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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]

Although mature black spruce forests are a dominant cover type in the boreal forest of North America, it is not clear how their carbon (C) budgets vary across the continent. The installation of an eddy covariance flux tower on an Old Black Spruce (OBS) site in eastern Canada (EOBS, Québec) provided a first opportunity to compare and contrast its annual (2004) and seasonal C exchange with two other pre-existing OBS flux sites from different climatic regions located in Saskatchewan [Southern OBS (SOBS)] and Manitoba [Northern OBS (NOBS)]. Although there was a relatively uniform seasonal pattern of net ecosystem productivity (NEP) among sites, EOBS had a lower total annual NEP than the other two sites. This was primarily because warmer soil under a thicker snowpack at EOBS appeared to increase winter C losses and low light suppressed both NEP and gross ecosystem productivity (GEP) in June. Across sites, greater total annual GEP and ecosystem respiration ( R) were associated with greater mean annual air temperatures and an earlier beginning of the growing season. Also, GEP at all three sites showed a stronger relationship with air temperature in spring and early summer compared with later in the growing season, highlighting the importance of springtime conditions to the C budget of these boreal ecosystems. The three sites had different parameter estimates describing the responses of R and GEP at the half hour time scale to near surface temperature and light, respectively. On the other hand, the responses of both R and GEP to temperature at the monthly scale did not differ among sites. These results suggest that a general parameterization could be sufficient at coarse time resolutions to model the response of C exchange to environmental factors of mature black spruce forests from different climatic regions. [ABSTRACT FROM AUTHOR]