Environmental controls over carbon dioxide and water vapor exchange of terrestrial vegetationIntroductionMethodsResults and discussionLight qualityTemperatureWaterConclusionsAcknowledgementsReferencesAgricultural and Forest Meteorology 113 (2002) 97–120Environmental controls over carbon dioxide and water vaporexchange of terrestrial vegetationB.E. Lawa,∗, E. Falgeb,L.Guc, D.D. Baldocchic, P. Bakwind, P. Berbigiere,K. Davisf, A.J. Dolmang,M.Falkh, J.D. Fuentesi, A. Goldsteinc, A. Granierj,A. Grellek, D. Hollingerl, I.A. Janssensm, P. Jarvisn, N.O. Jenseno, G. Katulp,Y. Mahliq, G. Matteuccir, T. Meyerss, R. Monsont, W. Mungeru, W. Oechelv,R. Olsonw, K. Pilegaardx, K.T. Paw Uh, H. Thorgeirssony, R. Valentinir, S. Vermaz,T. Vesalaa1, K. Wilsons, S. Wofsyua328 Richardson Hall, College of Forestry, Oregon State University, Corvallis, OR 97331-5752, USAbPflanzenökologie, Universität Bayreuth, 95440 Bayreuth, GermanycESPM, University of California, Berkeley, CA 94704, USAdNOAA/OAR, Climate Monitoring and Diagnostics Laboratory, 325 Broadway, Boulder, CO 80303, USAeINRA Centre de Bordeaux, Unite de Bioclimatologie, BP 81, 33833 Villenave d’ornon Cedex, FrancefDepartment of Soil, Water, and Climate, University of Minnesota, St. Paul, MN 55108, USAgAlterra, P.O. Box 47, 6700 AA Wageningen, The NetherlandshAtmospheric Science Group, LAWR, UC Davis, 122 Hoagland Hall, Davis, CA 95616, USAiDepartment of Environmental Science, University of Virginia, Charlottesville, VA, USAjCentre de Recherces de Nancy, Unite Ecophysiologie Forestieres, Equipe Bioclimatologie, 54280 Champenoux, FrancekDepartment of Ecology and Environmental Research, Swedish University of Agricultural Sciences, S-750 07 Uppsala, SwedenlUSDA Forest Service, 271 Mast Road, Durham, NH 03824, USAmDepartment of Biology, University of Antwerpen, Wilrijk, BelgiumnInstitute of Ecology and Resource Management, University of Edinburgh, Darwin Building, King’s Buildings, Edinburgh EH9 3JU, UKoRisoe National Laboratory, DK-4000 Roskilde, DenmarkpSchool of the Environment, Duke University, Box 90328, Durham, NC 27708-0328, USAqInstitute of Ecology and Resource Management, University of Edinburgh, Darwin Building, King’s Buildings,Mayfield Road, Edinburgh EH9 3JU, UKrDepartment of Forest Environment and Resources, University of Tuscia, I-01100 Viterbo, ItalysNOAA/ARL Atmospheric Turbulence and Diffusion Division, 456 South Illinois Avenue, Oak Ridge, TN 37831-2456, USAtDepartment of Environmental, Population, and Organismic Biology, University of Colorado, Campus Box 334, Boulder, CO 80309, USAuDepartment of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USAvDepartment of Biology, San Diego State University, San Diego, CA, USAwEnvironmental Science Division, Oak Ridge National Laboratory, Oak Ridge, TN, USAxPlant Biology and Biogeochemistry Department, Risoe National Laboratory, P.O. Box 49, DK-4000 Roskilde, DenmarkyDepartment of Environmental Research, Agricultural Research Institute, IS-112 Reykjavik, IcelandzDepartment of Agricultural Meteorology, University of Nebraska-Lincoln, 244 L.W. Chase Hall, P.O. Box 830728, Lincoln, NE 68583, USAa1Department of Physics, University of Helsinki, P.O. Box 9, FIN-00014 Helsinki, FinlandAccepted 3 April 2002∗Corresponding author. Tel.: +1-541-737-6111; fax: +1-541-737-1393.E-mail address: [email protected] (B.E. Law).0168-1923/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0168-1923(02)00104-198 B.E. Law et al./Agricultural and Forest Meteorology 113 (2002) 97–120AbstractThe objective of this research was to compare seasonal and annual estimates of CO2and water vapor exchange acrosssites in forests, grasslands, crops, and tundra that are part of an international network called FLUXNET, and to investigatingthe responses of vegetation to environmental variables. FLUXNETs goals are to understand the mechanisms controlling theexchangesof CO2, watervaporand energyacross aspectrum of timeand space scales, and to provideinformation for modelingof carbon and water cycling across regions and the globe. At a subset of sites, net carbon uptake (net ecosystem exchange,the net of photosynthesis and respiration) was greater under diffuse than under direct radiation conditions, perhaps becauseof a more efficient distribution of non-saturating light conditions for photosynthesis, lower vapor pressure deficit limitationto photosynthesis, and lower respiration associated with reduced temperature. The slope of the relation between monthlygross ecosystem production and evapotranspiration was similar between biomes, except for tundra vegetation, showing astrong linkage between carbon gain and water loss integrated over the year (slopes = 3.4gCO2/kgH2O for grasslands, 3.2for deciduous broadleaf forests, 3.1 for crops, 2.4 for evergreen conifers, and 1.5 for tundra vegetation). The ratio of annualecosystem respiration to gross photosynthesis averaged 0.83, with lower values for grasslands, presumably because of lessinvestment in respiring plant tissue compared with forests. Ecosystem respiration was weakly correlated with mean annualtemperature across biomes, in spite of within site sensitivity over shorter temporal scales. Mean annual temperature andsite water balance explained much of the variation in gross photosynthesis. Water availability limits leaf area index over thelong-term, and inter-annual climate variability can limit carbon uptake below the potential of the leaf area present.© 2002 Elsevier Science B.V. All rights reserved.Keywords: Gross ecosystem production; Ecosystem respiration; Net ecosystem exchange; Carbon balance; Eddy covariance1. IntroductionThe response of vegetation to the environment is akey global change issue that scientists are investigat-ing by means of measurements and models on short-and long-time scales. Previous comparisons of theresponses of terrestrial ecosystems of the world to theenvironment included measurements of abovegroundproduction in relation to temperature, precipitation,and empirical estimates of evapotranspiration (ET).For example, earlier work suggested that annual pro-ductivity increased with mean annual temperature andprecipitation (Lieth, 1972a,b; O’Neill and DeAngelis,1981). Concurrently, leaf-level studies suggested amechanism for optimal stomatal variation that reg-ulates the relationship between water loss throughtranspiration and carbon uptake through assimila-tion in