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Dennis D. Baldocchi, Jose D. Fuentes, David R. Bowling, Andrew A. Turnipseed, and Russell K. Monson

Abstract

The rate at which isoprene is emitted by a forest depends on an array of environmental variables, the forest’s biomass, and its species composition. At present it is unclear whether errors in canopy-scale and process-level isoprene emission models are due to inadequacies in leaf-to-canopy integration theory or the imperfect assessment of the isoprene-emitting biomass in the flux footprint. To address this issue, an isoprene emission model (CANVEG) was tested over a uniform aspen stand and a mixed-species, broad-leaved forest.

The isoprene emission model consists of coupled micrometeorological and physiological modules. The micrometeorological module computes leaf and soil energy exchange, turbulent diffusion, scalar concentration profiles, and radiative transfer through the canopy. Environmental variables that are computed by the micrometeorological module, in turn, drive physiological modules that calculate leaf photosynthesis, stomatal conductance, transpiration and leaf, bole and soil/root respiration, and rates of isoprene emission.

The isoprene emission model accurately predicted the diurnal variation of isoprene emission rates over the boreal aspen stand, as compared with micrometeorological flux measurements. The model’s ability to simulate isoprene emission rates over the mixed temperate forest, on the other hand, depended strongly upon the amount of isoprene-emitting biomass, which, in a mixed-species forest, is a function of the wind direction and the horizontal dimensions of the flux footprint. When information on the spatial distribution of biomass and the flux footprint probability distribution function were included, the CANVEG model produced values of isoprene emission that compared well with micrometeorological measurements. The authors conclude that a mass and energy exchange model, which couples flows of carbon, water, and nutrients, can be a reliable tool for integrating leaf-scale, isoprene emission algorithms to the canopy dimension over dissimilar vegetation types as long as the vegetation is characterized appropriately.

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Christoph A. Vogel, Dennis D. Baldocchi, Ashok K. Luhar, and K. Shankar Rao

Abstract

Several methods for estimating surface energy balance components over a vegetated surface are compared. These include Penman-Monteith, Deardorff, and multilayer canopy (CANWHT) models for evaporation. Measurements taken during the 1991 DOE-sponsored Boardman Area Regional Flux Experiment over a Well-irrigated, closed wheat canopy are used in the comparison. The relative performance of each model is then evaluated. It is found that the Penman-Monteith approach using a simple parameterization for stomatal conductance performs best for evaporation flux. The Deardorff model is found to have the best relative performance for sensible heat, while the CANWHT model gives the best results for net radiation and soil heat flux. The Priestley-Taylor model for evaporation and a resistance-analog equation for sensible heat flux are also tested.

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Piers Sellers, Forrest Hall, Hank Margolis, Bob Kelly, Dennis Baldocchi, Gerry den Hartog, Josef Cihlar, Michael G. Ryan, Barry Goodison, Patrick Crill, K. Jon Ranson, Dennis Lettenmaier, and Diane E. Wickland

The Boreal Ecosystem Atmosphere Study (BOREAS) is a largescale international field experiment that has the goal of improving our understanding of the exchanges of radiative energy, heat, water, CO2, and trace gases between the boreal forest and the lower atmosphere. An important objective of BOREAS is to collect the data needed to improve computer simulation models of the processes controlling these exchanges so that scientists can anticipate the effects of global change.

From August 1993 through September 1994, a continuous set of monitoring measurements—meteorology, hydrology, and satellite remote sensing—were gathered overthe 1000 × 1000 km BOREAS study region that covers most of Saskatchewan and Manitoba, Canada. This monitoring program was punctuated by six campaigns that saw the deployment of some 300 scientists and aircrew into the field, supported by 11 research aircraft. The participants were drawn primarily from U.S. and Canadian agencies and universities, although there were also important contributions from France, the United Kingdom, and Russia. The field campaigns lasted for a total of 123 days and saw the compilation of a comprehensive surfaceatmosphere flux dataset supported by ecological, trace gas, hydrological, and remote sensing science observations. The surface-atmosphere fluxes of sensible heat, latent heat, CO2, and momentum were measured using eddy correlation equipment mounted on a surface network of 10 towers complemented by four flux-measurement aircraft. All in all, over 350 airborne missions (remote sensing and eddy correlation) were flown during the 1994 field year.

Preliminary analyses of the data indicate that the area-averaged photosynthetic capacity of the boreal forest is much less than that of the temperate forests to the south. This is reflected in very low photosynthetic and carbon drawdown rates, which in turn are associated with low transpiration rates (less than 2 mm day−1 over the growing season for the coniferous species in the area). The strong sensible fluxes generated as a result of this often lead to the development of a deep dry planetary boundary layer overthe forest, particularly during the spring and early summer. The effects of frozen soils and the strong physiological control of evapotranspiration in the biome do not seem to be well represented in most operational general circulation models of the atmosphere.

Analyses of the data will continue through 1995 and 1996. Some limited revisits to the field are anticipated.

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Dennis Baldocchi, Eva Falge, Lianhong Gu, Richard Olson, David Hollinger, Steve Running, Peter Anthoni, Ch. Bernhofer, Kenneth Davis, Robert Evans, Jose Fuentes, Allen Goldstein, Gabriel Katul, Beverly Law, Xuhui Lee, Yadvinder Malhi, Tilden Meyers, William Munger, Walt Oechel, K. T. Paw U, Kim Pilegaard, H. P. Schmid, Riccardo Valentini, Shashi Verma, Timo Vesala, Kell Wilson, and Steve Wofsy

FLUXNET is a global network of micrometeorological flux measurement sites that measure the exchanges of carbon dioxide, water vapor, and energy between the biosphere and atmosphere. At present over 140 sites are operating on a long-term and continuous basis. Vegetation under study includes temperate conifer and broadleaved (deciduous and evergreen) forests, tropical and boreal forests, crops, grasslands, chaparral, wetlands, and tundra. Sites exist on five continents and their latitudinal distribution ranges from 70°N to 30°S.

FLUXNET has several primary functions. First, it provides infrastructure for compiling, archiving, and distributing carbon, water, and energy flux measurement, and meteorological, plant, and soil data to the science community. (Data and site information are available online at the FLUXNET Web site, http://www-eosdis.ornl.gov/FLUXNET/.) Second, the project supports calibration and flux intercomparison activities. This activity ensures that data from the regional networks are intercomparable. And third, FLUXNET supports the synthesis, discussion, and communication of ideas and data by supporting project scientists, workshops, and visiting scientists. The overarching goal is to provide information for validating computations of net primary productivity, evaporation, and energy absorption that are being generated by sensors mounted on the NASA Terra satellite.

Data being compiled by FLUXNET are being used to quantify and compare magnitudes and dynamics of annual ecosystem carbon and water balances, to quantify the response of stand-scale carbon dioxide and water vapor flux densities to controlling biotic and abiotic factors, and to validate a hierarchy of soil–plant–atmosphere trace gas exchange models. Findings so far include 1) net CO2 exchange of temperate broadleaved forests increases by about 5.7 g C m−2 day−1 for each additional day that the growing season is extended; 2) the sensitivity of net ecosystem CO2 exchange to sunlight doubles if the sky is cloudy rather than clear; 3) the spectrum of CO2 flux density exhibits peaks at timescales of days, weeks, and years, and a spectral gap exists at the month timescale; 4) the optimal temperature of net CO2 exchange varies with mean summer temperature; and 5) stand age affects carbon dioxide and water vapor flux densities.

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Sara H. Knox, Robert B. Jackson, Benjamin Poulter, Gavin McNicol, Etienne Fluet-Chouinard, Zhen Zhang, Gustaf Hugelius, Philippe Bousquet, Josep G. Canadell, Marielle Saunois, Dario Papale, Housen Chu, Trevor F. Keenan, Dennis Baldocchi, Margaret S. Torn, Ivan Mammarella, Carlo Trotta, Mika Aurela, Gil Bohrer, David I. Campbell, Alessandro Cescatti, Samuel Chamberlain, Jiquan Chen, Weinan Chen, Sigrid Dengel, Ankur R. Desai, Eugenie Euskirchen, Thomas Friborg, Daniele Gasbarra, Ignacio Goded, Mathias Goeckede, Martin Heimann, Manuel Helbig, Takashi Hirano, David Y. Hollinger, Hiroki Iwata, Minseok Kang, Janina Klatt, Ken W. Krauss, Lars Kutzbach, Annalea Lohila, Bhaskar Mitra, Timothy H. Morin, Mats B. Nilsson, Shuli Niu, Asko Noormets, Walter C. Oechel, Matthias Peichl, Olli Peltola, Michele L. Reba, Andrew D. Richardson, Benjamin R. K. Runkle, Youngryel Ryu, Torsten Sachs, Karina V. R. Schäfer, Hans Peter Schmid, Narasinha Shurpali, Oliver Sonnentag, Angela C. I. Tang, Masahito Ueyama, Rodrigo Vargas, Timo Vesala, Eric J. Ward, Lisamarie Windham-Myers, Georg Wohlfahrt, and Donatella Zona

Abstract

This paper describes the formation of, and initial results for, a new FLUXNET coordination network for ecosystem-scale methane (CH4) measurements at 60 sites globally, organized by the Global Carbon Project in partnership with other initiatives and regional flux tower networks. The objectives of the effort are presented along with an overview of the coverage of eddy covariance (EC) CH4 flux measurements globally, initial results comparing CH4 fluxes across the sites, and future research directions and needs. Annual estimates of net CH4 fluxes across sites ranged from −0.2 ± 0.02 g C m–2 yr–1 for an upland forest site to 114.9 ± 13.4 g C m–2 yr–1 for an estuarine freshwater marsh, with fluxes exceeding 40 g C m–2 yr–1 at multiple sites. Average annual soil and air temperatures were found to be the strongest predictor of annual CH4 flux across wetland sites globally. Water table position was positively correlated with annual CH4 emissions, although only for wetland sites that were not consistently inundated throughout the year. The ratio of annual CH4 fluxes to ecosystem respiration increased significantly with mean site temperature. Uncertainties in annual CH4 estimates due to gap-filling and random errors were on average ±1.6 g C m–2 yr–1 at 95% confidence, with the relative error decreasing exponentially with increasing flux magnitude across sites. Through the analysis and synthesis of a growing EC CH4 flux database, the controls on ecosystem CH4 fluxes can be better understood, used to inform and validate Earth system models, and reconcile differences between land surface model- and atmospheric-based estimates of CH4 emissions.

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