Figure 1. Location of research sites in the Ameriflux network (from Ameriflux web site,

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1 CONTEXT - AMERIFLUX NETWORK Figure 1. Location of research sites in the Ameriflux network (from Ameriflux web site, AMERIFLUX OBJECTIVES: Quantify spatial and temporal variation in carbon storage in plants and soils, and exchanges of carbon, water, and energy in major vegetation types across a range of disturbance histories and climatic conditions in the Americas. Advance understanding of processes regulating carbon assimilation, respiration, and storage, and linkages between carbon, water, energy and nitrogen through measurements and modeling. Produce high quality data for site-level analyses, synthesis activities, and the data archive. Science Questions The network role is to address the scientific uncertainties associated with global change. Our focus is to address these scientific questions: 1. What are the magnitudes of carbon storage and the exchanges of energy, CO 2 and water vapor in terrestrial systems? What is the spatial and temporal variability? 2. How is this variability influenced by vegetation type, phenology, changes in land use, management, and disturbance history, and what is the relative effect of these factors? 3. What is the causal link between climate and the exchanges of energy, CO 2 and water vapor for major vegetation types, and how does seasonal and inter-annual climate variability and anomalies influence fluxes? Page 1

2 4. What is the spatial and temporal variation of boundary layer CO 2 concentrations, and how does this vary with topography, climatic zone and vegetation? CONTEXT - FLORIDA FOREST TYPES Figure 2. Distribution of timberland in Florida as reported by the U.S. Forest Service Forest Inventory and Analysis Research Work Unit. About 47% of Florida's total land area is forested (Brown 1999). FLORIDA AMERIFLUX PROJECT QUESTIONS What is the pattern of net CO 2 exchange over stand development, and what ecosystem sources and sinks govern this pattern? How does the structure and function of even-aged, intensively-managed plantations compare with the structure and function of a naturally-regenerated, less intensively managed stand? What is the role of the plantation forest with regards to short-term and long-term carbon, water, and energy balances in the Coastal Plain landscape? If drought conditions persist over long periods of time, how will this prolonged stress affect carbon assimilation in the region, and what structural and functional tree and ecosystem attributes are important for driving these responses? Page 2

3 How does prescribed fire affect ecosystem carbon dynamics? How does NEE vary across the landscape, and what are the management and soils factors that control this variation? USEFUL TERMS AND BACKGROUND INFORMATION eddy covariance - A micrometeorological technique that utilizes high frequency (1 Hz) measurements of air turbulence and the concentration of an entity (such as CO 2 or water vapor) to infer the flux density of that entity to or from a surface. NEE or Net ecosystem exchange, or Net ecosystem production (NEP) The net uptake or release of carbon from an ecosystem. GEP or gross ecosystem production - The total carbon taken up or fixed by photosynthesis. R h or heterotrophic respiration - Carbon lost due to respiration of heterotrophic organisms. R a or autotrophic respiration - Carbon lost due to respiration of autotrophic organisms. R eco or ecosystem respiration = R h + R a LAI or leaf area index - area of leaves per unit area of ground stomatal conductance - the inverse of the resistance to gaseous diffusion through stomata transpiration - water that is evaporated from leaf internal mesophyll cells and lost through stomata evapotranspiration - total evaporation of water, includes transpiration as well as evaporation of free water from surfaces Factors affecting NEE NEE = GEP + R a + R h GEP - species composition genetic constraints plant physiology nutrient availability responses to stresses canopy architecture leaf area index vertical leaf area distribution horizontal leaf area distribution climate patterns of precipitation, temperature, humidity, solar radiation R a species composition plant physiology climate (esp. temperature) R h organic matter quantity and quality climate (esp. temperature, substrate moisture) species composition (?) Page 3

4 aboveground plant physiology (?) MEASUREMENTS Measurement Instrument Frequency eddy covariance "closed path" system utilizing 3-D sonic anemometer, infrared gas analyzer, pump continuous - 1 Hz meteorological - shortwave radiation photosynthetically active radiation net radiation air temperature air relative humidity precipitation soil temperature soil heat flux misc. continuous - half hourly tree sap flow Granier-type heat dissipation probes continuous - half hourly soil volumetric water content portable TDR and permanent capacitance probes weekly and continuous - half hourly soil CO 2 efflux closed-path transient system monthly with infrared gas analyzer leaf litter litter traps monthly leaf physiology light-saturated net photosynthesis A-Ci curves chlorophyll fluorescence leaf nitrogen concentration leaf water potential open-configuration photosynthesis system pulse-modulated fluorometer elemental analyzer pressure chamber periodic permanent plot inventory tree height tree diameter tree destructive harvest to enable prediction of component biomass yearly periodic Page 4

5 Eddy Covariance F c = NEE = w'c' Figure 3. Schematic diagram of eddy covariance measurements. A fast-response sonic anemometer measures fluctuations in "eddies" or packets of air as they move into and out of the canopy. A fast-response infrared gas analyzer simultaneously measures changes in carbon dioxide and water vapor concentrations or density. These measurements are combined to obtain shorttime-scale estimates of carbon and water. Atmosphere CO 2 Taken Up by Plant Photosynthesis + = or CO 2 Given Off by Respiration / Decay / Fire Net Uptake or Loss of CO 2 by Forest (NEE) FOREST Figure 4. Representation of the components of carbon uptake and loss (left side of equation) and the resulting net uptake or loss (NEE). Additional measurements (such as biomass increment, soil respiration, leaf-level photosynthesis) must be taken to understand how the arrows on the left side of the equation change and therefore influence NEE. Page 5

6 2 7 Apr 25-8 May 25 Net Ecosystem Exchange (µmol CO 2 m -2 s -1 ) Regen-Perm (Mize) Regen-Rov Photosynthetic Photon Flux Density (µmol m -2 s -1 ) Cumulative NEE (g C m -2 ) /11/5 4/18/5 4/25/5 5/2/5 5/9/5 Date Regen-Perm (Mize) Regen-Rov1 Figure 5: Response of half-hourly NEE to PPFD (left panel) and cumulative NEE (right panel) for two regenerating slash pine plantations. (Unpublished data used only for teaching purpose, do not reproduce or cite without permission from T.A. Martin). 6 5 Annual NEP NEP (MgC ha -1 yr -1 ) Year Figure6. ACMF annual net ecosystem production. (Unpublished data used for teaching purposes only, do not reproduce or cite without permission from T.A. Martin) Page 6

7 Net Carbon Exchange (kt / year) Summed 9 km 2 Study Area NEE Timber Harvest Fire Mining Total Figure 7. Annual net carbon exchange for a 9 km 2 area of north-central Florida (Binford et al. 26). Energy closure is estimated by measuring latent (λe) and sensible (H) heat fluxes through eddy covariance and net radiation (Rn) and soil heat flux (G) in the form: R n G = λe + H Where: λ E = λw'q' H = ρc p w't ' λ is the latent heat o vaporization, ρ is air density and C p is the heat capacity of the air, w Q and w T are the covariance between vertica wind speed (w) and fluctuations of water vapor (Q) and air temperature (T). Evapotranspiration ET: ET = λe λ For more information see: Gholz & Clark 22. Agricultural and Forest Meteorology 112: Clark et al. 24. Ecological Applications 14(4): Powell et al 25. Can J. For. Res. 35: Powell et al 28. Global Change Biology 14: Bracho et al 212. Ecological Monographs. Page 7