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Saskatoon, Canada

Grant R.F.,University of Alberta | Barr A.G.,Climate Research Branch | Black T.A.,University of British Columbia | Margolis H.A.,Laval University | And 2 more authors.
Tellus, Series B: Chemical and Physical Meteorology

Clearcutting strongly affects subsequent forest net ecosystem productivity (NEP). Hypotheses for ecological controls on NEP in the ecosystem model ecosys were tested with CO2 fluxes measured by eddy covariance (EC) in three postclearcut conifer chronosequences in different ecological zones across Canada. In the model, microbial colonization of postharvest fine and woody debris drove heterotrophic respiration (Rh), and hence decomposition, microbial growth, N mineralization and asymbiotic N2 fixation. These processes controlled root N uptake, and thereby CO2 fixation in regrowing vegetation. Interactions among soil and plant processes allowed the model to simulate hourly CO2 fluxes and annual NEP within the uncertainty of EC measurements from 2003 to 2007 over forest stands from 1 to 80 yr of age in all three chronosequences without site or speciesspecific parameterization. The model was then used to study the impacts of increasing harvest removals on subsequent C stocks at one of the chronosequence sites. Model results indicated that increasing harvest removals would hasten recovery of NEP during the first 30 yr after clearcutting, but would reduce ecosystem C stocks by about 15% of the increased removals at the end of an 80yr harvest cycle. © 2010 The Authors Tellus B © 2010 International Meteorological Institute in Stockholm. Source

Flanagan L.B.,University of Lethbridge | Cai T.,University of Lethbridge | Black T.A.,University of British Columbia | Barr A.G.,Climate Research Branch | And 2 more authors.
Agricultural and Forest Meteorology

Photosynthetic capacity in boreal coniferous forests varies on a seasonal basis in response to the strong fluctuations in environmental conditions, and this contributes significantly to temporal changes in the concentration and stable isotope composition of atmospheric carbon dioxide. Our objectives in this study were to compare measurements of seasonal variation in ecosystem-scale photosynthesis and the carbon isotope composition of ecosystem-respired CO 2 (δ R) with calculations done using a model that included seasonal changes in the temperature acclimation of photosynthesis. Our measurements and model calculations were conducted in three boreal coniferous forests during the main growing season months (May-September) in three different years (2004-2006) as part of the Fluxnet-Canada Research Network. We observed good agreement between measured and modeled ecosystem photosynthesis, with measured ecosystem photosynthesis based on eddy covariance measurements of net ecosystem CO 2 exchange. In addition, good agreement was observed between measured and modeled δ R values, which helped to provide validation of our calculations of ecosystem-scale carbon isotope discrimination. There were important seasonal changes in both ecosystem photosynthesis rate and carbon isotope discrimination that affect the concentration and stable isotope composition of atmospheric CO 2. The seasonal patterns of change in ecosystem photosynthesis and carbon isotope discrimination determined in this study closely matched the timing of measured changes in the concentration and isotope composition of atmospheric CO 2 recorded at Cold Bay, Alaska. © 2011 Elsevier B.V. Source

Ju W.,Nanjing University | Ju W.,University of Toronto | Chen J.M.,University of Toronto | Black T.A.,University of British Columbia | And 2 more authors.
Tellus, Series B: Chemical and Physical Meteorology

The variations of soil water content (SWC) and its influences on the carbon (C) cycle in Canada's forests and wetlands were studied through model simulations using the Integrated Terrestrial Ecosystem Carbon (InTEC) model. It shows that Canada's forests and wetlands experienced spatially and temporally heterogeneous changes in SWC from 1901 to 2000. SWC changes caused average NPP to decrease 40.8 Tg C yr-1 from 1901 to 2000, whereas the integrated effect of non-disturbance factors (climate change, CO2 fertilization and N deposition) enhanced NPP by 9.9%. During 1981-2000, the reduction of NPP caused by changes in SWC was 58.1 Tg C yr-1 whereas non-disturbance factors together caused NPP to increase by 16.6%. SWC changes resulted in an average increase of 4.1 Tg C yr-1 in the net C uptake during 1901-2000, relatively small compared with the enhancement in C uptake of 50.2 Tg C yr-1 by the integrated effect of non-disturbance factors. During 1981-2000, changes in SWC caused a reduction of 3.8 Tg C yr-1 in net C sequestration whereas the integrated factors increased net C sequestration by 54.1 Tg C yr-1. Increase in SWC enhanced C sequestration in all ecozones. © 2010 The Authors Journal compilation © 2010 Blackwell Munksgaard. Source

Chen B.,CAS Beijing Institute of Geographic Sciences and Nature Resources Research | Chen B.,University of British Columbia | Coops N.C.,University of British Columbia | Fu D.,CAS Beijing Institute of Geographic Sciences and Nature Resources Research | And 10 more authors.
Agricultural and Forest Meteorology

We describe an approach for evaluating the representativeness of eddy covariance flux measurements and assessing sensor location bias (SLB) based on footprint modelling and remote sensing. This approach was applied to the 12 main sites of the Fluxnet-Canada Research Network (FCRN)/Canadian Carbon Program (CCP) located along an east-west continental-scale transect, covering grassland, forest, and wetland biomes. For each site, monthly and annual footprint climatologies (i.e. monthly or annual cumulative footprints) were calculated using the Simple Analytical Footprint model on Eulerian coordinates (SAFE). The resulting footprint climatologies were then overlaid on to images of the Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) derived from LANDSAT Thematic Mapper (TM) imagery, which were used as surrogates of land surface fluxes to estimate SLB. Results indicate that (i) the sizes of annual footprint climatology increased exponentially with increasing cumulative footprint percentages and, for a given percentage of footprint climatology, the footprint areas were significantly different among the sites. Typically, the 90% annual footprint climatology areas varied from 1.1km2 to 5.0km2; (ii) using either NDVI or EVI as the flux surrogate, the SLB was less than 5% for most sites with respect to the reference area of interest (Ar) at 90% annual footprint climatology (scenario A) and a circular area with radius of 1km centred at the individual tower (scenario B), with several exceptions; (iii) the SLB decreased with increasing size of footprint climatology for all sites for both scenarios A and B; (iv) out of 12, eight flux towers represented most of the ecosystem surrounding the towers for an area of 0.3km2 up to 10km2 with a satisfactorily low bias of <5%, whereas four towers represented areas ranging from only 0.75 to 4km2; and (v) the seasonal differences in monthly SLB using NDVI as a flux surrogate were about 1-4% for most sites for both scenarios A and B. © 2010 Elsevier B.V. Source

Gaumont-Guay D.,Vancouver Island University | Black T.A.,University of British Columbia | Barr A.G.,Climate Research Branch | Griffis T.J.,University of Minnesota | And 4 more authors.
Agricultural and Forest Meteorology

Automated measurements of the net forest-floor CO2 exchange (NFFE) were made in a mature (130-year-old) boreal black spruce forest over an 8-year period (2002-2009) with the objectives of (1) quantifying the spatial and temporal (seasonal and interannual) patterns in NFFE, soil respiration (SR) and gross forest-floor photosynthesis (GFFP), and (2) better understanding the key climatic controls on each component at both time scales. Scaling-up of the component fluxes to the stand level showed that the feather moss community accounted for more than 85% of NFFE and SR, and more than 70% of GFFP. The remainder was partitioned almost equally between the sphagnum and lichen communities for all components fluxes, while the exposed mineral soil in hollows accounted for less than 1% of NFFE and SR. Soil temperature (Ts) was the dominant climate variable determining seasonal trends in NFFE and SR. The shape of the exponential response was, however, strongly modulated by soil water content (SWC) in the surface organic horizon, with reduced apparent temperature sensitivity at low SWC. A lowering of the water table depth also had an effect on NFFE and SR, although very weak, with increased CO2 loss from the hollows likely due to improved soil aeration. Air temperature (Ta) was the dominant climate variable determining seasonal trends in GFFP, while plant water status seemed to have played a minor role. Although not statistically significant (p=0.9907), annual totals of scaled-up NFFE varied from 505±121 to 601±144gCm-2y-1 over the 8-year period. The lowest NFFE was observed in 2004, the coldest and wettest year on record, while the highest was observed in 2005, a warmer year with slightly above-average precipitation. SR, by far the dominant component of the forest-floor CO2 exchange, closely followed the inter-annual trends in NFFE, while GFFP was lowest in 2004 and highest in 2003, also a cold year but with very low precipitation. Over the 8-year period, winter NFFE contributed 7% to annual NFFE while GFFP during the growing season reduced losses due to SR by 18%.While strong correlations were observed between the component fluxes and temperature (Ts or Ta) and SWC at the seasonal time scale, the mean annual values of these climate variables were poor predictors of the inter-annual trends when considered individually. Combining multiplicatively Ts and SWC for NFFE and SR, and Ta and SWC for GFFP, significantly increased the predictive ability of the models. The difference in predictability of the two time scales poses some interesting challenges for interpreting and modeling the long-term temporal trends in NFEE and its components. The results obtained in this relatively long-term study suggest that the inter-annual variability in the component fluxes was not driven by the mean annual climate conditions, but rather the shorter time scale changes in climate conditions, i.e. changes that occurred within days, weeks and/or seasons. Moreover, it appeared that the timing of the climatic changes within each year was also critical, spring and summer conditions having a far greater impact than fall and winter conditions in this stand. © 2013 Elsevier B.V. Source

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