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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]

Abstract: Forest harvest is a major disturbance in the boreal forest of eastern North America and should significantly impact biosphere–atmosphere interactions at the regional scale. During a cooler, wetter year (2004) and a warmer, drier year (2005), we compared carbon dioxide (CO2) fluxes over a mature black spruce stand (EOBS) with a site that was harvested in 2000 (HBS00) and that had similar soil parent material, site fertility, climate, and pre-harvest species composition. During the study period, EOBS was C neutral (0–6gCm−2 year−1), while HBS00 was a fairly strong C source (119–167gCm−2 year−1). Averaged over both years, gross ecosystem productivity (GEP) was 101% higher at EOBS compared to HBS00 (636gCm−2 year−1 versus 316gCm−2 year−1), while ecosystem respiration (R) was 37% higher at EOBS (633gCm−2 year−1 versus 462gCm−2 year−1). The mean between-site difference in annual NEP was six times greater than was the mean between-year difference, thus suggesting that the C budget of boreal black spruce forests is much more affected by developmental stage (i.e., stand age) than by between-year climate variability. Relative to the mature site, the harvested site had a more dynamic structure due to plant regrowth that induced greater between- and within-year variability in the response of GEP and R to environmental conditions over the 2-year study period. For example, maximum photosynthetic capacity was stable between years at EOBS (12.5μmolm−2 s−1), whereas it increased from 4.7 to 7.3μmolm−2 s−1 from 2004 to 2005 at HBS00. The photosynthetically active growing season started about a week later and finished a week earlier at HBS00 relative to EOBS. The earlier snowmelt at the harvested site did not promote an earlier start of the growing season at this site compared to the mature site. Although environmental conditions in spring have a significant influence on the annual C budget of mature sites, this does not seem to be the case for disturbed sites where mid-summer conditions are more important to the annual C balance. [Copyright &y& Elsevier]