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

TL;DR: The FLUXNET project as mentioned in this paper 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.

Abstract: 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.

Summary

1. Introduction

• Large-scale, multi-investigator projects have been the keystone of many scientific and technological advances in the twentieth century.
• Physicists have learned much about the structure of an atom's nucleus with particle accelerators.
• Astrophysicists now peer deep into space with the Hubble Space Telescope and an array of radio telescopes.
• Molecular biologists are decoding the structure of their DNA with the Human Genome project.
• The intent is to show how information from this network can aid ecologists, meteorologists, hydrologists, and biogeochemists to understand temporal and spatial variations that are associated with fluxes of carbon and water between the biosphere and atmosphere.

2. What is the problem?

• Over the past century, the states of the earth's atmosphere and biosphere have experienced much change.
• Potential consequences of elevated C0 2 concentrations include a warming of the earth's surface (Hansen et al. 1998) , melting of polar icecaps and a rising sea level, and an alteration of plant and ecosystem physiological functioning and plant composition (Amthor 1995; Norby et al. 1999) .
• A potential reduction of leaf transpiration, by stomatal closure, is compensated for in part by the entrainment of dry air from above the planetary boundary layer and elevated leaf temperatures and humidities (Jacobs and deBruin 1992) .
• Many agricultural lands have been transformed into suburban and urban landscapes, wetlands have been drained, and many tropical forests have been logged, burnt, and converted to pasture.
• Changes in land use alter the earth's radiation balance by changing its albedo, Bowen ratio (the ratio between the flux densities of sensible and latent heat exchange), leaf area index, and physiological capacity to assimilate carbon and evaporate water (Betts et al.

3. How to study biosphere C02

• Study of the earth's biogeochemistry and hydrology involves quantifying the flows of matter in and out of the atmosphere.
• Soil carbon surveys can be conducted too but, like biomass surveys, they require long intervals to resolve de-tectable differences in net carbon uptake or loss (Amundson et al. 1998) .
• Its application is generally restricted to periods when atmospheric conditions are steady and to locations with relatively flat terrain and vegetation that extends horizontally about 100 times the sampling height.
• Data from a network of eddy covariance measurement sites can also be used to improve and validate the algorithms being used by remote sensing scientists and ecosystem modelers.

4. History: What has been done?

• Micrometeorologists have been measuring C0 2 and water vapor exchange between vegetation and the atmosphere since the late 1950s and early 1960s.
• Several flux-gradient studies of C0 2 exchange were conducted over forests (Denmead 1969; Baumgartner 1969; Jarvis et al. 1976) , and one team ventured to assess the annual carbon balance of a salt marsh (Houghton and Woodwell 1980) .
• As more data were collected it became evident that the flux-gradient method was suffering from major deficiencies, when applied over tall forests.
• By the late 1980s and early 1990s, the further technological developments, such as larger data storage capacity and linear and nondrifting instruments, enabled scientists to make defensible measurements of eddy fluxes for extended periods.
• The AmeriFlux project was conceived in 1997, subsuming the several ongoing tower studies and initiating many new studies.

5. What is being done?

• The FLUXNET project serves as a mechanism for uniting the activities of several regional and continental networks into an integrated global network.
• The regional networks include AmeriFlux [which includes Large-scale Biosphere-Atmosphere Experiment (LBA) sites in Brazil], CarboEuroflux (which has subsumed Euroflux and Medeflu), AsiaFlux, and OzFlux (Australia, New Zealand).
• The global nature of FLUXNET extends the diversity of biomes, climate regions, and methods that are associated with the regional networks.
• The European networks also used a standard methodology, based on closed-path infrared spectrometers (Aubinet et al. 2000; Valentini et al. 2000) .
• FLUXNET has two operational components, a project office and a data archive office.

Methodology

• The eddy covariance method is used to assess trace gas fluxes between the biosphere and atmosphere at each site within the FLUXNET community (Aubinet et al. 2000; Valentini et al. 2000) .
• Positive flux densities represent mass and energy transfer into the atmosphere and away from the surface, and negative values denote the reverse; ecologists use an opposite sign convention where the uptake of carbon by the biosphere is positive.
• Turbulent fluctuations were computed as the difference between instantaneous and mean scalar quantities.
• When the thermal stratification of the atmosphere is stable or turbulent mixing is weak, material diffusing from leaves and the soil may not reach the reference height z r in a time that is short compared to the averaging time T, thereby violating the assumption of steady-state conditions and a constant flux layer.
• At present, routine corrections for advection may require a three-dimensional array of towers (Finnigan 1999) , rather than the application of a vertical velocity advection correction (see Lee 1998; Baldocchi et al. 2000) .

6. Instrumentation, data acquisition, and processing

• Typical instrumentation at FLUXNET field sites includes a three-dimensional sonic anemometer, to measure wind velocities and virtual temperature, and a fast responding sensor to measure C0 2 and water vapor.
• Application of the eddy covariance methods involves issues relating to site selection, instrument placement, sampling duration and frequency, calibration and postprocessing (Moore 1986; Baldocchi et al.
• In practice many of the FLUXNET sites are on undulating or gently sloping terrain, as this is where native vegetation exists.
• Agricultural scientists mount their sensors on small poles, while forest scientists use either walk-up scaffolding or low-profile radio towers.
• Some investigators place their gas transducer on the tower in a constant environment box to minimize the lag time from the sample port and the sensor.

a. Gap filling

• Most clients of eddy flux data, for example modelers, require uninterrupted time series.
• Gaps in the data record are attributed to system or sensor breakdown, periods when instruments are off-scale, when the wind is blowing through a tower, when spikes occur in the raw data, when the vertical angle of attack by the wind vector is too severe, and when data are missing because of calibration and maintenance.
• Other criteria used to reject data include applications of biological or physical constraints (lack of energy balance closure; Aubinet et al. 2000) and a meandering flux-footprint source area (e.g., Schmid 1994) .
• In addition, rejection probability for some sites is higher during nighttime because of calm wind conditions.
• Tests show that this observed level of data acceptance provides a statistically robust and oversampled estimate of the ensemble mean.

b. Accuracy assessment

• Errors associated with the application of the eddy covariance method can be random or biased (Moncrieff et al. 1996) .
• Fully systematic bias errors arise from errors due to sensor drift, limited spectral response of instruments, limited sampling duration and frequency, and calibration errors.
• This figure of merit is close to the "flux detection" level that many of us see with their field measurements.
• A cohort of carbon flux researchers suspect that they may be underevaluating the nighttime measure of CO, fluxes, despite attempts to measure the storage term (Black et al.

7. Recent findings

• As the authors write this report, 69 site-years of carbon dioxide and water vapor flux data have been published in the literature.
• Based on data in Fig. 3 , one may suspect that phenological timing of spring and autumn would be a major factor causing site variability among net carbon exchange of temperate deciduous forests.
• Weather events cause distinct periods of clear sky, overcast, and partly cloudy conditions, which alter the amount of available light to an ecosystem, how light is transmitted through a plant canopy, and the efficiency by which it is used to assimilate carbon (Gu et al. 1999) .
• Figure 9 indicates that the temperature at which peak gross C0 2 exchange occurs is several degrees lower for many European sites than for their North American counterparts.
• With regard to meteorology, FLUXNET can provide new information on how the biosphere affects the partitioning of net radiation into sensible (//) and latent heat (XE) exchange, as quantified by the Bo wen ratio.

Bulletin of the American Meteorological Society

• Productivity is assumed to diminish with age, and disturbances affect the relative ratio between canopy carbon uptake and soil respiration, and the ratio between canopy transpiration and soil evaporation.
• An attribute of a global network, such as FLUXNET, is the occurrence of sites with similar species and functionality but different ages.
• The young stand experiences greatest rates of uptake during the summer, and peak rates of the young stand can exceed peak rates of the old stand in the summer by 14% (-140 g C m~2 month -1 near month 30 vs -120 near month 18).
• Simulation modeling, which includes the effects of tree age, suggested that the milder winters and ample annual rainfall in the young stand allow it to have a higher leaf area than the old forest, allowing more carbon uptake through the year (Law et al. 2001, manuscript submitted to Global Change Biol.) .
• The index of plant physiological capacity is a function of the maximum carboxylation velocity of photosynthesis , leaf area index (LAI), and the fraction of absorbed visible sunlight (/par).

8. Conclusions

• A global network of long-term measurement sites has been established and is producing new information on how C0 2 and water vapor between the terrestrial biosphere and atmosphere vary across a broad spectrum of biomes, climates, and timescales.
• Gross primary productivity of forest ecosystems may not be constant, but may depend on plant architecture (e.g., on leaf area), foliage photosynthetic capacity, and the amount of sunlight absorbed.
• Concerning conifer forests, the conclusions are not as unifying.
• Seasonal and annual sums of net carbon exchange by boreal, semiarid, temperate, and humid conifers differ among one another and for different physiological reasons.
• The response of canopy-scale C0 2 exchange to sunlight varies with cloud cover as clouds alter the direction of incoming sunlight and how it penetrates into a canopy.

Full text

University of Nebraska - Lincoln University of Nebraska - Lincoln
DigitalCommons@University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln
Papers in Natural Resources Natural Resources, School of
2001
FLUXNET: A New Tool to Study the Temporal and Spatial FLUXNET: A New Tool to Study the Temporal and Spatial
Variability of Ecosystem-Scale Carbon Dioxide, Water Vapor, and Variability of Ecosystem-Scale Carbon Dioxide, Water Vapor, and
Energy Flux Densities Energy Flux Densities
Dennis Baldocchi
University of California, Berkeley
Eva Falge
Universitat Bayreuth, Bayreuth
Lianhong Gu
University of California, Berkeley
Richard Olson
Oak Ridge National Laboratory
David Hollinger
USDA Forest Service, Durham
See next page for additional authors
Follow this and additional works at: https://digitalcommons.unl.edu/natrespapers
Part of the Natural Resources and Conservation Commons, Natural Resources Management and
Policy Commons, and the Other Environmental Sciences Commons
Baldocchi, Dennis; Falge, Eva; Gu, Lianhong; Olson, Richard; Hollinger, David; Running, Steve; Anthoni,
Peter; Bernhofer, Ch.; Davis, Kenneth; Evans, Robert; Fuente, Jose; Goldstein, Allen; Katul, Gabriel; Law,
Beverly; Lee, Xuhui; Malhi, Yadvinder; Meyers, Tilden; Munge, William; Oechel, Walt; U, K.T. Paw; Pilegaard,
Kim; Schmid, H.P.; Valentini, Riccardo; Verma, Shashi; Vesala, Timo; Wilson, Kell; and Wofsy, Steve,
"FLUXNET: A New Tool to Study the Temporal and Spatial Variability of Ecosystem-Scale Carbon Dioxide,
Water Vapor, and Energy Flux Densities" (2001).
Papers in Natural Resources
. 1163.
https://digitalcommons.unl.edu/natrespapers/1163
This Article is brought to you for free and open access by the Natural Resources, School of at
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Authors Authors
Dennis Baldocchi, Eva Falge, Lianhong Gu, Richard Olson, David Hollinger, Steve Running, Peter Anthoni,
Ch. Bernhofer, Kenneth Davis, Robert Evans, Jose Fuente, Allen Goldstein, Gabriel Katul, Beverly Law,
Xuhui Lee, Yadvinder Malhi, Tilden Meyers, William Munge, Walt Oechel, K.T. Paw U, Kim Pilegaard, H.P.
Schmid, Riccardo Valentini, Shashi Verma, Timo Vesala, Kell Wilson, and Steve Wofsy
This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/
natrespapers/1163

FLUXNET: A
New
Tool to Study
the Temporal and Spatial
Variability
of Ecosystem-Scale
Carbon
Dioxide,
Water
Vapor, and
Energy
Flux
Densities
Dennis Baldocchi,
3
Eva Falge,
b
Lianhong
Gu,
a
Richard Olson,
c
David Hollinger,
d
Steve Running/
Peter Anthoni/
Ch.
Bernhofer,§ Kenneth Davis,
h
Robert Evans,
d
Jose
Fuentes/ Allen Goldstein,
3
Gabriel Katul,J Beverly Law/ Xuhui Lee,
k
Yadvinder
Malhi,
1
Tilden
Meyers,"
1
William Munger,"
Walt Oechel,
0
K. T.
Paw
U,P
Kim Pilegaard/
H. P.
Schmid/ Riccardo Valentini,
5
Shashi Verma/ Timo Vesala,
u
Kell Wilson,"
1
and Steve Wofsy"
ABSTRACT
FLUXNET is a global network of micrometeorological flux measurement sites that measure the exchanges of car-
bon 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 con-
tinents 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 infor-
mation 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 ex-
change models. Findings so far include 1) net C0
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 C0
2
exchange to sunlight doubles if the sky is cloudy rather than clear; 3) the spectrum of C0
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 C0
2
exchange varies with mean summer temperature; and 5) stand age affects carbon dioxide and water vapor
flux densities.
a
ESPM, University of California, Berkeley, Berkeley, California.
h
Pflanzenokologie, Universitat Bayreuth, Bayreuth, Germany.
"Environmental Science Division, Oak Ridge National Laboratory, Oak
Ridge, Tennessee.
d
USDA Forest Service, Durham, New Hampshire.
"School of Forestry, University of Montana, Missoula, Montana.
'Richardson Hall, Oregon State University, Corvallis, Oregon.
g
Technische Universitat Dresden, IHM Meteorologie, Tharandt, Germany.
•"Department of Meteorology, The Pennsylvania State University, Univer-
sity Park, Pennsylvania.
'Department of Environmental Science, University of Virginia,
Charlottesville, Virginia.
'School of the Environment, Duke University, Durham, North Carolina.
k
School of Forestry, Yale University, New Haven, Connecticut.
'Institute of Ecology and Resource Management, University of Edinburgh,
Edinburgh, United Kingdom.
m
NOAA/Atrnospheric Turbulence and Diffusion Division, Oak Ridge,
Tennessee.
"Department of Earth and Planetary Sciences, Harvard University, Cam-
bridge, Massachusetts.
"Department of Biology, San Diego State University, San Diego, California.
p
Land, Air and Water Resources, University of California, Davis, Davis,
California.
q
Plant Biology and Biogeochemistry Department, Risoe National Labo-
ratory, Roskilde, Denmark.
"Department of Geography, Indiana University, Bloomington, Indiana.
s
DISAFRI, Universita de Tuscia, Viterbo, Italy.
'School of Natural Resource Sciences, University of Nebraska at Lincoln,
Lincoln, Nebraska.
"Department of Physics, University of Helsinki, Helsinki, Finland.
Corresponding author address: Dennis Baldocchi, Department of Environ-
mental Science, Policy and Management, Ecosystems Science Division, 151
Hilgard Hall, University of California, Berkeley, Berkeley, CA 94720.
E-mail: baldocchi@nature.berkeley.edu
In final form 27 February 2001.
©2001 American Meteorological Society
Bulletin of the American Meteorological Society
2357

1. Introduction
Large-scale, multi-investigator projects have been
the keystone of many scientific and technological ad-
vances in the twentieth century. Physicists have
learned much about the structure of an atom's nucleus
with particle accelerators. Astrophysicists now peer
deep into space with the Hubble Space Telescope and
an array of radio telescopes. Molecular biologists are
decoding the structure of our DNA with the Human
Genome project.
Ecosystem scientists need a tool that assesses the
flows of carbon, water, and energy to and from the
terrestrial biosphere across the spectrum of time- and
space scales over which the biosphere operates (see
Running et al. 1999; Canadell et al. 2000). Similarly,
atmospheric scientists need a tool to quantify surface
energy fluxes at the land-atmosphere interface, as
these energy fluxes influence weather and climate
(Betts et al. 1996; Pielke et al. 1998). In this paper, we
report on such a tool. It consists of a global array of
micrometeorological towers that are measuring flux
densities of carbon dioxide, water vapor, and energy
between vegetation and the atmosphere on a quasi-
continuous and long-term (multiyear) basis. The ob-
jectives of this paper are to introduce the FLUXNET
project and describe its rationale, goals, measurement
methods, and geographic distribution. We also present
a sampling of new results that are being generated
through the project. The intent is to show how infor-
mation from this network can aid ecologists, meteo-
rologists, hydrologists, and biogeochemists to
understand temporal and spatial variations that are as-
sociated with fluxes of carbon and water between the
biosphere and atmosphere.
2. What is the problem?
Over the past century, the states of the earth's at-
mosphere and biosphere have experienced much
change. Since the dawn of the industrial revolution, the
mean global C0
2
concentration has risen from about
280 ppm to over 368 ppm (Keeling and Whorf 1994;
Conway et al. 1994). The secular rise in atmospheric
carbon dioxide concentrations is occurring due to im-
balances between the rates that anthropogenic and
natural sources emit C0
2
and the rate that biospheric
and oceanic sinks remove C0
2
from the atmosphere.
Superimposed on the secular trend of C0
2
is a record
of large interannual variability in the annual rate of
growth of atmospheric C0
2
. Typical values of inter-
annual variability are on the order of 0.5-3.0 ppm yr
1
.
On a mass basis, these values correspond with a range
between 1 and 5 gigatons C yr
-1
. Potential sources of
year-to-year changes in atmospheric C0
2
remain a
topic of debate. Studies of atmospheric
13
C and 0
2
dis-
tributions imply that the terrestrial biosphere plays an
important role in this interannual variability (Ciais
et al.
1995 ;
Keeling et al. 1996). Sources of this vari-
ability have been attributed to El Nino/La Nina events,
which cause regions of droughts or superabundant
rainfall (Conway et al. 1994; Keeling et al. 1995), and
alterations in the timing and length of the growing
season (Mynenietal. 1997a,b; Randersonetal. 1997).
Rising levels of C0
2
, and other greenhouse gases,
are of concern to scientists and policy makers because
they trap infrared radiation that is emitted by the
earth's surface. Potential consequences of elevated
C0
2
concentrations include a warming of the earth's
surface (Hansen et al. 1998), melting of polar icecaps
and a rising sea level, and an alteration of plant and
ecosystem physiological functioning and plant com-
position (Amthor 1995; Norby et al. 1999).
With regard to plants and ecosystems, short-term
experiments with elevated C0
2
show increased rates
of photosynthesis and plant growth and lowered sto-
matal conductance (Drake et al. 1996). The long-term
sustainability of enhanced rates of growth by plants,
however, depends on nutrient and water availability,
temperature, and light competition (Ceulemans and
Mousseau 1994; Norby et al. 1999). Warmer tempera-
tures, associated with elevated C0
2
, promote increased
rates of respiration when soil moisture is ample. But
if climate warming is associated with drying, there can
be reduced assimilation and lower rates of soil/root
respiration in temperate ecosystems. In the tundra and
boreal forest, warming and drying can lower water
tables, exposing organic peat to air and increasing its
rate of respiration (Goulden et al. 1998; Lindroth et al.
1998; Oechel et al. 2000).
How evaporation is impacted by elevated C0
2
de-
pends on the scale of study and concurrent changes in
leaf area index, canopy surface conductance, and the
depth of the planetary boundary layer. Theory indi-
cates that complex feedbacks among stomatal conduc-
tance, leaf temperature, and the air's vapor pressure
deficit cause responses at the leaf scale to differ from
that of the canopy scale. For instance, a potential re-
duction of leaf transpiration, by stomatal closure, is
compensated for in part by the entrainment of dry air
from above the planetary boundary layer and elevated
2382
Vol. 82,, No. 7
7,
November 2001

leaf temperatures and humidities (Jacobs and deBruin
1992). Hence, canopy evaporation may not be reduced
to the same degree as leaf transpiration in an elevated
C0
2
world.
In the meantime, the composition of the land sur-
face has changed dramatically to meet the needs of the
growing human population. Many agricultural lands
have been transformed into suburban and urban land-
scapes, wetlands have been drained, and many tropi-
cal forests have been logged, burnt, and converted to
pasture. In contrast, abandoned farmland in the north-
east United States and Europe is returning to forest
as those societies become more urban. Changes in land
use alter the earth's radiation balance by changing its
albedo, Bowen ratio (the ratio between the flux den-
sities of sensible and latent heat exchange), leaf area
index, and physiological capacity to assimilate carbon
and evaporate water (Betts et al. 1996; Pielke et al.
1998). Changing a landscape from forest to agricul-
tural crops, for instance, increases the surface's albedo
and decreases the Bowen ratio (Betts et al. 1996);
forests have a lower physiological capacity to assimi-
late carbon and a lower ability to transpire water, as
compared to crops (Kelliher et al. 1995; Baldocchi and
Meyers 1998). A change in the age structure of for-
ests due to direct (deforestation) or indirect (climate-
induced fires) disturbance alters its ability to acquire
carbon and transpire water (Amiro et al. 1999; Schulze
et al. 1999).
The issues identified above all require information
on fluxes of carbon, water, and energy at the earth's
surface and how these fluxes interact with the physi-
cal climate and physiological functioning of plants and
ecosystems. In the next section we identify numerous
ways to obtain this information and illustrate how a
network of long flux measurement sites can provide
particularly useful information to study these complex
problems.
3. How to study biosphere C0
2
exchange?
Study of the earth's biogeochemistry and hydrol-
ogy involves quantifying the flows of matter in and
out of the atmosphere. Numerous techniques exist for
studying biosphere-atmosphere C0
2
exchange, each
with distinct advantages and disadvantages. At the
continental and global scales, scientists assess carbon
dioxide sources and sinks using atmospheric inversion
models that ingest information on fields of C0
2
,
13
C,
and 0
2
concentration and wind (Tans et al. 1990; Ciais
et al. 1995; Denning et al. 1996; Fan et al. 1998). This
approach is subject to errors due to the sparseness of
the trace gas measurement network, their biased place-
ment in the marine boundary layer, and the accuracy
of the atmospheric transport models (Tans et al. 1990;
Denning et al. 1996; Fan et al. 1998).
Instruments mounted on satellite platforms view
the earth in total. Consequently, satellite-based instru-
ments offer the potential to evaluate surface carbon
fluxes on the basis of algorithms that can be driven by
reflected and emitted radiation measurements
(Running et al. 1999; Cramer et al. 1999). This ap-
proach, however, is inferential, so it is dependent on
the accuracy of the model algorithms, the frequency
of satellite images, and the spectral information con-
tained in the images.
At the landscape to regional scale we can use in-
struments mounted on aircraft (Crawford et al. 1996;
Desjardins et al. 1984, 1997) or boundary layer bud-
get methods (Denmead et al. 1996; Levy et al. 1999;
Yi et al. 2000) to assess carbon and water fluxes.
Aircraft-based eddy covariance flux density measure-
ments give good information on spatial patterns of
carbon and water fluxes across transects to tens to
hundreds of kilometers. However, they do not provide
information that is continuous in time, nor do they
provide insights on the physiological mechanisms that
govern carbon and water fluxes. Boundary layer bud-
get methods can be implemented using tall towers,
tethered balloons, or small aircraft. This method pro-
vides information on spatially integrated fluxes of tens
to hundreds of square kilometers, but it can be applied
only during ideal meteorological conditions (Denmead
et al. 1996). Routine application of this method is ham-
pered by a lack of data on entrainment and horizontal
advection.
One measure of carbon flux "ground truth" can be
provided by biomass surveys (Kauppi et al. 1992;
Gower et al. 1999). However, biomass surveys pro-
vide information on multiyear to decadal timescales,
so they do not provide information on shorter-term
physiological forcings and mechanisms. Furthermore,
forest inventory studies are labor intensive and are
inferential estimates of net carbon exchange. Such
studies rarely measure growth of small trees and below-
ground allocation of carbon. Instead biomass surveys
commonly assume that a certain portion of carbon is
allocated below ground (Gower et al. 1999). Soil car-
bon surveys can be conducted too but, like biomass
surveys, they require long intervals to resolve de-
Bulletin of the American Meteorological Society
2357