How does forest harvest influence carbon dioxide fluxes of black spruce ecosystems in eastern North America?

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 (0–6 g C m−2 year−1), while HBS00 was a fairly strong C source (119–167 g C m−2 year−1). Averaged over both years, gross ecosystem productivity (GEP) was 101% higher at EOBS compared to HBS00 (636 g C m−2 year−1 versus 316 g C m−2 year−1), while ecosystem respiration (R) was 37% higher at EOBS (633 g C m−2 year−1 versus 462 g C m−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 (12.5 μmol m−2 s−1), whereas it increased from 4.7 to 7.3 μmol m−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.