Kell B. Wilson

FLUXNET: A New Tool to Study the Temporal and Spatial Variability of Ecosystem-Scale Carbon Dioxide, Water Vapor, and Energy Flux Densities

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 CO 2 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 CO 2 exchange to sunlight doubles if the sky is cloudy rather than clear; 3) the spectrum of CO 2 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 CO 2 exchange varies with mean summer temperature; and 5) stand age affects carbon dioxide and water vapor flux densities.

Energy balance closure at FLUXNET sites

A comprehensive evaluation of energy balance closure is performed across 22 sites and 50 site-years in FLUXNET, a network of eddy covariance sites measuring long-term carbon and energy fluxes in contrasting ecosystems and climates. Energy balance closure was evaluated by statistical regression of turbulent energy fluxes (sensible and latent heat (LE)) against available energy (net radiation, less the energy stored) and by solving for the energy balance ratio, the ratio of turbulent energy fluxes to available energy. These methods indicate a general lack of closure at most sites, with a mean imbalance in the order of 20%. The imbalance was prevalent in all measured vegetation types and in climates ranging from Mediterranean to temperate and arctic. There were no clear differences between sites using open and closed path infrared gas analyzers. At a majority of sites closure improved with turbulent intensity (friction velocity), but lack of total closure was still prevalent under most conditions. The imbalance was greatest during nocturnal periods. The results suggest that estimates of the scalar turbulent fluxes of sensible and LE are underestimated and/or that available energy is overestimated. The implications on interpreting long-term CO2 fluxes at FLUXNET sites depends on whether the imbalance results primarily from general errors associated

Seasonal and interannual variability of energy fluxes over a broadleaved temperate deciduous forest in North America

The components of the surface energy balance were measured for 3 years over a broadleaved deciduous forest using the eddy covariance technique. Within years, the magnitude and distribution of fluxes was controlled by seasonal changes in solar radiation, drought, as well as leaf emergence and senescence. Evapotranspiration increased by a factor greater than five (from about 0.5 to 3 mm day 1 ) after leaves emerged in spring. Large decreases in sensible heat flux were observed over the same period (6 to 2 MJ day 1 ) despite increases in solar radiation. The most influential effect on annual fluxes was the occurrence and extent of drought, with lesser control exerted by differences in the timing of leaf expansion and leaf senescence. Average annual evapotranspiration over the period was 567 mm and ranged from 537 to 611 mm. The year with the lowest precipitation, soil moisture content and surface conductance also had the lowest evapotranspiration. Although evapotranspiration was quite sensitive to surface conductance and surface conductance was reduced substantially by drought, the correlation of low surface conductance and high humidity deficit reduced the effects of drought on evapotranspiration. Differences in net radiation among years were only a minor source of variability in evapotranspiration. In addition to surface conductance, other bulk parameters are calculated to describe the general exchange characteristics of this forest. ©2000 Elsevier Science B.V. All rights reserved.

Biometric and eddy-covariance based estimates of annual carbon storage in five eastern North American deciduous forests

Quantifying net carbon (C) storage by forests is a necessary step in the validation of carbon sequestration estimates and in assessing the possible role of these ecosystems in offsetting fossil fuel emissions. In eastern North America, five sites were established in deciduous forests to provide measurements of net ecosystem CO2 exchange (NEE) using micro-meteorological methods, and measures of major carbon pools and fluxes, using a combination of forest mensurational, eco-physiological, and other biometric methods. The five study sites, part of the AmeriFlux network, ranged across 10 ◦ of latitude and 18 ◦ of longitude, but were all of similar age, canopy height, and stand basal area. Here we present a cross-site synthesis of annual carbon storage estimates, comparing meteorological and biometric approaches, and also comparing biometric estimates based on analyses of autotrophic carbon pools and heterotrophic carbon fluxes (net ecosystem production, NEP) versus those based on measurements of change in two major carbon pools (� C). Annual above-ground net primary production (ANPP) varied nearly two-fold among sites and was strongly correlated with average annual temperature and with annual soil nitrogen mineralization (Nmin). Estimates of NEP ranged from 0.7 Mg C per hectare per year in northern lower Michigan to 3.5 Mg C per hectare per year in central Indiana, and were also well correlated with Nmin. There was less variation among sites in estimates

Correction of eddy-covariance measurements incorporating both advective effects and density fluxes.

Equations are presented to correct eddy-covariancemeasurements for both fluctuations in density andnon-zero mean advection, induced by convergence ordivergence of flow, and spatial source/sinkinhomogeneity, under steady-state and transientconditions. This correction collapses to theWebb–Pearman–Leuning expression ifthe mean vertical velocity is zero, and formally addsthe Webb–Pearman–Leuning expression to the correctionssuggested by Lee for conditions ofnon-zero vertical velocity and source/sink and meanscalar horizontal homogeneity. The equation requiresmeasurement of the mean vertical gradients of thescalar concentration of interest (air temperature,humidity, CO2) as well as an accurateestimation of the mean vertical velocity, in additionto the vertical eddy covariance of the scalar. Simplemethods for the approximation of sensor tilt andcomplex terrain flow angle are presented, to allowestimation of non-zero mean vertical velocities. Theequations are applied to data from a maize crop and aforest to give examples of when the correction issignificant. In addition, a term for thethermodynamic expansion energy associated with watervapour flux is derived, which implies that the sonictemperature derived sensible heat flux will accuratelyinclude this contribution.

Modeling CO2 and water vapor exchange of a temperate broadleaved forest across hourly to decadal time scales

Fluxes of carbon dioxide, water and energy between a temperate deciduous forest and the atmosphere were quantified across time scales of hours, days, seasons, years and decades. This exercise was performed using stand-level eddy covariance flux measurements and a biophysical model, CANOAK. The CANOAK model was tested with measurements of carbon dioxide, water vapor and energy flux densities we have been collecting since October 1994. Model calculations reproduced 80% of CO2 and water vapor flux variance that are contained in a year-long time series, when the model was forced with hourly weather data and seasonal information on plant structure and physiological capacity. Spectral analysis of measured and computed time series revealed that peak time scales of flux variance have periods of a day, half-week, season and year. We examined questions relating to inter-annual variability of mass and energy exchange by forcing the validated model with a decade-long meteorological record. Theoretical estimates of year-to-year variability of net ecosystem CO2 exchange were on the order of ±200 gC m−2 year. We also deduced that significant variance of water vapor and CO2 exchange occurs on the time scale of 5–6 years, the time scale associated with El Nino phenomena. Sensitivity tests performed with the model examined issues associated with model complex and parameterization issues. Of particular importance were the effects of leaf clumping and length of the growing season on canopy photosynthesis and net ecosystem CO2 exchange. Ignoring the effects of leaf clumping caused an error as large as 50% in the estimation of annual biosphere–atmosphere net carbon exchange. Each incremental day change in the length of the growing season altered the net ecosystem CO2 exchange by 5.9 gC m−2.

On measuring net ecosystem carbon exchange over tall vegetation on complex terrain.

To assess annual budgets of CO2 exchange betweenthe biosphere and atmosphere over representativeecosystems, long-term measurements must be made overecosystems that do not exist on ideal terrain. How tointerpret eddy covariance measurements correctlyremains a major task. At present, net ecosystemCO2 exchange is assessed, by members of themicrometeorological community, as the sum of eddycovariance measurements and the storage of CO2 inthe underlying air. This approach, however, seemsunsatisfactory as numerous investigators are reportingthat it may be causing nocturnal respiration fluxdensities to be underestimated.A new theory was recently published by Lee (1998, Agricultural and Forest Meteorology91: 39–50) for assessing net ecosystem-atmosphere CO2 exchange(Ne) over non-ideal terrain. Itincludes a vertical advection term. We apply thisequation over a temperate broadleaved forest growingin undulating terrain. Inclusion of the verticaladvection term yields hourly, daily and annual sums ofnet ecosystem CO2 exchange that are moreecologically correct during the growing season.During the winter dormant period, on the other hand,corrected CO2 flux density measurements of anactively respiring forest were near zero. Thisobservation is unrealistic compared to chambermeasurements and model calculations. Only duringmidday, when the atmosphere is well-mixed, domeasurements of Ne match estimatesbased on model calculations and chamber measurements. On an annual basis, sums of Newithout the advection correction were 40% too large,as compared with computations derived from a validatedand process-based model. With the inclusion of theadvection correction term, we observe convergencebetween measured and calculated values ofNe on hourly, daily and yearly time scales. We cannot, however, conclude that inclusion of aone-dimensional, vertical advection term into thecontinuity equation is sufficient for evaluatingCO2 exchange over tall forests in complexterrain. There is an indication that the neglected term,ū(∂ c¯/∂ x), isnon-zero and that CO2 may be leakingfrom the sides of the control volume during the winter. In this circumstance, forest floor CO2 effluxdensities exceed effluxes measured above the canopy.

Oak forest carbon and water simulations: model intercomparisons and evaluations against independent data

Models represent our primary method for integration of small-scale, process- level phenomena into a comprehensive description of forest-stand or ecosystem function. They also represent a key method for testing hypotheses about the response of forest ecosystems to multiple changing environmental conditions. This paper describes the eval- uation of 13 stand-level models varying in their spatial, mechanistic, and temporal com- plexity for their ability to capture intra- and interannual components of the water and carbon cycle for an upland, oak-dominated forest of eastern Tennessee. Comparisons between model simulations and observations were conducted for hourly, daily, and annual time steps. Data for the comparisons were obtained from a wide range of methods including: eddy covariance, sapflow, chamber-based soil respiration, biometric estimates of stand-level net primary production and growth, and soil water content by time or frequency domain reflectometry. Response surfaces of carbon and water flux as a function of environmental drivers, and a variety of goodness-of-fit statistics (bias, absolute bias, and model efficiency) were used to judge model performance. A single model did not consistently perform the best at all time steps or for all variables considered. Intermodel comparisons showed good agreement for water cycle fluxes, but considerable disagreement among models for predicted carbon fluxes. The mean of all model outputs, however, was nearly always the best fit to the observations. Not surprisingly, models missing key forest components or processes, such as roots or modeled soil water content, were unable to provide accurate predictions of ecosystem responses to short-term drought phenomenon. Nevertheless, an inability to correctly capture short-term physiolog- ical processes under drought was not necessarily an indicator of poor annual water and carbon budget simulations. This is possible because droughts in the subject ecosystem were of short duration and therefore had a small cumulative impact. Models using hourly time steps and detailed mechanistic processes, and having a realistic spatial representation of the forest ecosystem provided the best predictions of observed data. Predictive ability of all models deteriorated under drought conditions, suggesting that further work is needed to evaluate and improve ecosystem model performance under unusual conditions, such as drought, that are a common focus of environmental change discussions.

Energy Partitioning Between Latent And Sensible Heat Flux During The Warm Season At Fluxnet Sites

The warm season (mid-June through late August) partitioning between sensible (H) and latent (LE) heat flux, or the Bowen ratio (beta=H/LE), was investigated at 27 sites over 66 site years within the international network of eddy covariance sites (FLUXNET). Variability in beta across ecosystems and climates was analyzed by quantifying general climatic and surface characteristics that control flux partitioning. The climatic control on beta was quantified using the climatological resistance (R-i), which is proportional to the ratio of vapor pressure deficit (difference between saturation vapor pressure and atmospheric vapor pressure) to net radiation (large values of R-i decrease beta). The control of flux partitioning by the vegetation and underlying surface was quantified by computing the surface resistance to water vapor transport (R-c, with large values tending to increase beta). There was a considerable range in flux partitioning characteristics (R-c, R-i and beta) among sites, but it was possible to define some general differences between vegetation types and climates. Deciduous forest sites and the agricultural site had the lowest values of R-c and beta (0.25-0.50). Coniferous forests typically had a larger R-c and higher beta (typically between 0.50 and 1.00 but also much larger). However, there was notable variability in R-c and R-i between coniferous site years, most notably differences between oceanic and continental climates and sites with a distinct dry summer season (Mediterranean climate). Sites with Mediterranean climates generally had the highest net radiation, R-c, R-i, and beta. There was considerable variability in beta between grassland site years, primarily the result of interannual differences in soil water content and R-c

A spectral analysis of biosphere-atmosphere trace gas flux densities and meteorological variables across hour to multi-year time scales

The advent of long-term studies on CO2 and water vapor exchange provides us with new information on how the atmosphere and biosphere interact. Conventional time series analysis suggests that temporal fluctuations of weather variables and mass and energy flux densities occur on numerous time scales. The time scales of variance that are associated with annual time series of meteorological variables, scalar flux densities and their covariance with one another, however, remain unquantified. We applied Fourier analysis to time series (4 years in duration) of photon flux density, air temperature, wind speed, pressure and the flux densities of CO 2 and water vapor. At the daily time scale, strong spectral peaks occurred in the meteorological and flux density records at periods of 12 and 24 h, due to the daily rising and setting of the sun. At the synoptic time scale (3‐7 days) the periodic passage of weather fronts alter available sunlight and temperature. In turn, variations in these state variables affect carbon assimilation, respiration and transpiration. At the seasonal and semi-annual time scales, a broad spectral peak occurs due to seasonal changes in weather and plant functionality and phenology. In general, 21% of the variance of CO2 exchange is associated with the annual cycle, 43% of the variance is associated with the diurnal cycle and 9% is associated with the semi-annual time scale. A pronounced spectral gap was associated with periods 3‐4 weeks long. Interactions between CO2 flux density ( Fc) and sunlight, air temperature and latent heat flux density were quantified using co-spectral, coherence and phase angle analyzes. Coherent and in-phase spectral peaks occur between CO2 exchange rates and water vapor exchange on annual, seasonal and daily time scales. A 180 phase shift occurs between Fc and photon flux density (Qp) on seasonal and daily time scales because the temporal course of sunlight corresponds with the withdrawal of CO2 from the atmosphere, a flux that possesses a negative sign. Covariations between Fc and Tair experience a 180 phase shift with one another at the seasonal time scale because rising temperatures are associated with more carbon uptake. At daily time scales the phase angle between Fc and Tair is on the order of 130. This phase lag can be explained by the strong dependence of canopy photosynthesis on available light and the 2‐3 h lags, which occur between the daily course of sunlight and air temperature.