Saskatoon, Canada
Saskatoon, Canada

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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 | Year: 2011

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.


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 | Year: 2012

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.


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.
Remote Sensing of Environment | Year: 2012

We describe a pragmatic approach for evaluating the spatial representativeness of flux tower measurements based on footprint climatology modeling analyses of land cover and remotely sensed vegetation indices. The approach was applied to the twelve flux sites of the Canadian Carbon Program (CCP) that include grassland, wetland, and temperate and boreal forests across an east-west continental gradient. The spatial variation within the footprint area was evaluated by examining the spatial structure of Normalized Difference Vegetation Index (NDVI) and land cover using geostatistical analyses of frequency distribution, variogram and window size. The results show that at most sites (i) the percentages of the target vegetation functional type (dominant land cover) observed by the CCP towers were higher than 60%; (ii) to some extent, most of the CCP sites presented anisotropically distributed patterns of NDVI in the 90% annual footprint climatology area; and (iii) the land surface heterogeneity within the flux footprint area differed among sites. Overall, the forest sites had larger fine-scale spatial variation than the grassland and wetland sites. The coniferous boreal forest sites had greater spatial variability than the two wetland sites and a coniferous temperate forest site. We conclude that the combination of footprint modeling, semivariogram and window size techniques, together with moderate spatial resolution remotely-sensed image data, is a pragmatic approach for assessing the spatial representativeness of flux tower measurements. © 2012 Elsevier Inc.


Brummer C.,University of British Columbia | Brummer C.,Johann Heinrich Von Thunen Institute | Black T.A.,University of British Columbia | Jassal R.S.,University of British Columbia | And 16 more authors.
Agricultural and Forest Meteorology | Year: 2012

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 H 2O, the BC-DF49 site had the highest WUE of the twelve sites with values near 6.0gCkg -1 H 2O. Of the two peatland sites, AB-WPL, a western treed fen, had a significantly higher WUE (∼3.0gCkg -1 H 2O) than ON-EPL, an eastern ombrotrophic bog (∼1.8gCkg -1 H 2O), which was related to peatland productivity and plant functional type. © 2011 Elsevier B.V.


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 | Year: 2010

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.


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 | Year: 2014

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.


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 | Year: 2010

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.


Middleton E.M.,NASA | Huemmrich K.F.,University of Maryland Baltimore County | Landis D.R.,Global Science & Technology, Inc. | Black T.A.,University of British Columbia | And 2 more authors.
Remote Sensing of Environment | Year: 2016

This study evaluates a direct remote sensing approach from space for the determination of ecosystem photosynthetic light use efficiency (LUE), through measurement of vegetation reflectance changes expressed with the Photochemical Reflectance Index (PRI). The PRI is a normalized difference index based on spectral changes at a physiologically active wavelength (~ 531 nm) as compared to a reference waveband, and is only available from a very few satellites. These include the two Moderate-Resolution Imaging Spectroradiometers (MODIS) on the Aqua and Terra satellites each of which have a narrow (10 nm) ocean band centered at 531 nm. We examined several PRI variations computed with candidate reference bands, since MODIS lacks the traditional 570 nm reference band. The PRI computed using MODIS land band 1 (620–670 nm) gave the best performance for daily LUE estimation. Through rigorous statistical analyses over a large image collection (n = 420), the success of relating in situ daily tower-derived LUE to MODIS observations for northern forests was strongly influenced by satellite viewing geometry. LUE was calculated from CO2 fluxes (mol C mol− 1 absorbed quanta) measured at instrumented Canadian Carbon Program flux towers in four Canadian forests: a mature fir site in British Columbia, mature aspen and black spruce sites in Saskatchewan, and a mixed deciduous/coniferous forest site in Ontario. All aspects of the viewing geometry had significant effects on the MODIS-PRI, including the view zenith angle (VZA), the view azimuth angle, and the displacement of the view azimuth relative to the solar principal plane, in addition to illumination related variables. Nevertheless, we show that forward scatter sector views (VZA, 16°–45°) provided the strongest relationships to daily LUE, especially those collected in the early afternoon by Aqua (r2 = 0.83, RMSE = 0.003 mol C mol− 1 absorbed quanta). Nadir (VZA, 0° ± 15°) and backscatter views (VZA, − 16° to − 45°) had lower performance in estimating LUE (nadir: r2 ~ 0.62–0.67; backscatter: r2 ~ 0.54–0.59) and similar estimation error (RMSE = 0.004–0.005). When directional effects were not considered, only a moderately successful MODIS-PRI vs. LUE relationship (r2 = 0.34, RMSE = 0.007) was obtained in the full dataset (all views & sites, both satellites), but site-specific relationships were able to discriminate between coniferous and deciduous forests. Overall, MODIS-PRI values from Terra (late morning) were higher than those from Aqua (early afternoon), before/after the onset of diurnal stress responses expressed spectrally. Therefore, we identified ninety-two Terra-Aqua “same day” pairs, for which the sum of Terra morning and Aqua afternoon MODIS-PRI values (PRIsum) using all available directional observations was linearly correlated with daily tower LUE (r2 = 0.622, RMSE = 0.013) and independent of site differences or meteorological information. Our study highlights the value of off-nadir directional reflectance observations, and the value of pairing morning and afternoon satellite observations to monitor stress responses that inhibit carbon uptake in Canadian forest ecosystems. In addition, we show that MODIS-PRI values, when derived from either: (i) forward views only, or (ii) Terra/Aqua same day (any view) combined observations, provided more accurate estimates of tower-measured daily LUE than those derived from either nadir or backscatter views or those calculated by the widely used semi-operational MODIS GPP model (MOD17) which is based on a theoretical maximum LUE and environmental data. Consequently, we demonstrate the importance of diurnal as well as off-nadir satellite observations for detecting vegetation physiological processes. © 2016

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