J.M. Chen

Publications (85)98.69 Total impact

• Article: Terrestrial biosphere models need better representation of vegetation phenology: results from the North American Carbon Program Site Synthesis

  A. D. Richardson, R. S. Anderson, M. A. Arain, A. G. Barr, G. Bohrer, G. S. Chen, J. M. Chen, P. Ciais, K. J. Davis, A. R. Desai, [......], B. Poulter, B. M. Raczka, D. M. Ricciuto, A. K. Sahoo, K. Schaefer, H. Q. Tian, R. Vargas, H. Verbeeck, J. F. Xiao, Y. K. Xue
  [show abstract] [hide abstract]
  ABSTRACT: Phenology, by controlling the seasonal activity of vegetation on the land surface, plays a fundamental role in regulating photosynthesis and other ecosystem processes, as well as competitive interactions and feedbacks to the climate system. We conducted an analysis to evaluate the representation of phenology, and the associated seasonality of ecosystem-scale CO2 exchange, in 14 models participating in the North American Carbon Program Site Synthesis. Model predictions were evaluated using long-term measurements (emphasizing the period 20002006) from 10 forested sites within the AmeriFlux and Fluxnet-Canada networks. In deciduous forests, almost all models consistently predicted that the growing season started earlier, and ended later, than was actually observed; biases of 2 weeks or more were typical. For these sites, most models were also unable to explain more than a small fraction of the observed interannual variability in phenological transition dates. Finally, for deciduous forests, misrepresentation of the seasonal cycle resulted in over-prediction of gross ecosystem photosynthesis by +160 145 g C m-2 yr-1 during the spring transition period and +75 +/- 130 g C m-2 yr-1 during the autumn transition period (13% and 8% annual productivity, respectively) compensating for the tendency of most models to under-predict the magnitude of peak summertime photosynthetic rates. Models did a better job of predicting the seasonality of CO2 exchange for evergreen forests. These results highlight the need for improved understanding of the environmental controls on vegetation phenology and incorporation of this knowledge into better phenological models. Existing models are unlikely to predict future responses of phenology to climate change accurately and therefore will misrepresent the seasonality and interannual variability of key biosphereatmosphere feedbacks and interactions in coupled global climate models.
  Global Change Biology 02/2013; 18:566-584. · 6.86 Impact Factor
• Source

  Available from: Frank Berninger

  Article: Interannual variability of net carbon exchange in forest is regulated by growing season and carbon uptake phenology in autumn

  Wu C, J.M. Chen, A. Black, D.T. Price, W.A. Kurz, A.R. Desai, Alemu Gonsamo, R.S. Jassal, C.M. Gough, G. Bohrer, D. Dragoni, M. Herbst, B. Gielen, F. Berninger, T. Vesala, I. Mammarella, K. Pilegaard, P.D. Blanken
  Global Ecology and Biogeography 01/2013; · 5.14 Impact Factor
• Source

  Available from: Harry McCaughey

  Article: Terrestrial biosphere models need better representation of vegetation phenology: results from the North American Carbon Program Site Synthesis

  A. D. Richardson, R. S. Anderson, M. A. Arain, A. G. Barr, G. Bohrer, G. S. Chen, J. M. Chen, P. Ciais, K. J. Davis, A. R. Desai, [......], B. Poulter, B. M. Raczka, D. M. Ricciuto, A. K. Sahoo, K. Schaefer, H. Q. Tian, R. Vargas, H. Verbeeck, J. F. Xiao, Y. K. Xue
  [show abstract] [hide abstract]
  ABSTRACT: Phenology, by controlling the seasonal activity of vegetation on the land surface, plays a fundamental role in regulating photosynthesis and other ecosystem processes, as well as competitive interactions and feedbacks to the climate system. We conducted an analysis to evaluate the representation of phenology, and the associated seasonality of ecosystem-scale CO2 exchange, in 14 models participating in the North American Carbon Program Site Synthesis. Model predictions were evaluated using long-term measurements (emphasizing the period 20002006) from 10 forested sites within the AmeriFlux and Fluxnet-Canada networks. In deciduous forests, almost all models consistently predicted that the growing season started earlier, and ended later, than was actually observed; biases of 2 weeks or more were typical. For these sites, most models were also unable to explain more than a small fraction of the observed interannual variability in phenological transition dates. Finally, for deciduous forests, misrepresentation of the seasonal cycle resulted in over-prediction of gross ecosystem photosynthesis by +160 145 g C m-2 yr-1 during the spring transition period and +75 +/- 130 g C m-2 yr-1 during the autumn transition period (13% and 8% annual productivity, respectively) compensating for the tendency of most models to under-predict the magnitude of peak summertime photosynthetic rates. Models did a better job of predicting the seasonality of CO2 exchange for evergreen forests. These results highlight the need for improved understanding of the environmental controls on vegetation phenology and incorporation of this knowledge into better phenological models. Existing models are unlikely to predict future responses of phenology to climate change accurately and therefore will misrepresent the seasonality and interannual variability of key biosphereatmosphere feedbacks and interactions in coupled global climate models.
  Global Change Biology 11/2012; 18:566-584. · 6.86 Impact Factor
• Article: Fine-scale spatial genetic structure of an endangered marsh herb, Caldesia grandis (Alismataceae).

  X-L Yue, J-M Chen, Y-H Guo, Q-F Wang
  [show abstract] [hide abstract]
  ABSTRACT: The endangered marsh herb, Caldesia grandis, is native to China. We investigated the spatial structure of the genetic variation of three populations of C. grandis using RAPD markers and spatial autocorrelation analysis, based on the method of equal distance interval. A total of 157 individuals were sampled from four patches collected from the region of Hunan and Yunnan Provinces, China. Among the polymorphic bands generated by seven selective primers, polymorphic bands with frequencies ranging from 20 to 80% were used to calculate Moran's I spatial autocorrelation coefficient for each patch. We found significant spatial structure of genetic variation in the three patches in Bei Hai (BH) (patches BH-1 and BH-2) and Guai Hu (GH) (patch GH-1) populations of C. grandis (with significant positive autocorrelation within the short distance class). In contrast, the genetic variation in the Lang Pan Hu (LPH) population (patch LPH-1) was found to be randomly distributed. The different spatial distribution patterns may be attributed to environment differences. These results have implications for the conservation and management of this species, especially for sampling strategies for ex situ conservation.
  Genetics and molecular research: GMR 05/2012; 11(3):2412-21. · 1.18 Impact Factor
• Article: Newly diagnosed acute lymphoblastic leukemia in China (I): abnormal genetic patterns in 1346 childhood and adult cases and their comparison with the reports from Western countries.

  B Chen, Y-Y Wang, Y Shen, W-N Zhang, H-Y He, Y-M Zhu, H-M Chen, C-H Gu, X Fan, J-M Chen, [......], C-L Jiang, X-Q Weng, X-X Zhang, S-M Xiong, Z-X Shen, H Jiang, L-J Gu, Z Chen, J-Q Mi, S-J Chen
  [show abstract] [hide abstract]
  ABSTRACT: It has been generally acknowledged that the diagnosis, treatment and prognosis evaluation of leukemia largely rely on an adequate identification of genetic abnormalities. A systemic analysis of genetic aberrations was performed in a cohort of 1346 patients with newly diagnosed acute lymphoblastic leukemia (ALL) in China. The pediatric patients had higher incidence of hyperdiploidy and t(12;21) (p13;q22)/ETV6-RUNX1 than adults (P<0.0001); in contrast, the occurrence of Ph and Ik6 variant of IKZF1 gene was much more frequent in adult patients (all P<0.0001). In B-ALL, the existence of Ik6 and that of BCR-ABL were statistically correlated (P<0.0001). In comparison with Western cohorts, the incidence of t(9;22) (q34;q11)/BCR-ABL (14.60%) in B-ALL and HOX11 expression in T-ALL (25.24%) seemed to be much higher in our group, while the incidence of t(12;21) (p13;q22)/ETV6-RUNX1 (15.34%) seemed to be lower in Chinese pediatric patients. The occurrence of hyperdiploidy was much lower either in pediatric (10.61% vs 20-38%) or adult patients (2.36% vs 6.77-12%) in our study than in Western reports. In addition, the frequencies of HOX11L2 in adult patients were much higher in our cohort than in Western countries (20.69% vs 4-11%). In general, it seems that Chinese ALL patients bear more adverse prognostic factors than their Western counterparts do.
  Leukemia: official journal of the Leukemia Society of America, Leukemia Research Fund, U.K 02/2012; 26(7):1608-16. · 8.30 Impact Factor
• Article: Isolation and characterization of polymorphic microsatellite loci from the invasive plant Solidago canadensis (Asteraceae).

  S-Y Zhao, S-G Sun, Y-H Guo, J-M Chen, Q-F Wang
  [show abstract] [hide abstract]
  ABSTRACT: Solidago canadensis, a clonal herb originally from North America (common name: Canada goldenrod), is an invasive species in many countries. We developed microsatellite primers for this species. Eleven polymorphic loci were generated and primers were designed. Polymorphism of these 11 loci was assessed in 35 plants from two populations (Wuhan and Shanghai) in China. The number of alleles per locus ranged from 3 to 14. The observed and expected heterozygosities varied from 0.0732 to 0.7391 and from 0.1177 to 0.8687, respectively. These microsatellite markers will be useful tools for studies of population genetics in the native and invasive range of this species.
  Genetics and molecular research: GMR 01/2012; 11(1):421-4. · 1.18 Impact Factor
• Source

  Available from: Michael Sprintsin

  Article: Evaluation of leaf-to-canopy upscaling methodologies against carbon flux data in North America

  Michael Sprintsin, Jing M Chen, Ankur Desai, Christopher M Gough, Citation : Sprintsin, J M Chen, A Desai, C M Gough
  [show abstract] [hide abstract]
  ABSTRACT: 1] Despite the wide acceptance of the "big-leaf" upscaling strategy in evapotranspiration modeling (e.g., the Penman-Monteith model), its usefulness in simulating canopy photosynthesis may be limited by the underlying assumption of homogeneous response of carbon assimilation light-response kinetics through the canopy. While previous studies have shown that the separation of the canopy into sunlit and shaded parts (i.e., two-leaf model) is typically more effective than big-leaf models for upscaling photosynthesis from leaf to canopy, a systematic comparison between these two upscaling strategies among multiple ecosystems has not been presented. In this study, gross primary productivity was modeled using two-leaf and big-leaf upscaling approaches in the Boreal Ecosystem Productivity Simulator for shrublands, broadleaf, and conifer forest types. When given the same leaf-level photosynthetic parameters, the big-leaf approach significantly underestimated canopy-level GPP while the two-leaf approach more closely predicted both the magnitude and day-to-day variability in eddy covariance measurements. The underestimation by the big-leaf approach is mostly caused by its exclusion of the photosynthetic contributions of shaded leaves. Tests of the model sensitivity to a foliage clumping index revealed that the contribution of shaded leaves to the total simulated productivity can be as high as 70% for highly clumped stands and seldom decreases below $40% for less-clumped canopies. Our results indicate that accurate upscaling of photosynthesis across a broad array of ecosystems requires an accurate description of canopy structure in ecosystem models. (2012), Evaluation of leaf-to-canopy upscaling methodologies against carbon flux data in North America, J. Geophys. Res., 117, G01023, doi:10.1029/2010JG001407.
  Journal of Geophysical Reseach-Biogeosciences. 01/2012;
• Source

  Available from: biogeosciences.net

  Article: Recent global CO 2 flux inferred from atmospheric CO 2 observations and its regional analyses

  F Deng, J M Chen
  [show abstract] [hide abstract]
  ABSTRACT: The net surface exchange of CO 2 for the years 2002–2007 is inferred from 12 181 atmospheric CO 2 con-centration data with a time-dependent Bayesian synthesis in-version scheme. Monthly CO 2 fluxes are optimized for 30 re-gions of the North America and 20 regions for the rest of the globe. Although there have been many previous multiyear in-version studies, the reliability of atmospheric inversion tech-niques has not yet been systematically evaluated for quanti-fying regional interannual variability in the carbon cycle. In this study, the global interannual variability of the CO 2 flux is found to be dominated by terrestrial ecosystems, particu-larly by tropical land, and the variations of regional terrestrial carbon fluxes are closely related to climate variations. These interannual variations are mostly caused by abnormal mete-orological conditions in a few months in the year or part of a growing season and cannot be well represented using annual means, suggesting that we should pay attention to finer tem-poral climate variations in ecosystem modeling. We find that, excluding fossil fuel and biomass burning emissions, terres-trial ecosystems and oceans absorb an average of 3.63 ± 0.49 and 1.94 ± 0.41 Pg C yr −1 , respectively. The terrestrial up-take is mainly in northern land while the tropical and south-ern lands contribute 0.62 ± 0.47, and 0.67 ± 0.34 Pg C yr −1 to the sink, respectively. In North America, terrestrial ecosystems absorb 0.89 ± 0.18 Pg C yr −1 on average with a strong flux density found in the south-east of the continent.
  Biogeosciences 01/2011; 8:3263-3281. · 3.86 Impact Factor
• Article: Observation of curative efficacy and prognosis following combination chemotherapy with celecoxib in the treatment of advanced colorectal cancer.

  C-H Jin, A-H Wang, J-M Chen, R-X Li, X-M Liu, G-P Wang, L-Q Xing
  [show abstract] [hide abstract]
  ABSTRACT: This study investigated the antitumour and chemosensitizing effects of celecoxib in the treatment of advanced colorectal cancer. A total of 90 patients were randomly divided into two groups: group CF was treated with a combination of celecoxib and the folinic acid-fluorouracil-oxaliplatin (FOLFOX4) regimen; and group F was treated with the FOLFOX4 regimen alone. Immunohisto chemical analysis of tumour tissues for cyclooxygenase-2 (COX-2) protein was performed. With regard to short-term efficacy, the response and disease control rates were significantly greater in group CF than group F. A log-rank test showed that the 3-year survival rate was significantly greater in group CF than group F. It was concluded that the addition of celecoxib to the FOLFOX4 regimen increased the short-term efficacy and the 3-year survival rate, and improved the quality of life of patients with advanced colorectal cancer. The antitumour and chemo sensitizing effects of celecoxib appeared to be independent of COX-2.
  The Journal of international medical research 01/2011; 39(6):2129-40. · 0.90 Impact Factor
• Article: Band-gap narrowing of TiO2 doped with Ce probed with x-ray absorption spectroscopy

  S. W. Chen, J. M. Lee, K. T. Lu, C. W. Pao, J. F. Lee, T. S. Chan, J. M. Chen
  [show abstract] [hide abstract]
  ABSTRACT: The electronic structure of TiO2 doped with Ce was investigated with x-ray absorption and photoluminescence (PL) spectra. Ce doping narrows the band gap of TiO2, as evidenced by a redshift in the absorption edge in the O K-edge spectrum and PL. Narrowing is ascribed to the formation of an impurity band, composed of Ce 4f (Ce3+/Ce4+) and oxygen defect states, that induces tailing of the conduction band. Band gap narrowing extends the photoactivity of TiO2 to visible light. The impurity band located below the minimum of the conduction band traps excited electrons and suppresses recombination, enhancing the photocatalytic activity.
  Applied Physics Letters 07/2010; 97(1):012104-012104-3. · 3.84 Impact Factor
• Article: On the relationship between water table depth and water vapor and carbon dioxide fluxes in a minerotrophic fen

  O. SONNENTAG, G. Van Der KAMP, A. G. BARR, J. M. CHEN
  [show abstract] [hide abstract]
  ABSTRACT: The focus of this study is the relationship between water table depth (WTD) and water vapor [evapotranspiration (ET)] and carbon dioxide [CO2; net ecosystem exchange (NEE)] fluxes in a fen in western Canada. We analyzed hydrological and eddy covariance measurements from four snow-free periods (2003–2006) with contrasting meteorological conditions to establish the link between daily WTD and ET and gross ecosystem CO2 exchange (GEE) and ecosystem respiration (Reco; NEE=Reco−GEE), respectively: 2003 was warm and dry, 2004 was cool and wet, and 2005 and 2006 were both wet. In 2003, the water table (WT) was below the ground surface. In 2004, the WT rose above the ground surface, and in 2005 and 2006, the WT stayed well above the ground surface. There were no significant differences in total ET (∼316 mm period−1), but total NEE was significantly different (2003: 8 g C m−2 period−1; 2004: −139 g C m−2 period−1; 2005: −163 g C m−2 period−1; 2006: −195 g C m−2 period−1), mostly due to differences in total GEE (2003: 327 g C m−2 period−1; 2004: 513 g C m−2 period−1; 2005: 411 g C m−2 period−1; 2006: 556 g C m−2 period−1). Variation in ET is mostly explained by radiation (67%), and the contribution of WTD is only minor (33%). WTD controls the compensating contributions of different land surface components, resulting in similar total ET regardless of the hydrological conditions. WTD and temperature each contribute about half to the explained variation in GEE up to a threshold ponding depth, below which temperature alone is the key explanatory variable. WTD is only of minor importance for the variation in Reco, which is mainly controlled by temperature. Our study implies that future peatland modeling efforts explicitly consider topographic and hydrogeological influences on WTD.
  Global Change Biology 05/2010; 16(6):1762 - 1776. · 6.86 Impact Factor
• Article: Heart Transplantation in the United States, 1998–2007

  J. D. Vega, J. Moore, S. Murray, J. M. Chen, M. R. Johnson, D. B. Dyke
  [show abstract] [hide abstract]
  ABSTRACT: This article highlights trends in heart transplantation from 1998 to 2007, using data from the Organ Procurement and Transplantation Network (OPTN) and the Scientific Registry of Transplant Recipients (SRTR). The number of candidates actively awaiting heart transplantation has declined steadily, from 2525 in 1998 to 1408 in 2007, a 44% decrease. Despite this decline, a larger proportion of patients are listed as either Status 1A or 1B, likely secondary to increased use of mechanical circulatory support. During this time, the overall death rate among patients awaiting heart transplantation fell from 220 to 142 patients per 1000 patient-years at risk; this likely reflects better medical and surgical options for those with end-stage heart failure. This trend was noted across all racial groups, both sexes, all disease etiologies (retransplantation excepted) and all status groups. Recipient numbers were relatively stable over the past decade. In 2007, 2207 transplants were performed, although the proportion of patients transplanted as Status 1A shifted from 34% to 50%. A trend toward transplanting more patients above 65 years of age was seen. Adjusted patient (and graft) survival at 3 months, 1, 5 and 10 years after transplantation has gradually, but significantly, improved during the same period; current patient survival estimates are 93%, 88%, 74% and 55%, respectively.
  American Journal of Transplantation 03/2009; 9(4p2):932 - 941. · 6.39 Impact Factor
• Source

  Available from: ubc.ca

  Article: Daily heterotrophic respiration model considering the diurnal temperature variability in the soil

  J M Chen, S E Huang, W Ju, D Gaumont-Guay, T A Black
  [show abstract] [hide abstract]
  ABSTRACT: 1] In daily, monthly, and annual respiration models for regional and global applications, the diurnal variation of temperature is generally ignored. As the effect of temperature on respiration is nonlinear, this ignorance may cause considerable errors in respiration estimation, but these errors have not yet been systematically investigated. This is in fact a central issue in temporal scaling of ecosystem models which are often applied in time steps equal to or larger than a day. In this study, we develop an integrated daily heterotrophic respiration model, and demonstrate first theoretically the importance of considering the diurnal amplitude of soil temperature and the vertical soil carbon distribution pattern in daily respiration estimation using the daily mean temperature. Measurements of soil respiration with roots exclusion made in a mature black spruce site in Saskatchewan, Canada, in July–September 2004 are used to validate the model. Daily heterotrophic respiration rates were underestimated by up to 15%, with a mean value of 4.5%, when only the mean daily temperature was used. This underestimation occurred under the conditions that the diurnal temperature amplitude in the forest was less than 12°C and the vertical distribution of organic carbon in the top 15–30 cm was uniform. Based on the integrated daily model, this underestimation at the same site would be 38% if the amplitude increases to 20°C, and in soils with steep vertical carbon distributions with a 20°C diurnal amplitude, it can increase to 44%. The magnitude of this underestimation is theoretically proportional to [ln(Q 10)] 2 . During the experimental period, the value of Q 10 for heterotrophic respiration was found to be 4.0–4.5. If Q 10 = 2.0, this underestimation is reduced to about 10% at a diurnal temperature amplitude of 20°C.
  J. Geophys. Res. 01/2009; 114.
• Source

  Available from: utoronto.ca

  Article: Spatially explicit simulation of peatland hydrology and carbon dioxide exchange: influence of mesoscale topography

  O Sonnentag, J M Chen, N T Roulet, W Ju, A Govind
  [show abstract] [hide abstract]
  ABSTRACT: 1] Carbon dynamics in peatlands are controlled, in large part, by their wetness as defined by water table depth and volumetric liquid soil moisture content. A common type of peatland is raised bogs that typically have a multiple-layer canopy of vascular plants over a Sphagnum moss ground cover. Their convex form restricts water supply to precipitation and water is shed toward the margins, usually by lateral subsurface flow. The hydraulic gradient for lateral subsurface flow is governed by the peat surface topography at the mesoscale ($200 m to 5 km). To investigate the influence of mesoscale topography on wetness, evapotranspiration (ET), and gross primary productivity (GPP) in a bog during the snow-free period, we compare the outputs of a further developed version of the daily Boreal Ecosystem Productivity Simulator (BEPS) with observations made at the Mer Bleue peatland, located near Ottawa, Canada. Explicitly considering mesoscale topography, simulated total ET and GPP correlate well with measured ET (r = 0.91) and derived gross ecosystem productivity (GEP; r = 0.92). Both measured ET and derived GEP are simulated similarly well when mesoscale topography is neglected, but daily simulated values are systematically underestimated by about 10% and 12% on average, respectively, due to greater wetness resulting from the lack of lateral subsurface flow. Owing to the differences in moss surface conductances of water vapor and carbon dioxide with increasing moss water content, the differences in the spatial patterns of simulated total ET and GPP are controlled by the mesotopographic position of the moss ground cover.
  J. Geophys. Res. 01/2008; 113.
• Article: LAI inversion algorithm based on directional reflectance kernels

  S. Tang, J.M. Chen, Q. Zhu, X. Li, M. Chen, R. Sun, Y. Zhou, F. Deng, D. Xie
  [show abstract] [hide abstract]
  ABSTRACT: Leaf area index (LAI) is an important ecological and environmental parameter. A new LAI algorithm is developed using the principles of ground LAI measurements based on canopy gap fraction. First, the relationship between LAI and gap fraction at various zenith angles is derived from the definition of LAI. Then, the directional gap fraction is acquired from a remote sensing bidirectional reflectance distribution function (BRDF) product. This acquisition is obtained by using a kernel driven model and a large-scale directional gap fraction algorithm. The algorithm has been applied to estimate a LAI distribution in China in mid-July 2002. The ground data acquired from two field experiments in Changbai Mountain and Qilian Mountain were used to validate the algorithm. To resolve the scale discrepancy between high resolution ground observations and low resolution remote sensing data, two TM images with a resolution approaching the size of ground plots were used to relate the coarse resolution LAI map to ground measurements. First, an empirical relationship between the measured LAI and a vegetation index was established. Next, a high resolution LAI map was generated using the relationship. The LAI value of a low resolution pixel was calculated from the area-weighted sum of high resolution LAIs composing the low resolution pixel. The results of this comparison showed that the inversion algorithm has an accuracy of 82%. Factors that may influence the accuracy are also discussed in this paper.
  Journal of Environmental Management. 05/2007;
• Article: Measurements and simulation of forest leaf area index and net primary productivity in Northern China

  P. Wang, R. Sun, J. Hu, Q. Zhu, Y. Zhou, L. Li, J.M. Chen
  [show abstract] [hide abstract]
  ABSTRACT: Large scale process-based modeling is a useful approach to estimate distributions of global net primary productivity (NPP). In this paper, in order to validate an existing NPP model with observed data at site level, field experiments were conducted at three sites in northern China. One site is located in Qilian Mountain in Gansu Province, and the other two sites are in Changbaishan Natural Reserve and Dunhua County in Jilin Province. Detailed field experiments are discussed and field data are used to validate the simulated NPP. Remotely sensed images including Landsat Enhanced Thematic Mapper plus (ETM+, 30 m spatial resolution in visible and near infrared bands) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER, 15 m spatial resolution in visible and near infrared bands) are used to derive maps of land cover, leaf area index, and biomass. Based on these maps, field measured data, soil texture and daily meteorological data, NPP of these sites are simulated for year 2001 with the boreal ecosystem productivity simulator (BEPS). The NPP in these sites ranges from 80 to 800 g C m−2 a−1. The observed NPP agrees well with the modeled NPP. This study suggests that BEPS can be used to estimate NPP in northern China if remotely sensed images of high spatial resolution are available.
  Journal of Environmental Management 05/2007; · 3.24 Impact Factor
• Article: Topographically Driven Lateral Water Fluxes and Their Influence on Carbon Assimilation of a Black Spruce Ecosystem.

  A. Govind, J. M. Chen, H. Margolis, P. Y. Bernier
  [show abstract] [hide abstract]
  ABSTRACT: Current estimates of ecophysiological indicators overlook the effects of topographically-driven lateral flow of soil water. We hypothesize that topographically driven lateral water flows over the landscape have significant influence on the terrestrial carbon cycle. To this end, we simulated the hydrological controls on carbon cycle processes in a black spruce forest in central Quebec, Canada, using the Boreal Ecosystem Productivity Simulator (BEPS) at a daily time step. We accounted for lateral surface and subsurface flows in BEPS by incorporating a distributed, process-oriented hydrological procedure. The results show that modeled dynamics of ecophysiological processes such as evapotranspiration (ET) and photosynthesis (GPP) are consistent with the spatial variation of land cover, topography, soil texture, and leaf area index. Simulated ET and GPP averaged within the footprint of an eddy covariance tower in the watershed agree well with flux measurements with R2=0.77 and 0.83 for ET and GPP, respectively. For ET simulation, much of the remaining discrepancies are found in the winter when the model underestimates snow sublimation. For GPP, there is an underestimation in the fall coinciding with a mid growing season drought, showing the high sensitivity of the model to the soil water status. The key processes controlling primary production were hydraulic limitations for water transfer from soil, roots, stems and leaves through stomatal conductance. Therefore, a further understanding of soil water dynamics is warranted. Comparison with the soil water content of the footprint- averaged unsaturated zone showed that the model captured the annual trend. We also simulated the variations in the water table as well as the mid growing season drought, with a reasonable accuracy(R2=0.68). The foot print average water budget reveals that the annual precipitation of 835mm is partitioned into 282mm of ET, 541 mm of subsurface runoff, and 6 mm of storage change. To test the influence of topographically driven lateral water flow on the carbon cycle, we made three hydrological modeling scenarios viz. 1) explicit hydrological simulation including lateral water routing, 2) bucket model with implicit runoff calculations and 3) a control run, where the lateral water flow was turned off in the model. Bucket model overestimated GPP as much as 25% as opposed to explicit simulations because there was no topographical constrain on runoff. Flat areas dominated with mineral soils shows the highest overestimation because of an increase in stomatal conductance. Control simulation, on the other hand, underestimated GPP as much as 15% as opposed to explicit routing because of rapid soil saturation, which decreases stomatal conductance. These results suggest that lateral water flow does play a significant role in the terrestrial carbon cycle and should be accounted for in ecological models. For details please see http://ajit.govind.googlepages.com/agu2006
  AGU Fall Meeting Abstracts. 11/2006; -1:06.
• Article: Characterization for Fusion Candidate Vanadium Alloys

  T Muroga, T Nagasaka, J M Chen, Z Y Xu, Q Y Huang, Y C Wu
  [show abstract] [hide abstract]
  ABSTRACT: This paper summarizes recent achievements in the characterization of candidate vanadium alloys obtained for fusion in the framework of the Japan-China Core University Program. National Institute for Fusion Science (NIFS) has a program of fabricating high-purity V-4Cr-4Ti alloys. The resulting products (NIFS-HEAT-1,2), were characterized by various research groups in the world including Chinese partners. South Western Institute of Physics (SWIP) fabricated a new V-4Cr-4Ti alloy (SWIP-Heat), and carried out a comparative evaluation of hydrogen embrittlement of NIFS-HEATs and SWIP-Heat. The tensile test of hydrogen-doped alloys showed that the NIFS-HEAT maintained the ductility to relatively high hydrogen levels. The comparison of the data with those of previous studies suggested that the reduced oxygen level in the NIFS-HEATs should be responsible for the increased resistance to hydrogen embrittlement. Based on the chemical analysis data of NIFS-HEATs and SWIP-Heats, neutron-induced activation was analyzed in Institute of Plasma Physics (IPP-CAS) as a function of cooling time after the use in the fusion first wall. The results showed that the low level of Co dominates the activity up to 50 years followed by a domination of Nb or Nb and Al in the respective alloys. It was suggested that reduction of Co and Nb, both of which are thought to have been introduced via cross-contamination into the alloys from the molds used should be crucial for reducing further the activation.
  Plasma Science and Technology 10/2006; 6(4):2395. · 0.41 Impact Factor
• Article: High-resolution XANES study of Eu(Ba1−xRx)2Cu3O7+δ (R = Eu, Pr)

  J M Chen, R S Liu, J M Lee, K T Lu, T J Yang, M J Kramer, R W McCallum
  [show abstract] [hide abstract]
  ABSTRACT: We have probed the distribution of hole carriers in Eu(Ba1−xEux)2Cu3O7 + δ (x = 0–0.2) and Eu(Ba1−xPrx)2Cu3O7 + δ (x = 0–0.25) by O K-edge and Cu L-edge x-ray absorption spectra. Upon Eu and Pr substitution at the Ba site in Eu(Ba1 − xRx)2Cu3O7 + δ, the concentration of holes in the CuO2 planes becomes greatly diminished. The depletion rate of hole carriers within the CuO2 planes in Eu(Ba1 − xPrx)2Cu3O7 + δ is greater than that in Eu(Ba1 − xEux)2Cu3O7 + δ. The rate of Tc suppression with Pr doping in Eu(Ba1−xPrx)2Cu3O7 + δ is accordingly greater than that in Eu(Ba1 − xEux)2Cu3O7 + δ.
  New Journal of Physics 09/2006; 8(9):215. · 4.18 Impact Factor
• Article: Enhanced production of excited neutrals following core-to-Rydberg excitation in molecules: SiCl4

  R A Rosenberg, C-R Wen, K Tan, J-M Chen
  [show abstract] [hide abstract]
  ABSTRACT: Using monochromatized synchrotron radiation in the range 103 to 112 eV we have monitored the dispersed UV/optical fluorescence resulting from excitation of a Si 2p electron in SiCl4. The main features in the fluorescence spectrum have been identified as emission from the SiCl4+ C state and from excited Si atoms. Features in the fluorescence excitation spectra are assigned to excitation of a Si 2p electron to unoccupied valence and Rydberg orbitals. For the SiCl4+ C state emission there is significant enhancement in the yield following excitation to valence orbitals and very little enhancement as a result of Rydberg excitation. The opposite is true for emission from excited Si atoms. Enhancement in the SiCl4+ C state production upon valence excitation results from autoionization of the core-excited state. The relatively large yield of excited Si atoms following Rydberg excitation is due to the greater probability of the core-excited Rydberg state decaying, via a resonant Auger process, to highly excited, unbound states of SiCl4+. The molecular ion then fragments before the excited Rydberg electron can relax.
  Physica Scripta 09/2006; 41(4):475. · 1.20 Impact Factor