters, except for two used in empirical canopy conductance–assimilation, and leaf area–sapwood relationships, and then validated using observed eddy covariance flux data. Leaf Rubisco-N dynamics that are associated with soil–plant N cycling, and depend on canopy temperature, enabled the model to simulate site-specific gross ecosystem productivity (GEP) reasonably well for all seven sites. Overall GEP simulations had relatively smaller differences compared with observations vs. ecosystem respiration (RE), which was the sum of many plant and soil components with larger variability and/or uncertainty associated with them. Both observed and simulated data showed that, on an annual basis, boreal forest sites were either carbon-neutral or a weak C sink, ranging from 30 to 180 g C m−2 yr−1; while temperate forests were either a medium or strong C sink, ranging from 150 to 500 g C m−2 yr−1, depending on forest age and climatic regime. Model sensitivity tests illustrated that air temperature, among climate variables, and aboveground biomass, among major C stocks, were dominant factors impacting annual NEP. Vegetation biomass effects on annual GEP, RE and NEP showed similar patterns of variability at four boreal and three temperate forests. Air temperature showed different impacts on GEP and RE, and the response varied considerably from site to site. Higher solar radiation enhanced GEP, while precipitation differences had a minor effect. Magnitude of forest litter content and soil organic matter (SOM) affected RE. SOM also affected GEP, but only at low levels of SOM, because of low N mineralization that limited soil nutrient (N) availability. The results of this study will help to evaluate the impact of future climatic changes and/or forest C stock variations on C uptake and loss in forest ecosystems growing in diverse environments. [ABSTRACT FROM AUTHOR]