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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]

Abstract: The effects of climatic factors and vegetation type on evapotranspiration (E) and water use efficiency (WUE) were analyzed using tower-based eddy-covariance (EC) data for nine mature forest sites, two peatland sites and one grassland site across an east–west continental-scale transect in Canada during the period 2003–2006. The seasonal pattern of E was closely linked to growing-season length and rainfall distribution. Although annual precipitation (P) during the observation period was highly variable among sites (250−1450mm), minimum annual E was not less than 200mm and was limited to 400−500mm where annual P exceeded 700mm. Site-specific interannual variability in E could be explained by either changes in total P or variations in solar irradiance. A highly positive linear correlation was found between monthly mean values of E and net radiation (R n ) at the grassland site (AB-GRL), the two peatland sites (AB-WPL and ON-EPL), and only one of the forest sites (coastal Douglas-fir, BC-DF49) whereas a hysteretic relationship at the other forest sites indicated that E lagged behind the typical seasonal progression of R n . Results of a cross-correlation analysis between daily (24-h) E and R n revealed that site-specific lag times were between 10 and 40 days depending on the lag of vapour pressure deficit (D) behind R n and the decoupling coefficient, Ω. There was significant seasonal variation in daytime mean dry-foliage Priestley–Taylor α with maxima occurring in the growing season at all sites except BC-DF49 where it was relatively constant (∼0.55) throughout all years. Annual means of daytime dry-foliage α mostly ranging between 0.5 and 0.7 implied stomatal limitation to transpiration. Increasing D significantly decreased canopy conductance (g c ) at the forest sites but had little effect at the peatland and grassland sites, while variation in soil water content caused only minor changes in g c . At all sites, a strong linear correlation between monthly mean values of gross primary production (GPP) and E resulted in water use efficiency being relatively constant. While at most sites, WUE was in the range of 2.6–3.6gCkg−1 H2O, the BC-DF49 site had the highest WUE of the twelve sites with values near 6.0gCkg−1 H2O. Of the two peatland sites, AB-WPL, a western treed fen, had a significantly higher WUE (∼3.0gCkg−1 H2O) than ON-EPL, an eastern ombrotrophic bog (∼1.8gCkg−1 H2O), which was related to peatland productivity and plant functional type. [Copyright &y& Elsevier]