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Abstract: Heterotrophic respiration from agricultural soils can be differentiated as originating from microbial decomposition of recent litter inputs or crop residue carbon (CRC) and resident soil organic carbon (SOC) pools of varying age and stages of decomposition. Our objective was to determine the relative contributions of these pools to respiration in a northern agroecosystem where the non-growing season is long. A tunable diode laser trace gas analyzer was used to determine atmospheric stable C isotope ratio (δ 13C) values and 12CO2 and 13CO2 fluxes over an agricultural field in the Red River Valley of southern Manitoba, Canada. Measurement campaigns were conducted in the fall of 2006 and spring of 2007 following harvest of a maize (C4) crop from soil having SOC derived from previous C3 crops. Stable CO2 isotopologue gradients were measured from the center of four 200×200m experimental plots, and fluxes were calculated using the aerodynamic flux gradient method. The soil in two of the experimental plots underwent intensive tillage, while the other two plots were managed using a form of reduced tillage. Approximately 70% and 20–30% of the total respiration flux originated from the maize C4-CRC during the fall of 2006 and spring of 2007, respectively. At least 25% of the maize residue was lost to respiration during this non-growing period. No difference in the partitioning of heterotrophic respiration into that derived from CRC and SOC was detected between the intensive tillage and recently established reduced tillage treatments at the site. [Copyright &y& Elsevier]

Abstract: Net carbon dioxide ecosystem exchange (NEE) was measured over three years (2006–2008) in a maize/faba/spring-wheat crop rotation in the Red River Valley of southern Manitoba, Canada using the flux-gradient method. The turbulent transfer coefficients were obtained using a similarity-theory, eddy-covariance based aerodynamic method and the carbon dioxide vertical gradient was measured with a tunable diode laser analyzer. The system measured 30-min-average fluxes over each of four plots sequentially to test reduced and intensive tillage treatments. Annual cumulative NEE was −720, 70 and −2400kgCha−1 yr−1 in the maize, faba and spring-wheat crop years, respectively. Enhanced respiration in the faba year was largely responsible for the loss, whereas the large gain in the spring-wheat year was a combination of high photosynthetic activity and reduced respiration. The three years experienced sequentially cooler and wetter years that likely affected the crops. After accounting for harvest removals, the net ecosystem loss was 510, 3140 and −480(gain)kgCha−1 yr−1 for the respective crop years, summing to a three-year loss of 3170kgCha−1. A recent conversion from intensive tillage to a reduced tillage practice showed no difference because of relatively large variability among the four plots. [Copyright &y& Elsevier]

Abstract: Fire and harvesting are major forest renewal processes in the Canadian boreal forest. The eddy covariance method was used to compare ecosystem fluxes of carbon dioxide between harvested and burned boreal forest sites in Saskatchewan, Canada. The harvest chronosequence had sites harvested in 2002 (HJP02), 1994 (HJP94) and 1975 (HJP75), whereas the fire chronosequence sites were burned in 1998 (F98), 1989 (F89), 1977 (F77) and 1929 (OJP). All sites were dominated by jack pine prior to the disturbance. During 2004 and 2005, net ecosystem production showed an average carbon gain (gCm−2 year−1) at F89=84, HJP75=80, HJP94=14 and OJP=20. The other sites lost carbon (gCm−2 year−1) at HJP02=−139, F98=−20, and F77=−58. Gross ecosystem production (GEP), ecosystem respiration (R e) and evapotranspiration tended to be greater at the burned sites than the harvested sites. The F89 and F77 sites had the strongest response of GEP to photosynthetically active radiation, and the strongest response of R e to soil temperature at the 2-cm depth. HJP02 had the weakest responses, followed by HJP94. This apparent greater ecosystem activity at the burned sites is likely caused by local differences in soil moisture and nutrients, differences in vegetation development, and differences in the decomposition of coarse woody debris. [Copyright &y& Elsevier]

Abstract: Fire in the boreal forest renews forest stands and changes the ecosystem properties. The successional stage of the vegetation determines the radiative budget, energy balance partitioning, evapotranspiration and carbon dioxide flux. Here, we synthesize energy balance measurements from across the western boreal zone of North America as a function of stand age following fire. The data are from 22 sites in Alaska, Saskatchewan and Manitoba collected between 1998 and 2004 for a 150-year forest chronosequence. The summertime albedo immediately after a fire is about 0.05, increasing to about 0.12 for a period of about 30 years and then averaging about 0.08 for mature coniferous forests. A mature deciduous (aspen) forest has a higher summer albedo of about 0.16. Wintertime albedo decreases from a high of 0.7 for 5- to 30-year-old forests to about 0.2 for mature forests (deciduous and coniferous). Summer net radiation normalized to incoming solar radiation is lower in successional forests than in more mature forests by about 10%, except for the first 1–3 years after fire. This reduction in net radiative forcing is about 12–24Wm−2 as a daily average in summer (July). The summertime daily Bowen ratio exceeds 2 immediately after the fire, decreasing to about 0.5 for 15-year-old forests, with a wide range of 0.3–2 for mature forests depending on the forest type and soil water status. The magnitude of these changes is relatively large and may affect local, regional and perhaps global climates. Although fire has always determined stand renewal in these forests, increased future area burned could further alter the radiation balance and energy partitioning, causing a cooling feedback to counteract possible warming from carbon dioxide released by boreal fires. [Copyright &y& Elsevier]

Abstract: Fire and harvesting are important disturbances in the boreal forest, driving net biome production. Measurements of net ecosystem production (NEP) over mature forest stands have been made from flux towers for about a decade at the Boreal Ecosystem Research and Monitoring Sites (BERMS) in central Saskatchewan, Canada. Over the last few years, the network of towers has been expanded to include stands that were recently disturbed by fire or harvesting. These new towers are part of Fluxnet-Canada, a national network that ties into the international Fluxnet network. Data from 2001 and 2002 show that forests burned in 1998 (F98) or harvested in 1994 (HJP94) were net carbon sources to the atmosphere with annual NEP values in the range of −50 to −130gCm−2 y−1. A site burned in 1989 (F89) was a carbon sink of about 68gCm−2 y−1 in 2002. NEP at a mature jack pine site (SOJP) ranged from 41 to −23gCm−2 y−1 in this period, whereas, an old black spruce site (SOBS) ranged from 68 to 21gCm−2 y−1. A mature aspen site (SOA) had the greatest NEP with 361gCm−2 y−1 in 2001 and 139gCm−2 y−1 in 2002, the contrast between years attributed to differences in spring temperature and leaf emergence, as well as a long-term drought at the site. Evapotranspiration (ET) followed similar patterns although the F89 site had the highest ET. The deciduous tree components of SOA and F89 are likely responsible for lower summer Bowen ratios with greater energy going into ET, compared to coniferous sites. These data indicate that there is still a broad gap in our knowledge of carbon fluxes to forest stands between about 10 and 50 years of age. This is especially true in understanding the dynamics of the growth of successional vegetation along with heterotrophic respiration, highlighted by predictions of more boreal fire disturbance in the future. [Copyright &y& Elsevier]

Estimates direct emissions of carbon from Canadian forest fires for all Canada and for each ecozone for the period 1959?1999. Basis of the estimates on a data base of large fires for the country and calculations of fuel consumption for each fire using the Canadian Forest Fire Behaviour Prediction System; Fire locations and start dates to estimate prevailing fire weather and fuel type for each of about 11 000 fires; Mean area-weighted fuel consumption for all fires; Mean annual estimate of direct carbon emissions; Individual years, which ranged from 3 to 115 Tg C?year?1. results, including that these direct fire emissions represent about 18% of the current carbon dioxide emissions from the Canadian energy sector, on average, but vary from 2 to 75% among years. Conclusion that post-fire effects cause an additional loss of carbon and changes to the forest sink condition.

Simultaneous biometric measurements of aboveground net primary production (ANPP) and eddy-covariance measurements of gross primary production (GPP) were made at 18 forest stands with 80 site-years of data across Canada – to assess the fraction of photosynthesis that is used to produce plant tissues and the consistency of carbon allocation patterns across forest ecosystems. The stands included boreal and temperate forests and spanned very young to mature stand ages. Across all sites, ANPP averaged 298±138gCm−2 yr−1 (mean±1 s.d.), with the highest values for temperate white pine plantations (307–630gCm−2 yr−1) and harvested Douglas-fir stands (219–459gCm−2 yr−1), and the lowest values for boreal harvested jack pine stands (97–185gCm−2 yr−1). ANPP more than doubled from newly established (≤12 years) to young (13–25 years) stands, then stabilized in young to mature (≥51 years) stands but with diverging trends among species. Inter-site variations in ANPP and GPP were closely related to site characteristics, in particular, to leaf area index, which explained 66% of the variation in ANPP and 80% of the variation in GPP, and absorbed photosynthetically active radiation, which explained 80% of the variation in ANPP and 82% of the variation in GPP. Both ANPP and GPP were also positively correlated with mean annual air temperature, mean annual precipitation, and total soil nitrogen in the upper 10cm of the mineral soil. ANPP was strongly, positively correlated with GPP and the ANPP/GPP ratio was relatively constant (0.29±0.06), with no consistent differences among species or age classes. The results support the use of a constant ANPP/GPP ratio as a reasonable assumption in models of forest productivity for boreal and northern temperate forests. A similar conclusion is reached for the NPP/GPP ratio when published values of belowground NPP are considered. [ABSTRACT FROM AUTHOR]

Abstract: Proper management and accounting of forest carbon requires good knowledge of how disturbances and climate affect the carbon dynamics of different stand types. We have investigated such relationships by measuring, over a 5-year period (2003–2007), the net ecosystem productivity (NEP), gross ecosystem productivity (GEP) and ecosystem respiration (ER) of 26 forest sites in Canada using the eddy covariance technique. The study included black spruce, jack pine, Douglas-fir, aspen, boreal mixedwood and white pine forest ecosystems ranging in age from 1- to 153-years. The dataset included six chronosequences (one afforested plantation, three harvested and two burned). Following planting, the afforested white pine stands quickly became carbon sinks and offset initial carbon losses after 4years. Depending on forest type, the other forest stands were carbon sources for 10–18years following a disturbance, offset initial carbon losses after 19–47years, and showed net total gains ranging from 38 to 86MgCha−1 at 80years. Peak NEP ranged from 0.9 to 2.9MgCha−1 year−1 at ages of 35–55years except for the afforested white pine where it was 6.9MgCha−1 year−1 at 15–20years. Stepwise regression and Pearson correlation analyses indicated that the GEP and ER of mature stands (>70years old) were driven mainly by climate, while fluxes of young stands (<19years old) were driven by both leaf area index and climate. Although stand age of the afforested white pine plantations did not affect the GEP growing season lengths, the growing season length of the other forests increased with age until about 20years and this coincided with the switch from carbon source to sink. With the exception of the afforested white pine, peak GEP/ER ratios of the youngest sites occurred later in the growing season compared to older sites. The strong influence of stand age on the seasonal dynamics of GEP fluxes needs to be considered to avoid confounding the impacts of climate change with those of disturbance. These age-related seasonality effects are continental in scope and should be important in interpreting the time series of atmospheric CO2 concentration measurements at regional and global scales. [Copyright &y& Elsevier]

Abstract: Direct estimation of photosynthetic light-use efficiency (LUE) from space would be of significant benefit to LUE-based models which use inputs from remote sensing to estimate terrestrial productivity. The Photochemical Reflectance Index (PRI) has shown promise in tracking LUE at the leaf- to small canopy levels, but its use at regional to global scales still remains a challenge. In this study, we used different formulations of PRI calculated from the MODIS ocean band centered at 531 nm and a set of alternative reference bands at 488, 551, and 678 nm to explore the relationship between PRI and LUE where LUE was measured at eight eddy covariance flux towers located in the boreal forest of Saskatchewan, Canada. The magnitude and variability of LUE was significantly lower at the times when useful MODIS ocean band images were available (i.e. around midday under clear-sky conditions) relative to the rest of the growing season. PRI678 (reference band at 678 nm) showed the strongest relationship (r 2 =0.70) with LUE90a (i.e. 90-minute mean LUE calculated using Absorbed Photosynthetically Active Radiation, APAR), but only when all sites were combined. Overall, the relationships between the MODIS PRIs and LUE90a were always stronger for observations closer to the backscatter direction and there were no significant differences in the strength of the correlations whether LUE was calculated based on incident PAR or on APAR. Predictions of ecosystem photosynthesis at the time of the MODIS overpasses were significantly improved by multiplying either PAR or APAR by MODIS PRI (r 2 improved from 0.09 to 0.44 and 0.54 depending on the PRI formulation). We used our PRI-LUE model to create a regional LUE90a map for the three cover types covering 47,500 km2 around the flux sites. The MODIS PRI-derived LUE90a map appeared to capture more realistic spatial heterogeneity of LUE across the landscape compared to a daily LUE map derived using the look-up table in the MODIS GPP (MOD17) algorithm. While our LUE map is only a snapshot of minimum regional LUE90a values, with appropriate gap-filling methods it could be used to improve regional-scale monitoring of GPP. Moreover, the strong relationship between midday and daily LUE on clear days (r 2 =0.93) indicates that instantaneous MODIS observations of LUE90a could be used to estimate daily LUE. Finally, pixel shadow fraction from the 5-Scale geometric-optical model was closely related to both MODIS PRI and tower-derived LUE suggesting that differences in stand leaf area and in diffuse illumination among flux sites play a role in the relationship we observed between LUE and MODIS PRI. [Copyright &y& Elsevier]