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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: Carbon (C) cycling in the boreal forest is driven by both natural and human disturbances, but there is little information on the impact of forest management practices on ecosystem-level C balance. We evaluated the C balance of a recently harvested boreal site in eastern Canada by measuring CO2 fluxes with the eddy covariance technique for 1 year before the application of a scarification treatment (mechanical site preparation) and for 1 year after the treatment was applied to approximately 40% of the study area. Net annual exchange indicated a source of 111gCm−2 year−1 before scarification that increased to 175gCm−2 year−1 after treatment. Annual gross ecosystem productivity (GEP) and net C flux varied between years, but there was no significant difference for ecosystem respiration. Since the differences in climate between years did not explain the changes in the site C balance and daytime net C sequestration under non-limiting environmental conditions was generally lower after the treatment, the large difference in C emissions was most likely due to decreased GEP resulting from the destruction of approximately 60% of the living aboveground vegetation within the scarified areas. Although daily NEP was almost always negative throughout the year, a net daily C sink was observed during a 2-week period in late summer 2003 when air temperatures were approximately 8°C cooler than preceding weeks. An analysis of the residuals from light–response regressions showed that soil water content and vapor pressure deficit were the second most important variables explaining morning and afternoon net C flux in 2003 and 2004, respectively. [Copyright &y& Elsevier]

Net ecosystem productivity (NEP) during August 2003 was measured by using eddy covariance above 17 forest and 3 peatland sites along an east–west continental-scale transect in Canada. Measured sites included recently disturbed stands, young forest stands, intermediate-aged conifer stands, mature deciduous stands, mature conifer stands, fens, and an open shrub bog. Diurnal courses of NEP showed strong coherence within the different ecosystem categories. Recently disturbed sites showed the weakest diurnal cycle; and intermediate-aged conifers, the strongest. The western treed fen had a more pronounced diurnal pattern than the eastern shrub bog or the Saskatchewan patterned fen. All but three sites were clearly afternoon C sinks. Ecosystem respiration was highest for the young fire sites. The intermediate-aged conifer sites had the highest maximum NEP (NEPmax) and gross ecosystem productivity (GEPmax), attaining rates that would be consistent with the presence of a strong terrestrial C sink in regions where these types of forest are common. These results support the idea that large-scale C cycle modeling activities would benefit from information on the age-class distribution and disturbance types within larger grid cells. Light use efficiency followed a pattern similar to that of NEPmax and GEPmax. Four of the five recently disturbed sites and all three of the peatland sites had low water use efficiencies. [ABSTRACT FROM AUTHOR]

The impact of freezing temperatures and exposure times during budbreak and shoot elongation on 10-month-old containerized white spruce seedlings [ Picea glauca (Moench) Voss] was studied. Seedlings were exposed to 0, -2, -4, -6, -8 and -10°C for 0, 1, 2, 3, 4, 5 and 6 h at four stages of budbreak and shoot elongation (stage 1: unswollen bud; stage 2: swollen bud; stage 3: bud scales parted by emerging needles; stage 4: 1-5 cm of terminal shoot growth). Seedling and terminal bud survival, viability of first-year needles (developed before frost exposure), terminal shoot length, stem diameter and aerial dry mass were measured 90 days after freezing exposure. Frost severity had a much greater influence on seedling survival, damage and subsequent growth than exposure time. The bud development stage at which freezing temperatures were applied also affected survival and subsequent growth. [ABSTRACT FROM AUTHOR]

The root systems of 2-yr-old Picea glauca, Picea mariana and Pinus banksiana seedlings were submitted to various frost temperatures during an artificial frost to induce different levels of root damage. Frost-damaged and control seedlings were placed in a greenhouse under high and low soil moisture regimes. Seedling growth and physiology were evaluated periodically. Seedling survival was reduced when root damage reached levels of 60-80%. Root systems of all three species showed partial to total recovery by the end of the experiment. In general, root freezing damage caused reductions in seedling growth, with these reductions becoming less significant over time. Root damage had little to no effect on black spruce and jack pine seedling physiology, while white spruce CO[sub 2] uptake decreased with increasing root damage. Shoot nitrogen content of all three species decreased slightly with increasing root damage. [ABSTRACT FROM AUTHOR]

Examines the effects of induced root freezing on the growth and physiology of boreal conifer seedlings. Decrease of stomatal conductance; Reduction in nitrogen concentration of current-year foliage; Indication of reduced in annual height increment of white spruce.

Whole root systems of 2-yr-old containerized white spruce [Picea glauca (Moench) Voss], black spruce [Picea mariana (Mill.) B.S.P.] and jack pine (Pinus banksiana Lamb.) seedlings, with intact root plugs, were exposed to various frost temperatures, which a preliminary test indicated would induce approximately 0 (control), 20, 40, 60, 80 and nearly 100% frost damage. Damage to root systems was evaluated using: (1) two measures of electrolyte leakage (relative conductivity and total tissue leakage after autoclaving); (2) water loss after pressurization; (3) chlorophyll fluorescence (F[sub v]/[sub Fm], maximal PSII photochemical efficiency) measured 4, 21 and 30 days after the beginning of seedling regrowth and (4) live root dry mass measured 21 days after the artificial frost and 60 days after the beginning of regrowth. Seedling survival and growth after the artificial frost were evaluated using live root dry mass measured after 60 days of regrowth and new shoot length, stem diameter, and root and shoot dry mass. Live root dry mass, dead tissue leakage, jack pine root water loss and fluorescence measurements were all significantly correlated with one or more of the growth variables and the number of significant correlations varied with species. Dead tissue leakage measurements appear to be the most promising method for evaluating root damage to 2-yr-old well-developed root systems of these species. [ABSTRACT FROM AUTHOR]

Abstract: A boreal bog located in the James Bay lowlands, Canada, was instrumented with an open-path gas analyzer to monitor the turbulent fluxes of methane throughout the summer of 2012. The mostly continuous eddy covariance measurements permitted the study of methane dynamics at the hourly, daily and seasonal scales. To exclude data segments for which the biological methane fluxes were underestimated due to inefficient atmospheric transport under stable stratification, we applied a novel approach based on both the atmospheric stability parameter ζ = z/L and the friction velocity u ∗, where z is the measurement height and L the Obukhov length. The field measurements revealed the existence of at least one sustained ebullition event, triggered by low barometric pressures, a declining water table and increasing mechanical turbulence – suggesting that large-scale release of methane bubbles can be an important transport mechanism of methane in boreal bogs. The validity of similarity scaling for atmospheric methane under convective conditions was also assessed and the normalized standard deviations of methane concentrations did not scale well with ζ, highlighting the heterogeneity in natural methane production and release across the bog. Overall the hourly emissions ranged between −2.0 and 32.1 mg CH4 m−2 h−1, with a summertime mean of 2.4 mg CH4 m−2 h−1. At the daily scale, the two main controls on methane emissions were found to be the water table position and the peat temperature at 0.3 m under the surface. Contrary to other studies, seasonal methane emissions peaked when the water table was at its maximum distance from the surface, around mid-August. No clear diurnal pattern could be found in methane emissions, indicating that methane was produced quite deep within the peat. The seasonal emissions were estimated at 4.4 g CH4 m−2, and compared well with other observations over similar landscapes using different measurement techniques. Given that methane releases and transport are greatly affected by local characteristics such as climate and vegetative cover, this study emphasizes the need for further in situ continuous measurements of methane fluxes across northern peatlands. [ABSTRACT FROM AUTHOR]

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]

In northern landscapes, peatlands are widespread and their hydrological processes are complex. Furthermore, they are typically remote, limiting the amount and accuracy of in situ measurements. This is especially the case for evapotranspiration ET, which strongly influences watershed hydrology. The objective of this paper is to demonstrate the validity of the bulk-transfer approach to estimate ET over boreal peatlands. The simplicity of the model relies on four assumptions: (i) near-neutral atmospheric conditions; (ii) wet surface; (iii) constant momentum roughness length depending on vegetation height; and (iv) constant water vapor roughness length, with the last two assumptions implying a constant water vapor transfer coefficient C E. Using eddy covariance data from three Canadian peatlands-Necopastic (James Bay, Québec), Mer Bleue (Ottawa, Ontario), and Western Peatland (Athabasca, Alberta)-this paper shows that these sites are characterized by frequent occurrences of near-neutral atmospheric conditions, especially the Necopastic site, with nearly 76% of the 30-min data segments occurring under near-neutral stratification. The analysis suggests these near-neutral conditions occur as a result of strong mechanical turbulence and weak buoyancy effects. The bulk-transfer approach gives promising results for 30-min and daily ET in terms of mean error and correlation, with performances similar to the Penman equation, without requiring net radiation data. The accuracy of the approach is likely related to the number of near-neutral periods and the elevated position of the water table, which backs up the wet surface assumption. [ABSTRACT FROM AUTHOR]

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]

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]

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]