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669 results on '"OECHEL, WALTER C."'

151. Exchange of carbon dioxide across twelve northern peatland and tundra sites

Author

Lund, Magnus, Roulet, Nigel T., Lindroth, Anders, Lafleur, Peter M., Christensen, Torben R., Aurela, Mika, Chojnicki, Bogdan H., Flanagan, Lawrence B., Humphreys, Elyn R., Laurila, Tuomas, Oechel, Walter C., Olejnik, Janusz, Rinne, Janne, Schubert, Per, and Nilsson, Mats B.

Subjects
Carbon dioxide, Peatland, Respiration, Wetland, Eddy covariance, Net ecosystem exchange, Photosynthesis, Tundra, Carbon, Mire
Published
2009

152. CO2 exchange and evapotranspiration across dryland ecosystems of southwestern North America.

Author

Biederman, Joel A., Scott, Russell L., Bell, Tom W., Bowling, David R., Dore, Sabina, Garatuza‐Payan, Jaime, Kolb, Thomas E., Krishnan, Praveena, Krofcheck, Dan J., Litvak, Marcy E., Maurer, Gregory E., Meyers, Tilden P., Oechel, Walter C., Papuga, Shirley A., Ponce‐Campos, Guillermo E., Rodriguez, Julio C., Smith, William K., Vargas, Rodrigo, Watts, Christopher J., and Yepez, Enrico A.

Subjects
ARID regions, EVAPOTRANSPIRATION, VEGETATION & climate, PHOTOSYNTHESIS, REMOTE sensing
Abstract

Global-scale studies suggest that dryland ecosystems dominate an increasing trend in the magnitude and interannual variability of the land CO2 sink. However, such analyses are poorly constrained by measured CO2 exchange in drylands. Here we address this observation gap with eddy covariance data from 25 sites in the water-limited Southwest region of North America with observed ranges in annual precipitation of 100-1000 mm, annual temperatures of 2-25°C, and records of 3-10 years (150 site-years in total). Annual fluxes were integrated using site-specific ecohydrologic years to group precipitation with resulting ecosystem exchanges. We found a wide range of carbon sink/source function, with mean annual net ecosystem production ( NEP) varying from -350 to +330 gCm−2 across sites with diverse vegetation types, contrasting with the more constant sink typically measured in mesic ecosystems. In this region, only forest-dominated sites were consistent carbon sinks. Interannual variability of NEP, gross ecosystem production ( GEP), and ecosystem respiration (Reco) was larger than for mesic regions, and half the sites switched between functioning as C sinks/C sources in wet/dry years. The sites demonstrated coherent responses of GEP and NEP to anomalies in annual evapotranspiration ( ET), used here as a proxy for annually available water after hydrologic losses. Notably, GEP and Reco were negatively related to temperature, both interannually within site and spatially across sites, in contrast to positive temperature effects commonly reported for mesic ecosystems. Models based on MODIS satellite observations matched the cross-site spatial pattern in mean annual GEP but consistently underestimated mean annual ET by ~50%. Importantly, the MODIS-based models captured only 20-30% of interannual variation magnitude. These results suggest the contribution of this dryland region to variability of regional to global CO2 exchange may be up to 3-5 times larger than current estimates. [ABSTRACT FROM AUTHOR]

Published
2017
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153. Mapping Arctic Tundra Vegetation Communities Using Field Spectroscopy and Multispectral Satellite Data in North Alaska, USA.

Author

Davidson, Scott J., Santos, Maria J., Sloan, Victoria L., Watts, Jennifer D., Phoenix, Gareth K., Oechel, Walter C., and Zona, Donatella

Subjects
SPECTRUM analysis, BOTANY, CLIMATE change, REMOTE sensing, ELECTROMAGNETIC wave absorption
Abstract

The Arctic is currently undergoing intense changes in climate; vegetation composition and productivity are expected to respond to such changes. To understand the impacts of climate change on the function of Arctic tundra ecosystems within the global carbon cycle, it is crucial to improve the understanding of vegetation distribution and heterogeneity at multiple scales. Information detailing the fine-scale spatial distribution of tundra communities provided by high resolution vegetation mapping, is needed to understand the relative contributions of and relationships between single vegetation community measurements of greenhouse gas fluxes (e.g., ∼1 m chamber flux) and those encompassing multiple vegetation communities (e.g., ∼300 m eddy covariance measurements). The objectives of this study were: (1) to determine whether dominant Arctic tundra vegetation communities found in different locations are spectrally distinct and distinguishable using field spectroscopy methods; and (2) to test which combination of raw reflectance and vegetation indices retrieved from field and satellite data resulted in accurate vegetation maps and whether these were transferable across locations to develop a systematic method to map dominant vegetation communities within larger eddy covariance tower footprints distributed along a 300 km transect in northern Alaska. We showed vegetation community separability primarily in the 450-510 nm, 630-690 nm and 705-745 nm regions of the spectrum with the field spectroscopy data. This is line with the different traits of these arctic tundra communities, with the drier, often non-vascular plant dominated communities having much higher reflectance in the 450-510 nm and 630-690 nm regions due to the lack of photosynthetic material, whereas the low reflectance values of the vascular plant dominated communities highlight the strong light absorption found here. High classification accuracies of 92% to 96% were achieved using linear discriminant analysis with raw and rescaled spectroscopy reflectance data and derived vegetation indices. However, lower classification accuracies (∼70%) resulted when using the coarser 2.0 m WorldView-2 data inputs. The results from this study suggest that tundra vegetation communities are separable using plot-level spectroscopy with hand-held sensors. These results also show that tundra vegetation mapping can be scaled from the plot level (<1 m) to patch level (<500 m) using spectroscopy data rescaled to match the wavebands of the multispectral satellite remote sensing. We find that developing a consistent method for classification of vegetation communities across the flux tower sites is a challenging process, given the spatial variability in vegetation communities and the need for detailed vegetation survey data for training and validating classification algorithms. This study highlights the benefits of using fine-scale field spectroscopy measurements to obtain tundra vegetation classifications for landscape analyses and use in carbon flux scaling studies. Improved understanding of tundra vegetation distributions will also provide necessary insight into the ecological processes driving plant community assemblages in Arctic environments. [ABSTRACT FROM AUTHOR]

Published
2016
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157. A model-data intercomparison of CO2 exchange across North America: results from the North American Carbon Program site synthesis

Author

Schwalm, Christopher R., Williams, Christopher A., Schaefer, Kevin, Anderson, Ryan S., Arain, M. Altaf, Baker, Ian T., Barr, Alan G., Black, T. Andrew, Chen, Guangsheng, Chen, Jing M., Ciais, Philippe, Davis, Kenneth J., Desai, Ankur R., Dietze, Michael C., Dragoni, Danilo, Fischer, Marc L., Flanagan, Larry B., Grant, Robert F., Gu, Lianhong, Hollinger, David Y., Izaurralde, R. Cesar, Kucharik, Christopher J., Lafleur, Peter M., Law, Beverly E., Li, Longhui, Li, Zhengpeng, Liu, Shuguang, Lokupitiya, Erandathie, Luo, Yiqi, Ma, Siyan, Margolis, Hank, Matamala, Roser, McCaughey, Harry, Monson, Russell K., Oechel, Walter C., Peng, Changhui, Poulter, Benjamin, Price, David T., Ricciuto, Daniel M., Riley, William J., Sahoo, Alok Kumar, Sprintsin, Michael, Sun, Jianfeng, Tian, Hanqin, Tonitto, Christina, Verbeeck, Hans, Verma, Shashi B., Schwalm, Christopher R., Williams, Christopher A., Schaefer, Kevin, Anderson, Ryan S., Arain, M. Altaf, Baker, Ian T., Barr, Alan G., Black, T. Andrew, Chen, Guangsheng, Chen, Jing M., Ciais, Philippe, Davis, Kenneth J., Desai, Ankur R., Dietze, Michael C., Dragoni, Danilo, Fischer, Marc L., Flanagan, Larry B., Grant, Robert F., Gu, Lianhong, Hollinger, David Y., Izaurralde, R. Cesar, Kucharik, Christopher J., Lafleur, Peter M., Law, Beverly E., Li, Longhui, Li, Zhengpeng, Liu, Shuguang, Lokupitiya, Erandathie, Luo, Yiqi, Ma, Siyan, Margolis, Hank, Matamala, Roser, McCaughey, Harry, Monson, Russell K., Oechel, Walter C., Peng, Changhui, Poulter, Benjamin, Price, David T., Ricciuto, Daniel M., Riley, William J., Sahoo, Alok Kumar, Sprintsin, Michael, Sun, Jianfeng, Tian, Hanqin, Tonitto, Christina, Verbeeck, Hans, and Verma, Shashi B.

Abstract

Our current understanding of terrestrial carbon processes is represented in various models used to integrate and scale measurements of CO2 exchange from remote sensing and other spatiotemporal data. Yet assessments are rarely conducted to determine how well models simulate carbon processes across vegetation types and environmental conditions. Using standardized data from the North American Carbon Program we compare observed and simulated monthly CO2 exchange from 44 eddy covariance flux towers in North America and 22 terrestrial biosphere models. The analysis period spans ∼220 site‐years, 10 biomes, and includes two large‐scale drought events, providing a natural experiment to evaluate model skill as a function of drought and seasonality. We evaluate models’ ability to simulate the seasonal cycle of CO2 exchange using multiple model skill metrics and analyze links between model characteristics, site history, and model skill. Overall model performance was poor; the difference between observations and simulations was ∼10 times observational uncertainty, with forested ecosystems better predicted than nonforested. Model‐data agreement was highest in summer and in temperate evergreen forests. In contrast, model performance declined in spring and fall, especially in ecosystems with large deciduous components, and in dry periods during the growing season. Models used across multiple biomes and sites, the mean model ensemble, and a model using assimilated parameter values showed high consistency with observations. Models with the highest skill across all biomes all used prescribed canopy phenology, calculated NEE as the difference between GPP and ecosystem respiration, and did not use a daily time step.

Published
2010

161. A multi-scale comparison of modeled and observed seasonal methane emissions in northern wetlands.

Author

Xiyan Xu, Riley, William J., Koven, Charles D., Billesbach, Dave P., Chang, Rachel Y.-W., Commane, Róisín, Euskirchen, Eugénie S., Hartery, Sean, Yoshinobu Harazono, Hiroki Iwata, McDonald, Kyle C., Miller, Charles E., Oechel, Walter C., Poulter, Benjamin, Raz-Yaseef, Naama, Sweeney, Colm, Torn, Margaret, Wofsy, Steven C., Zhen Zhang, and Zona, Donatella

Subjects
WETLANDS, METHANE & the environment, BIOGEOCHEMISTRY, HYDROLOGY, COMPARATIVE studies
Abstract

Wetlands are the largest global natural methane (CH4) source, and emissions between 50 and 70° N latitude contribute 10-30% to this source. Predictive capability of land models for northern wetland CH4 emissions is still low due to limited site measurements, strong spatial and temporal variability in emissions, and complex hydrological and biogeochemical dynamics. To explore this issue, we compare wetland CH4 emission predictions from the Community Land Model 4.5 (CLM4.5-BGC) with siteto regional-scale observations. A comparison of the CH4 fluxes with eddy flux data highlighted needed changes to the model's estimate of aerenchyma area, which we implemented and tested. The model modification substantially reduced biases in CH4 emissions when compared with CarbonTracker CH4 predictions. CLM4.5 CH4 emission predictions agree well with growing season (May-September) CarbonTracker Alaskan regional-level CH4 predictions and sitelevel observations. However, CLM4.5 underestimated CH4 emissions in the cold season (October-April). The monthly atmospheric CH4 mole fraction enhancements due to wetland emissions are also assessed using the Weather Research and Forecasting-Stochastic Time-Inverted Lagrangian Transport (WRF-STILT) model coupled with daily emissions from CLM4.5 and compared with aircraft CH4 mole fraction measurements from the Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE) campaign. Both the tower and aircraft analyses confirm the underestimate of cold-season CH4 emissions by CLM4.5. The greatest uncertainties in predicting the seasonal CH4 cycle are from the wetland extent, coldseason CH4 production and CH4 transport processes. We recommend more cold-season experimental studies in highlatitude systems, which could improve the understanding and parameterization of ecosystem structure and function during this period. Predicted CH4 emissions remain uncertain, but we show here that benchmarking against observations across spatial scales can inform model structural and parameter improvements. [ABSTRACT FROM AUTHOR]

Published
2016
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162. A multi-scale comparison of modeled and observed seasonal methane cycles in northern wetlands.

Author

Xiyan Xu, Riley, William J., Koven, Charles D., Billesbach, Dave P., Chang, Rachel Y.-­W., Commane, Róisín, Euskirchen, Eugénie S., Hartery, Sean, Yoshinobu Harazono, Hiroki Iwata, McDonald, Kyle C., Miller, Charles E., Oechel, Walter C., Poulter, Benjamin, Raz-Yaseef, Naama, Sweeney, Colm, Torn, Margaret, Wofsy, Steven C., Zhen Zhang, and Zona, Donatella

Subjects
METHANE cycle (Biogeochemistry), WETLANDS, GREENHOUSE gas mitigation, HYDROLOGY, EDDY flux, CHEMICAL reduction
Abstract

Wetlands are the single largest global natural methane (CH4) source, and emissions between 50°N and 70°N latitude contribute 10-30% to this source. Predictive capability of northern wetland CH4 emissions is still low due to limited site measurements, strong spatial and temporal variability in emissions, and complex hydrological and biogeochemical dynamics. To explore this issue, we compare wetland CH4 emission predictions from the Community Land Model 4.5 (CLM4.5-BGC) with site to regional scale observations. A comparison of the CH4 fluxes with eddy flux data highlighted needed changes to the model's estimate of aerenchyma area, which we implemented and tested. The model modification substantially reduced biases in CH4 emissions when compared with CarbonTracker CH4 predictions. CLM4.5 CH4 emission predictions agree well with growing season (May-September) CarbonTracker Alaskan regional-level CH4 predictions and site-level observations. However, CLM4.5 underestimated CH4 emissions in the cold season (October-April). The monthly CH4 mole fraction enhancements due to wetland emissions are also assessed using the WRF-STILT Lagrangian transport model coupled with daily emission priors from CLM4.5 and compared with aircraft CH4 mole fraction measurements from the Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE) campaign. Both the tower and aircraft analyses confirm the underestimate of cold season CH4 emissions by CLM4.5. The greatest uncertainties in predicting the seasonal CH4 cycle are from the wetland extent, cold season CH4 production and CH4 transport processes. We recommend more cold-season experimental studies in high latitude systems, which could improve understanding and parameterization of ecosystem structure and function during this period. Predicted CH4 emissions remain uncertain, but we show here that benchmarking against observations across spatial scales can inform model structural and parameter improvements. [ABSTRACT FROM AUTHOR]

Published
2016
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163. On the use of MODIS EVI to assess gross primary productivity of North American ecosystems

Author

Sims, Daniel A., Rahman, Abdullah F., Cordova, Vicente D., El-Masri, Bassil Z., Baldocchi, Dennis D., Flanagan, Larry B., Goldstein, Allen H., Hollinger, David Y., Misson, Laurent, Monson, Russell K., Oechel, Walter C., Schmid, Hans P., Wofsy, Steven C., Xu, Liukang, Sims, Daniel A., Rahman, Abdullah F., Cordova, Vicente D., El-Masri, Bassil Z., Baldocchi, Dennis D., Flanagan, Larry B., Goldstein, Allen H., Hollinger, David Y., Misson, Laurent, Monson, Russell K., Oechel, Walter C., Schmid, Hans P., Wofsy, Steven C., and Xu, Liukang

Abstract

Carbon flux models based on light use efficiency (LUE), such as the MOD17 algorithm, have proved difficult to parameterize because of uncertainties in the LUE term, which is usually estimated from meteorological variables available only at large spatial scales. In search of simpler models based entirely on remote-sensing data, we examined direct relationships between the enhanced vegetation index (EVI) and gross primary productivity (GPP) measured at nine eddy covariance flux tower sites across North America. When data from the winter period of inactive photosynthesis were excluded, the overall relationship between EVI and tower GPP was better than that between MOD17 GPP and tower GPP. However, the EVI/GPP relationships vary between sites. Correlations between EVI and GPP were generally greater for deciduous than for evergreen sites. However, this correlation declined substantially only for sites with the smallest seasonal variation in EVI, suggesting that this relationship can be used for all but the most evergreen sites. Within sites dominated by either evergreen or deciduous species, seasonal variation in EVI was best explained by the severity of summer drought. Our results demonstrate that EVI alone can provide estimates of GPP that are as good as, if not better than, current versions of the MOD17 algorithm for many sites during the active period of photosynthesis. Preliminary data suggest that inclusion of other remote-sensing products in addition to EVI, such as the MODIS land surface temperature (LST), may result in more robust models of carbon balance based entirely on remote-sensing data

Published
2006

168. Direct observations of the effects of aerosol loading on net ecosystem CO2 exchanges over different landscapes

Author

Niyogi, Dev, Chang, Hsin-I, Saxena, V.K., Holt, Teddy, Alapaty, Kiran, Booker, Fitzgerald, Chen, Fei, Davis, Kenneth J., Holben, Brent, Matsui, Toshihisa, Meyers, Tilden, Oechel, Walter C., Pielke Sr., Roger A., Wells, Randy, Wilson, Kell, Xue, Yongkang, Niyogi, Dev, Chang, Hsin-I, Saxena, V.K., Holt, Teddy, Alapaty, Kiran, Booker, Fitzgerald, Chen, Fei, Davis, Kenneth J., Holben, Brent, Matsui, Toshihisa, Meyers, Tilden, Oechel, Walter C., Pielke Sr., Roger A., Wells, Randy, Wilson, Kell, and Xue, Yongkang

Abstract

We present the first direct, multisite observations in Support of the hypothesis that atmospheric aerosols affect the regional terrestrial carbon cycle. The daytime growing season (summer) CO2 flux observations from six sites (forest, grasslands, and croplands) with collocated aerosol and surface radiation measurements were analyzed for high and low diffuse radiation; effect of cloud cover; and effect of high and low aerosol optical depths (AOD). Results indicate that, aerosols exert a significant impact on net CO2 exchange, and their effect may be even more significant than that due to clouds. The response appears to be a general feature irrespective of the landscape and photosynthetic pathway. The CO2 sink increased with aerosol loading for forest and crop lands, and decreased for grassland. The cause for the difference in response between vegetation types is hypothesized to be canopy architecture. Copyright 2004 by the American Geophysical Union.

Published
2004

174. Assessing net ecosystem carbon exchange of U.S. terrestrial ecosystems by integrating eddy covariance flux measurements and satellite observations

Author

Xiao, Jingfeng, primary, Zhuang, Qianlai, additional, Law, Beverly E., additional, Baldocchi, Dennis D., additional, Chen, Jiquan, additional, Richardson, Andrew D., additional, Melillo, Jerry M., additional, Davis, Kenneth J., additional, Hollinger, David Y., additional, Wharton, Sonia, additional, Oren, Ram, additional, Noormets, Asko, additional, Fischer, Marc L., additional, Verma, Shashi B., additional, Cook, David R., additional, Sun, Ge, additional, McNulty, Steve, additional, Wofsy, Steven C., additional, Bolstad, Paul V., additional, Burns, Sean P., additional, Curtis, Peter S., additional, Drake, Bert G., additional, Falk, Matthias, additional, Foster, David R., additional, Gu, Lianhong, additional, Hadley, Julian L., additional, Katul, Gabriel G., additional, Litvak, Marcy, additional, Ma, Siyan, additional, Martin, Timothy A., additional, Matamala, Roser, additional, Meyers, Tilden P., additional, Monson, Russell K., additional, Munger, J. William, additional, Oechel, Walter C., additional, Paw, U. Kyaw Tha, additional, Schmid, Hans Peter, additional, Scott, Russell L., additional, Starr, Gregory, additional, Suyker, Andrew E., additional, and Torn, Margaret S., additional

Published
2011
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175. A model-data intercomparison of CO2exchange across North America: Results from the North American Carbon Program site synthesis

Author

Schwalm, Christopher R., primary, Williams, Christopher A., additional, Schaefer, Kevin, additional, Anderson, Ryan, additional, Arain, M. Altaf, additional, Baker, Ian, additional, Barr, Alan, additional, Black, T. Andrew, additional, Chen, Guangsheng, additional, Chen, Jing Ming, additional, Ciais, Philippe, additional, Davis, Kenneth J., additional, Desai, Ankur, additional, Dietze, Michael, additional, Dragoni, Danilo, additional, Fischer, Marc L., additional, Flanagan, Lawrence B., additional, Grant, Robert, additional, Gu, Lianhong, additional, Hollinger, David, additional, Izaurralde, R. César, additional, Kucharik, Chris, additional, Lafleur, Peter, additional, Law, Beverly E., additional, Li, Longhui, additional, Li, Zhengpeng, additional, Liu, Shuguang, additional, Lokupitiya, Erandathie, additional, Luo, Yiqi, additional, Ma, Siyan, additional, Margolis, Hank, additional, Matamala, Roser, additional, McCaughey, Harry, additional, Monson, Russell K., additional, Oechel, Walter C., additional, Peng, Changhui, additional, Poulter, Benjamin, additional, Price, David T., additional, Riciutto, Dan M., additional, Riley, William, additional, Sahoo, Alok Kumar, additional, Sprintsin, Michael, additional, Sun, Jianfeng, additional, Tian, Hanqin, additional, Tonitto, Christina, additional, Verbeeck, Hans, additional, and Verma, Shashi B., additional

Published
2010
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178. Variability in exchange of CO2 across 12 northern peatland and tundra sites

Author

LUND, MAGNUS, primary, LAFLEUR, PETER M., additional, ROULET, NIGEL T., additional, LINDROTH, ANDERS, additional, CHRISTENSEN, TORBEN R., additional, AURELA, MIKA, additional, CHOJNICKI, BOGDAN H., additional, FLANAGAN, LAWRENCE B., additional, HUMPHREYS, ELYN R., additional, LAURILA, TUOMAS, additional, OECHEL, WALTER C., additional, OLEJNIK, JANUSZ, additional, RINNE, JANNE, additional, SCHUBERT, PER, additional, and NILSSON, MATS B., additional

Published
2009
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184. On the use of MODIS EVI to assess gross primary productivity of North American ecosystems

Author

Sims, Daniel A., primary, Rahman, Abdullah F., additional, Cordova, Vicente D., additional, El-Masri, Bassil Z., additional, Baldocchi, Dennis D., additional, Flanagan, Lawrence B., additional, Goldstein, Allen H., additional, Hollinger, David Y., additional, Misson, Laurent, additional, Monson, Russell K., additional, Oechel, Walter C., additional, Schmid, Hans P., additional, Wofsy, Steven C., additional, and Xu, Liukang, additional

Published
2006
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190. Site-level evaluation of satellite-based global terrestrial gross primary production and net primary production monitoring

Author

Turner, David P., primary, Ritts, William D., additional, Cohen, Warren B., additional, Maeirsperger, Thomas K., additional, Gower, Stith T., additional, Kirschbaum, Al A., additional, Running, Steve W., additional, Zhao, Maosheng, additional, Wofsy, Steven C., additional, Dunn, Allison L., additional, Law, Beverly E., additional, Campbell, John L., additional, Oechel, Walter C., additional, Kwon, Hyo Jung, additional, Meyers, Tilden P., additional, Small, Eric E., additional, Kurc, Shirley A., additional, and Gamon, John A., additional

Published
2005
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191. Direct observations of the effects of aerosol loading on net ecosystem CO2 exchanges over different landscapes

Author

Niyogi, Dev, primary, Chang, Hsin‐I, additional, Saxena, V. K., additional, Holt, Teddy, additional, Alapaty, Kiran, additional, Booker, Fitzgerald, additional, Chen, Fei, additional, Davis, Kenneth J., additional, Holben, Brent, additional, Matsui, Toshihisa, additional, Meyers, Tilden, additional, Oechel, Walter C., additional, Pielke, Roger A., additional, Wells, Randy, additional, Wilson, Kell, additional, and Xue, Yongkang, additional

Published
2004
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