1. science
2. chemistry

Proposal Cover Sheet
NASA Research Announcement, NRA-00-OES-08
Research Area: Carbon Cycle Science
Title: FLUXNET, A Global Network of Carbon, Water and Energy Flux Measurement
Sites: A Continuation Proposal for the Project Science Office
Principal Investigator: Dennis Baldocchi,
Department: Environmental Science, Policy and Management
Institution: University of California, Berkeley
Address: 151 Hilgard Hall
City: Berkeley
State: CA
Zip: 94720-3110
Telephone: 510-642-2874
Fax: 510-642-5098
Email: [email protected].
Collaborating Researchers:
Name
Richard J. Olson
Steve Running
David Hollinger
(AmeriFlux)
Riccardo Valentini
(CARBOEURO
FLUX)
Susumu Yamamoto
(AsiaFlux)
Ray Leuning
(OzFlux)
Institution & E-mail
Address
Oak Ridge National
Laboratory
[email protected]
University of Montana
[email protected]
USDA-Forest Service
[email protected]
Univerisity of Tuscia
[email protected]
National Institute for
Environmental Studies
[email protected]
CSIRO Land and Water
[email protected].
au
Address & Telephone
Oak Ridge National Laboratory
P.O. Box 2008, Mail Stop 6407
Oak Ridge, TN 37830-6407,865-574-7819
School of Forestry University of Montana,
Missoula, MT 59812, 406-243-6311
USDA Forest Service,271 Mast Rd.
Durham, NH 03824, 603-868-7673
University of Tuscia
Department of Forest Science and
Environment, 01100 Viterbo. ITALY, +390761-357394
National Institute for Resources and
Environment, Ibaraki, Japan+81-298-618360
CSIRO Land and Water
PO Box 1666 Canberra ACT 2601
AUSTRALIA, +61 2 6246 5557
ABSTRACT
FLUXNET is a global network of long-term micrometeorological flux measurement sites
at which the exchanges of carbon dioxide, water vapor and energy between the biosphere
and atmosphere are measured. Through FLUXNET, the data streams from various
regional flux measurement networks (AmeriFlux, CarboEurope, AsiaFlux, and OzFlux)
are combined and a mechanism for synthesizing data across sites and biomes is realized.
The goals of FLUXNET include quantifying temporal dynamics, spatial patterns and
biotic and abiotic forcings of carbon dioxide, water vapor and energy fluxes and
providing data for the validation of remote sensing products that are inferring fluxes of
carbon dioxide. The FLUXNET project is supported under NASA’s EOS Validation
Program, until December, 2001. We are soliciting funds to renew FLUXNET and to
update and expand upon its goals.
At present over 140 flux measurement sites are registered and operating on a continuous
basis. Vegetation under study includes temperate conifer and broadleaved forests,
tropical and boreal forests, crops, wetlands, grasslands, chaparral, and tundra. Research
sites exist on five continents and their latitudinal distribution ranges from 70 degrees
north to 30 degrees south. Over thirty years of data have been archived.
FLUXNET serves several primary functions that relate to the objectives of the
solicitation for Carbon Cycle Science. With regards to the ‘improvement and evaluation
of carbon cycling models’, integrated datasets produced by the FLUXNET project
provide ground truth for algorithms that are using information generated by instruments
mounted on satellites, e.g. TERRA, to generate regional and global estimates of carbon
dioxide uptake by ecosystems. We will also use FLUXNET data to evaluate response of
stand-scale carbon dioxide and water vapor flux densities to controlling biotic (plant
functional type, leaf area, photosynthetic capacity) and abiotic (light, temperature, soil
moisture) factors across a spectrum of time scales. Regarding the need to ‘advance our
ability to estimate global and regional primary productivity’, we intend to use
FLUXNET-derived data to quantifying the seasonal and inter-annual variability of
ecosystem-scale carbon balances across an array of field sites and ecosystem functional
types. We will also use data to test the performance of biophysical models that predict
carbon and water fluxes over a spectrum of time scales.
To meet our objectives, we sponsor a postdoctoral research associate, organize regular
electronic communications, convene workshops, and provide travel support for visiting
scientists to attend workshops and conduct analyzes at the FLUXNET project office.
Data and site information are available from the FLUXNET website, http://wwweosdis.ornl.gov/FLUXNET. The FLUXNET website is maintained by the FLUXNET
Data Information System (DIS) at Oak Ridge National Lab. Funding for this activity is
being solicited under a companion proposal headed by Richard Olson. The FLUXNET
project office provides scientific guidance to FLUXNET DIS to ensure data flow, quality
assurance and control. To ensure intra-network comparability, FLUXNET supports the
circulation of a reference set of flux measurement instrumentation to sites across the
component sub-networks.
TECHNICAL PLAN
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 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). This tool
exists in the form of a global array of micrometeorological towers, called FLUXNET, at
which the exchanges of carbon dioxide, water vapor and energy between the biosphere
and atmosphere are being measured on a quasi-continuous and long-term (multi-year)
basis. The FLUXNET project is currently being supported under NASA’s EOS
Validation Program, until December 2001. We are writing this proposal to solicit
funding to renew FLUXNET and to expand and extend its goals.
FLUXNET serves several unique functions. It provides the framework and mechanism
for uniting the activities and databases of several regional and continental carbon flux
measurement networks, e.g. AmeriFlux, CarboEurope (formerly EuroFlux), AsiaFlux,
OzFlux and CanadaNet, into an integrated global network. The global nature of
FLUXNET extends and expands the number of climates and biomes that are associated
with the regional networks, as well as methods. At present over 140 micrometeorological
sites are measuring ecosystem scale carbon dioxide flux densities on a nearly continuous
basis. Vegetation under study includes temperate conifer and broadleaved forests,
tropical and boreal forests, crops, wetlands, grasslands, chaparral, and tundra. Research
sites exist on five continents and their latitudinal distribution ranges from 70 degrees
north to 30 degrees south.
2. Statement of Research
This proposal, to fund the FLUXNET Project Office, addresses several of the objectives
cited in the solicitation for Carbon Cycle Science. With regards to the ‘improvement and
evaluation of carbon cycling models’, data derived from the FLUXNET project provide
ground truth for algorithms that are using information generated by instruments mounted
on satellites, e.g. MODIS, to generate regional and global estimates of carbon dioxide
uptake by ecosystems. We intend to use FLUXNET data to evaluate the responses of
stand-scale carbon dioxide and water vapor flux densities to controlling biotic (plant
functional type, leaf area, photosynthetic capacity) and abiotic (light, temperature, soil
moisture) factors across a spectrum of time scales. We also intend to use FLUXNET data
to test the performance of biophysical carbon/water flux models across a spectrum of
time scales and use such models to develop algorithms that can be forced with data taken
from sensors on satellite platforms. Regarding the need to ‘advance our ability to
estimate global and regional primary productivity’, we intend to use FLUXNET-derived
data to quantifying the seasonal and inter-annual variability of ecosystem-scale carbon
balances across an array of field sites and ecosystem functional types.
3. Background
Over the past century, the states of the Earth’s atmosphere and biosphere have
experienced much change. Since the dawn of the industrial revolution, the mean global
CO2 concentration has risen from about 280 ppm to over 368 ppm ((Keeling and Whorf,
1996). The secular rise in atmospheric carbon dioxide concentrations is occurring due to
imbalances between the rates that anthropogenic and natural sources emit CO2 and the
rate that biospheric and oceanic sinks remove CO2 out of the atmosphere.
Superimposed on the secular trend of CO2 is a record of large inter-annual variability in
the annual rate of growth of atmospheric CO2. Typical values are on the order of 0.5 to 3
ppm yr-1. On a mass basis, these values correspond with a range between 1 and 5 Gt C
yr-1. Potential sources of year-to-year changes in CO2 remain a topic of debate. Sources
of this variability have been attributed to El Nino/La Nina events, which cause regions of
droughts or superabundant rainfall (Keeling et al., 1995), and alterations in the timing and
length of the growing season (Myneni et al, 1997; Randerson et al., 1997).
In the meantime, the composition of the land surface has changed dramatically to meet
the needs of the growing human population. Many agricultural lands have transformed
into suburban and urban landscapes, wetlands have been drained and many tropical
forests have been logged, burnt and converted to pasture. In contrast, abandoned
farmland in the northeast United States and Europe is returning to forest as those societies
become more urban. Changing a landscape from forest to agricultural crops, for instance,
increases the surface’s albedo and decreases the Bowen ratio. Forests have a different
physiological capacity to assimilate carbon and transpire water, compared to crops. A
change in the age structure of forests due to direct (deforestation) or indirect (climateinduced fires) disturbance alters it ability to acquire carbon and transpire water (Schulze
et al. 1999).
Study of the Earth’s biogeochemistry and hydrology involves quantifying the flows of
matter in and out of the atmosphere with an array of methods (Canadell et al., 2000). At
the global scale, scientists assess carbon dioxide sources and sinks using inversion
modeling of CO2, 13C and O2 concentration and wind fields (Ciais et al., 1995; Denning et
al., 1996). At regional and continental scales, this approach is subject to errors due to the
sparseness of the flask concentration network and their biased placement in the marine
boundary layer (Denning et al., 1996).
Instruments mounted on satellites platforms, view the Earth in total or regions in fine
detail (1 km to 30 m resolution). They offer the potential to evaluate surface carbon
fluxes on the basis of algorithms that are driven reflected and emitted radiation
measurements (Running et al. 1999; Cramer et al., 1999). This approach, however, is
inferential, so it is dependent on the accuracy of the model algorithms, the frequency of
satellite images and the spectral information contained in the images. Application of
remote sensing technology to assess carbon cycle science is highly dependent on ground
truth data to generate, parameterize, and validate models and algorithms that use remotely
sensed data.
One measure of carbon flux ‘ground truth’ can be provided by biomass surveys (Kauppi
et al., 1992; Gower et al., 1999). However, biomass surveys provide information on
decadal time scales, so they do not provide information on shorter-term physiological
forcings and mechanisms that are needed by satellite-driven algorithms. Furthermore,
forest inventory studies are labor intensive and 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 a certain portion of
carbon is allocated below ground (Gower et al., 1999). Soil carbon surveys can be
conducted, but like biomass surveys, they require long intervals to resolve detectable
differences in net carbon uptake or loss.
The eddy covariance method, a micrometeorological technique, provides a direct measure
of net carbon and water fluxes between vegetated canopies and the atmosphere
(Baldocchi et al., 1988; Aubinet et al. 2000). It is a non-intrusive method that has been
automated. The eddy covariance method is able to measure fluxes quasi-continuously
over short and long time scales (hour, days, seasons and years) with minimal disturbance
to the underlying vegetation. The eddy covariance method is able to sample a relatively
large area of land. Typical footprints have longitudinal length scales of 100 to 2000 m.
With the eddy covariance methods we can measure, at the stand scale, how ecosystems
respond to environmental forcings and we can quantify their dynamic variability across a
multitude of time scales. Response functions, generated by a network of carbon flux
measurement sites can be used to validate and improve upon algorithms being used by
remote sensors and global and regional scale modelers to scale carbon and water fluxes
from landscape to regional and continental scales. Direct flux measurements also
identify new properties that emerge as we transcend scales from the leaf to canopy and
canopy to landscape scales.
Micrometeorologists have been making measurements of CO2 and water vapor exchange
between vegetation and the atmosphere since the late 1950's and early 1960's. Yet, only
recently have we had the technology available that enables us to make continuous flux
measurements at numerous sites. Wofsy et al. (1993), at Harvard Forest, were among the
first investigators to measure carbon dioxide and water vapor fluxes continuously over a
forest over the course of a year with the eddy covariance method. Spurred by this
pioneering study, a handful of other towers were soon established and operating by 1993
in North America (Oak Ridge, TN, Greco and Baldocchi, 1996) and in Japan (Yamamoto
et al., 1999) and Europe by 1994 (Valentini et al., 1996).
The concept of a global network of long-term flux measurement sites had a genesis as
early as the 1993, as noted in the Science Plan of the IGBP/BAHC (International
Geosphere-Biosphere Program/Biospheric Aspects of the Hydrological Cycle). Formal
discussion of the concept among the international science community occurred at the
1995 La Thuile workshop (Baldocchi et al., 1996). At this meeting, the flux
measurement community discussed the possibility, problems and pitfalls associated with
making long-term flux measurements. After the La Thuile meeting there was
acceleration in the establishment of flux tower sites and regional flux measurement
networks. The EUROFLUX project started in 1996 (Aubinet et al., 2000; Valentini et al.,
2000). The AmeriFlux project was conceived in 1997, subsuming the cited on-going
tower studies and initiating new studies. With anticipation of the Earth Observation
Satellite (EOS/TERRA), NASA decided to fund the global-scale FLUXNET project, as a
means of validating EOS products, in 1998. As we write this proposal over 30 site-years
of carbon dioxide flux data have been compiled and are accessible on-line for further
analysis and synthesis, and more are being processed.
The measurements provided by the FLUXNET project are only one component of an
integrated tool that assesses the carbon balance of ecosystems (see Canadell et al., 2000).
To perform spatial and temporal integrations, flux measurement data will need to be
linked with model computations and the models will be driven with satellite products,
forming a linkage between surface carbon flux measurements and their assessment from
space.
We, as a scientific community, possess a hierarchy and rapidly growing number of
biophysical models that assess carbon dioxide and water vapor exchange between
vegetation and the atmosphere (Sellers et al., 1997). The models vary in conceptual
complexity, by how they treat the spatial and temporal variability of the canopy
microclimate, the plant and community structure and the functionality of the ecosystem.
With regards to the spatial domain, biophysical trace gas flux models treat the canopy as:
1) a big-leaf; 2) a big-leaf with dual sources; 3) two layers (one for plants and the other
for soil); 4) multiple layers and 5) a three-dimensional array of plants and canopy
elements. With respect to temporal variability, carbon-water flux models with
micrometeorological origins focus on radiative and turbulence transfer within and above
the vegetative canopy and how stomata respond to environmental and physiological
forcings (Baldocchi and Harley, 1995; Leuning et al., 1995; Gu et al., 1999). These
models are capable of simulating the response of canopy-scale carbon and water fluxes to
short-term weather variations over the diurnal course of fine summer days. Models based
on ecological and biogeochemical principles focus on how carbon and water fluxes may
vary on daily, monthly and annual time scales (Running and Hunt, 1993; Cramer et al.,
1999). These models account for changes in plant, soil and root carbon pools by
understanding their respective sizes and turnover times, but tend to oversimplify or
ignore the effects of microclimate variability within a plant canopy. Biogeochemical
models that extend to longer time scales need to consider disturbance by fire and land use
change, nitrogen balance of plants and the soil and changes in stand composition,
hydrological balance and climate.
As we write this proposal, at the start of the new millennium, the study at Harvard Forest
has been ongoing for a decade and studies at Oak Ridge, TN, Prince Albert,
Saskatchewan and Thompson Manitoba have over five years of data. Consequently, data
sets with over 50,000 hours of flux measurements are now available to the modeling
community. Tests of coupled carbon-water flux models on multi-year time scales plus
are starting to be produced, yet the total number of papers that address and test models on
this time scale is meager (Law et al., 2000; Baldocchi and Wilson, in press). With the
availability of new and longer data sets new questions arise about the applicability and
accuracy of coupled carbon-water flux models. The new applications of coupled carbonwater flux models needs to consider the seasonal dynamics of biology, as well as
meteorology, and reproduce observed phenology. This feature requires the dynamic
adjustment of many model parameters that have been considered static in past
applications, such as leaf area index, photosynthetic capacity, basal rates of enzyme
kinetics and plant-water relations on stomatal conductance, photosynthesis and soil
respiration. We are on the verge of being able to combine the best attributes of
biophysical and biogeochemical models and to develop mechanistically based algorithms
that can be assessed with remotely sensed measurements. Combining hierarchal model
analysis (with regards to processes, time and space scales) with FLUXNET data is one
way to achieve this goal.
4. Objectives
The research objectives that FLUXNET and its Project Office intend to accomplish are:
1) to quantify the spatial differences in net biosphere-atmosphere carbon dioxide and
water vapor exchange rates that are experienced within and across ecological and climatic
gradients; this information can provide ground truth data for calculations of carbon
exchange that are generated by regional and global carbon balance models that are forced
with remote sensing information
2) to examine temporal dynamics and variability (seasonal, inter-annual) of carbon, water
and energy flux densities; this analysis is needed to examine the influences of
phenology, droughts, heat spells, El Niño, length of growing season, phenology and
presence or absence of snow on canopy-scale fluxes and to assess the temporal variability
of stand-scale model parameters being used by algorithms that are forced by remote
sensing products, such as light use efficiency;
3) to quantify the sensitivities of carbon dioxide and water vapor fluxes to changes in
insolation, temperature, humidity, soil moisture, photosynthetic capacity, nutrition,
canopy structure and ecosystem functional type; this analysis will allows us to quantify
scale-dependent and non-linear response functions for the generate empirical algorithms
that can be applied by regional and global carbon balance models that are forced with
remote sensing information.
4) test the performance of biophysical models across the ecological and climatic range of
sites in FLUXNET; validated biophysical models can be used to develop
mechanistically-based simpler algorithms that can be assessed by sensors residing on
satellites; citing one example, we have used this approach to examine how gross
photosynthetic production scales with leaf area index, the fraction of absorbed light and
photosynthetic capacity (Figure 1)
1600
CANOAK, 1997
2
r =0.935
-2
-1
Canopy Ps (gC m yr )
1400
1200
1000
800
600
400
0
100
200
300
400
500
Figure 1 Model
calculations of canopy
photosynthesis as a
function of leaf area
index, fpar and
photosynthetic
capacity (Vcmax, a
function of leaf
nitrogen) (after
Baldocchi and Wilson,
in press).
Vcmax LAI/fpar
5) compile and
synthesize of
ecophysiological metadata, such a photosynthetic capacity, leaf area and species
transects, to assess FLUXNET measurements and to drive process-level carbon flux
models; this information, combined with flux footprint information, is needed to
extrapolate tower flux measurements to the kilometer scale and match assessments from
coarser-scale remote sensing indices such as NDVI.
5. Scientific Relevance
FLUXNET addresses many aspects of the primary questions that are associated with this
solicitation. FLUXNET is compiling information on ecosystem scale carbon, water and
energy fluxes that can quantify how the global Earth system is changing, across a
spectrum of time and space scales. FLUXNET products are also quantifying and
identifying many primary forcings on the earth system. If the Earth System is truly
experiencing global warming, data provided by FLUXNET can act as a ‘canary in the
coal mine’, by quantifying ecosystem metabolism before, during and after the change.
Information obtained via FLUXNET also addresses the secondary-tier questions relating
to how terrestrial ecosystems respond to environmental change and how they affect the
global carbon cycle.
FLUXNET is particularly relevant with respect to the validation and defensibility of
many of NASA’s satellite based products. For example, MODIS derived maps of net
primary productivity depend on FLUXNET products for validation and verification.
With regard to carbon cycle science, the concept of a global flux measurement network is
endorsed in the US Carbon Cycle Plan (Sarmiento and Wofsy, eds, 1999). FLUXNET
serves as an instrument that enables NASA to develop, improve and evaluate terrestrial
carbon cycle models and it provides data and new algorithms for the estimate of regional
and global primary productivity by instruments on satellite-based platforms.
6. Expected Significance
FLUXNET does not intend to measure spatially integrated carbon and water fluxes at the
regional to continental scale. Instead, we endeavor to derive information on the temporal
and spatial controls of these fluxes at the landscape scale and combine this information
with satellite-derived products and geographical information systems to deduce spatially
integrated fluxes that are representative.
We cite three examples showing how information derived from FLUXNET can be
combined with satellite-based information to improve our understanding of the carbon
cycle. Multi-year carbon flux records allow us to examine inter-annual time scales, the
timing of leaf expansion may be advanced or delayed by a month due to large-scale
climatic features that can be associated with El Niño-La Niña cycles (Myneni et al., 1997;
Keeling et al., 1996). The FLUXNET project can produce direct information on the
impact of changing growing season length on net ecosystem carbon dioxide exchange.
Initial data are indicating that each additional day of growing season length affects net
ecosystem CO2 exchange of temperate deciduous forests by 6 g m-2 (Figure 2).
Combining the data in Figure 2 with remote sensing data (eg. Myneni et al., 1998) will
allow us to understand how CO2 exchange is perturbed across larger space scales.
Temperate Deciduous Forests
0
-100
NEE (g C m-2 year-1)
-200
-300
-400
Figure 2 Measurements and
computations of the relation
between length of growing
season and net ecosystem
CO2 exchange of
broadleaved deciduous
forests. The relation is linear,
it accounts for over 80% of
the variance and a coupled
biophysical model is able to
capture this behavior. (after
Baldocchi and Wilson, in
press).
-500
The second example,
involves the assessment
CANOAK, Oak Ridge, TN
of light use efficiency, as
-700
Published Measurements, r =0.89
used by many satellite
-800
algorithms to determine
120
140
160
180
200
220
240
gross primary
Days with NEE < 0
productivity (Cramer et
al., 1999). The
accumulating body of
FLUXNET data is showing that light use efficiency varies by a factor of two whether the
sky is clear or cloudy (Gu et al., 1999; Figure 3). This type of information is not widely
utilized by the remote sensing community, hence, their current estimates of regional
carbon exchange may be wrong.
-600
2
Figure 3 Seasonal
trend of initial
quantum yield of
canopy scale CO2
exchange as a function
of clear and cloudy
conditions. The data
are from an aspen
stand and were
acquired by TA Black
and colleagues. (after
Gu et al. in
preparation)
5
Initial quantum yield
(mol CO2/mol photon)*100
Diffuse PAR
Direct PAR
4
3
2
The third example
relates to the
seasonal forcing of
0
biophysical models
180
200
220
240
260
280
for computing
Day of Year
carbon exchange.
At present many
models may vary
weather inputs and leaf area index seasonally on the basis of remote sensing
measurements, but they assume static values for features such as photosynthetic capacity.
We are now using FLUXNET data to examine how well model calculations, based on
certain parameterization schemes, reproduce the power spectrum of measured carbon
fluxes (Figure 4). Identifying periods of low and high coherence between measured and
modeled fluxes across an array of plant functional types will enable us to refine how we
model carbon fluxes across a spectrum of time scales. Global assessments of carbon
fluxes using satellite-based sensors cannot proceed credibly without this necessary
intermediate step.
1
Figure 4
Comparison of the
power spectra of
CO2 flux density
measured over a
deciduous forest and
computed with a
biophysical model
(after Baldocchi and
Wilson, in press).
1997, Temperate Deciduous Forest
10
nSFc/σ2Fc
1
Fcwpl
CANOAK
0.1
7. Technical
Approach
0.01
0.001
0.0001
0.001
0.01
0.1
-1
Frequency (cycles hr )
1
FLUXNET has two functioning components, a project office and a data archive office.
The Project Office houses the principal investigator and a postdoctoral scientist. This
proposal is soliciting funds to continue the operation and activities of project office.
One function of the FLUXNET Project Office is to provide the leadership that enables to
various regional flux measurement networks (AmeriFlux, CarboEurope, AsiaFlux,
OzFlux) to operate as an integrated global network. This involves the fostering of
communication and collaboration among network leaders (co-investigators on this
project) and participants to ensure that data streams are combined, the data are
documented and they meet high and quantified standards. Science is still a people
oriented business and scientists tend to be reluctant to distribute data. Operations based
out of the FLUXNET Project Office have been instrumental in coaxing scientists to
submit data. We were able to get access to EUROFLUX data a year before the release of
its CDROM and we have been instrumental in getting many teams to release data to
FLUXNET DIS in near real-time (eg two week delay).
To ensure intra-network comparability, FLUXNET’s second function involves travel
support for the circulation of a reference set of flux measurement instrumentation to sites
across the component sub-networks. A roving set of reference field instruments will
travel to candidate sites in each regional network (contracted through the lab of Dr. David
Hollinger). Through the calibration activity we will be able to assess if carbon or energy
fluxes from site A, in North America, differs from Site B, in Europe, whether or not the
difference is methodological or actual.
The formation of a global network facilitates the acquisition of data across a wide range
of canopy height, plant functionality, climate conditions and topography. The third
function of FLUXNET is to provide a mechanism for synthesizing data across sites and
biomes, with the objective of understanding temporal dynamics, spatial patterns and
biotic and abiotic forcings of carbon dioxide, water vapor and energy fluxes. To meet
this objective, we sponsor a postdoctoral research associate, convene workshops,
organize regular electronic communication of ideas and research methods with the
FLUXNET community through our list server and provide travel support for visiting
scientists to workshops and to spend time and conduct research at the FLUXNET Project
Office. It is our experience working on past integrated projects (such as BOREAS) that
cross-site syntheses are not conducted unless there is a dedicated research office that is
responsible for this activity. Participating scientists tend to be too busy studying their
own site to be able to examine data across sites.
The fourth function of the FLUXNET Project Office is to provide scientific advice and
guidance to the FLUXNET Data Information System (DIS) (http://wwweosdis.ornl.gov/FLUXNET/) in regards to processing, archiving, documenting and
accessing data (see proposal by R. Olson et al, Fluxnet: a global flux database to advance
carbon cycle science). The FLUXNET Project Office has been very instrumental in
developing gap-filling routines (Falge et al., 2000), which are being used by FLUXNET
DIS. The FLUXNET Project Office also produced the first FLUXNET CD-ROM, in
June 2000, which contained over thirty site years of data.
The fifth function of FLUXNET is to ensure that NASA’s satellite-derive products of
carbon flows are validated against ground truth data. Tower flux data are used to test
model algorithms by season and plant functional type, and are used to identify new
emerging processes that have been ignored so far.
The sixth function of the FLUXNET Project Office is to represent the network
internationally, as FLUXNET has been incorporated into the plans of several
international projects such as International Geosphere Biosphere Program (IGBP/BAHC)
(http://www.pik-potsdam.de/~bahc/), the Global Terrestrial Observing System (GTOS)
(http://www.wmo.ch/web/gcos/gcoshome.html) and the International Biodiversity
Observation Year of Diversitas (http://www.nrel.colostate.edu/IBOY/index2.html)
a. Experimental Measurements
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). Vertical flux densities of CO2 (Fc), latent (λ
λE) and sensible heat (H)
between vegetation and the atmosphere are proportional to the mean covariance between
vertical velocity (w') and the respective scalar (c') fluctuations (eg. CO2, water vapor, and
temperature). Scalar concentration fluctuations are measured with open and closed path
infrared gas analyzers. Three-dimensional sonic anemometers are used to measure wind
velocities and standardized data processing routines are used to compute flux covariances
(Baldocchi et al., 1988; Aubinet et al., 2000).
As we attempt to apply the eddy covariance method in a network mode, errors can arise
from biases that are introduced by the sparse spatial density and low representativeness of
sites and errors due to differences in calibration and data processing. The former error
may be overcome by stratified sampling. In its current configuration the network has
good coverage of sites across a spectrum of temperature and precipitation zones (Running
et al., 1999). Systematic sampling errors can be quantified and compensated for by
employing uniform measurement methods (Valentini et al., 2000), and by intercalibrating and cross checking of data processing methods (Aubinet et al., 2000).
Our community is interested in assessing the net uptake of carbon dioxide by the
biosphere, not the flux across some arbitrary level. 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 zr in a time that is small compared to the
averaging time, thereby violating the assumption of steady state. Under such conditions
the storage term becomes non-zero, so it must be added to the eddy covariance
measurement to represent the balance of material flowing into and out of the soil and
vegetation. While the storage term is small over short crops, it is an important quantity
over taller forests. At present, several teams of investigators are applying an empirical
correction to compensate for the underestimate of nighttime flux measurements. This
correction is based on CO2 flux density measurements obtained during windy periods or
by replacing data with a temperature-dependent respiration function. Most researchers
presume that data from windy periods represent conditions when the storage and drainage
of CO2 is minimal (Aubinet et al., 2000).
Most clients of eddy flux data require uninterrupted time series. It is the intent of the
micrometeorological community to collect eddy covariance data 24 hours a day and 365
days a year. However, missing data in the archived records is a common feature. The
average data coverage during a year is between 65 % and 75% due to system failures or
data rejection (Falge et al., 2000). 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, if the vertical angle of attack by the
wind vector is too severe and when data are missing because of calibration and
maintenance. Other sources of missing data arise from farming operations and other
management activities (e.g., prescribed burn of grasslands). Rejection criteria applied to
the data vary among the flux tower groups. Data might be rejected, when stationarity tests
or integral turbulence characteristics fail, when precipitation limits the performance of
open path sensors, during sensor calibration, or when spikes occur in instrument readings.
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. If the wind is coming from a non-preferred direction as may occur over
mixed stands, a certain portion of the data will need to be screened. In addition, rejection
probability for some sites is higher during nighttime because of calm wind conditions.
b. Model Calculations
Process-based biophysical models will be used to compute fluxes of carbon, water and
energy over a spectrum of plant functional types and across a spectrum of time scales (eg.
Baldocchi and Harley, 1995; Baldocchi and Wilson, in press; Leuning et al., 1995; Gu et
al., 1999). Detailed multi-layered models will be used to develop simpler, process based
algorithms that can be implemented indices sensed by satellites, such as fpar (e.g. Figure
1).
CANVEG is one example a one-dimensional, multi-layer biosphere-atmosphere gas
exchange model that computes water vapor, CO2 and sensible heat flux densities and the
microclimate within and above vegetation (Baldocchi and Harley, 1995); though the
models of Gu et al. (1999) and Leuning et al. (1995) use similar principles. The model
consists of coupled micrometeorological and eco-physiological modules. The
micrometeorological modules compute leaf and soil energy exchange, turbulent
diffusion, scalar concentration profiles and radiative transfer through the canopy. To
account for non-linear effects computations are performed on sunlit and shaded leaf
fractions. Environmental variables, computed with the micrometeorological module
(light, wind, temperature, humidity, CO2), in turn, drive the physiological modules that
compute leaf photosynthesis, stomatal conductance, transpiration and leaf, bole and
soil/root respiration. Biochemical models are used for photosynthesis and stomatal
conductance and photosynthesis are coupled. Inputs include weather variables, leaf area
index and photosynthetic capacity. The model has been described and tested for summer
length studies in Baldocchi and Harley (1995) and Baldocchi (1997). A brief overview is
provided below.
The conservation budget for a passive scalar provides the foundation for computing
scalar fluxes and their local ambient concentrations. If we assume that the a canopy is
horizontally homogeneous and environmental conditions are steady, the scalar
conservation equation was evaluated as an equality between the change, with height, of
the vertical turbulent flux (F) and the diffusive source/sink strength. The diffusive source
strength was computed using a resistance-analog relationship:
S(C, z) = - a(z)
(C(z) - Ci )
r b (z) + r s (z)
(1)
where a(z) is the leaf area density (m2 m-3), (C(z) - Ci) is the potential difference of scalar
concentration or heat content between air outside the laminar boundary layer of leaves and
the air within the stomatal cavity (mol mol-1), rb is the boundary layer resistance to
molecular diffusion (mol-1 m2 s1), and rs is the stomatal resistance (mol-1 m2 s1).
The light environment on sunlit and shaded leaves in a canopy is very distinct and the
response of many biophysical processes to that light is highly non-linear. We evaluate
Equation 1 at particular levels in the canopy on the basis of the radiation balance on the
sunlit (psun) and shaded (pshade) leaf fractions.
The transfer of photons through the
canopy was simulated to evaluate the flux densities of visible, near infrared and longwave
radiation, the probability of sunlit and shaded leaves, as well as photosynthesis, stomatal
conductance, and leaf and soil energy balances. The radiative transfer model was derived
from probabilistic theory. The radiative transfer model assumes that foliage is randomly
distributed in space and the sun is a point source. In this case the probability of beam
penetration was calculated using a Poisson distribution:
P 0 = exp (-
LG
)
sin β
(2)
where L is leaf area index, β is the solar elevation angle and G is the foliage orientation
function. G represents the direction cosine between the sun and the mean leaf normal.
For the ideal case, in which leaves have a spherical angle distribution, G is constant and
equals one-half. Typically, native vegetation has clumped foliage. In these
circumstances, the Poisson probability density function is inadequate for computing
probabilities of photon transmission through vegetation. Instead, the Markov model can
be employed to compute the probability of beam penetration:
L GΩ
(3)
)
sin β
where Ω is a clumping factor and ranges between zero and one. Most satellite based carbon
flux algorithms ignore this effect, yet our research shows it can lead to a 20% increase in
P 0 = exp (-
computed net carbon flux (Baldocchi and Harley, 1995; Baldocchi and Wilson, in press), an
observation that has been verified with eddy covariance data.
The interdependence between sources and sinks (S(C,z)) and scalar concentrations (C(z))
requires the use of a turbulent diffusion model. A Lagrangian turbulence transfer scheme is
used to compute turbulent transport and diffusion. A dispersion matrix was computed
using a stochastic differential equation that tracked the diffusion of an ensemble of fluid
parcels. The algorithm accounts for inhomogeneity of turbulence statistics inside the
canopy and the impact of atmospheric thermal stability on the variance of vertical
velocity fluctuations. A turbulence diffusion model, based on the Lagranian framework,
is preferred to ‘K-theory’ because counter-gradient transport occurs in canopies. We
have developed simple ‘resistance’ based parameterizations of the dispersion matrix, to
enable routine application, that accounts for thermal stratification in the canopy. Leaf
boundary layer resistances for molecular compounds were computed using flat plate
theory and accommodations for free and forced convection are made.
Leaf temperature was calculated by solving the leaf energy balance. This information was
used to determine enzymatic rates associated with carboxylation, electron transport, and
respiration and to evaluate transpiration, sensible heat fluxes and infrared emission.
Soil constitutes the lowest boundary of a canopy-scale, water vapor, CO2 and trace gas
exchange model. Flux densities of convective and conductive heat transfer and evaporation
at the soil/litter boundary and soil temperature profiles were computed using a ten layer
numerical soil heat transfer model. Surface energy fluxes were computed using an
analytical solution to a surface’s energy balance.
The biochemical equations for the carbon exchange processes are taken from Farquhar et al.,
(1980). Leaf photosynthesis (A) is a function of the carboxylation (Vc), oxygenation (Vo,
photorespiration) and dark respiration (Rd) rates of CO2 exchange between the leaf and the
atmosphere (all have units of µmol m-2 s-1).
A = V c - 0.5 V o - Rd
(4)
The term, Vc - 0.5 Vo , is a function of the minimum of two processes, the rate of
carboxylation when ribulose bisphosphate (RuBP) is saturated and the carboxylation rate
when RuBP regeneration is limited by electron transport. Stomatal conductance is
computed using algorithms that link it to photosynthesis (e.g. Leuning et al., 1995).
Respiration provides energy for metabolism and synthesis. At the leaf level, we assessed
dark respiration at a reference temperature is as a function of photosynthetic capacity or
its surrogate leaf nitrogen. Kinetic coefficients for photosynthesis and respiration are
adjusted for temperature.
The model will generate environmental and biological response functions that can be
implemented with remotely sensed information, such as light response curves with
different integration times (Leuning et al, 1995) or a function of clear and cloudy days
(Baldocchi and Harley, 1995; Gu et al., 1999) (as shown in Figures 1 –3), light use
efficiencies, compartmental fluxes that depend on leaf area, photosynthetic capacity and
absorbed light. These response functions will be tested with data from the FLUXNET
database, for application to numerous functional types, and will be implemented in other
research programs, such as NASA carbon science proposals by Steve Running and by Inez
Fung.
c. Facilities and Equipment
At present over 140, instrumented tower flux sites are operating over various ecosystems
across the globe. Most tower flux sites are funded by the European Community
(EUROFLUX), US. DOE (NIGEC and TCP programs: AmeriFlux), NASA and NOAA
(GCIP); additional information on these networks is available at on-line web sites:
EUROFLUX, http://www.unitus.it/eflux/euro.html
AmeriFlux, http://www.ornl.esd.gov/programs/NIGEC).
AsiaFlux http://www-cger.nies.go.jp/moni/flux/asia_flux/main.html
FLUXNET does not fund tower sites directly, but depends upon institutional support
associated with the funding of the AmeriFlux (NIGEC, DOE, NASA, NOAA, Canadian
National Research Council, LBA), the EUROFLUX and MEDEFLU (European
Commision), AsiaFlux (Environment Agency, Ministry of Agriculture, Forestry and
Fisheries, Ministry of International Trade and Industry and Ministry of Education,
Science, Sports and Culture; Japan) and OzFlux (CSIRO, Australia) networks.
The individual flux tower sites within the FLUXNET sphere of influence represent a
considerable investment by local and regional government agencies. The cost of the
whole network is on the order of several millions of dollars per year; about $50k start-up
costs and about $50 to 100k per year for professional and staff support. The value-added
costs to NASA for funding FLUXNET project far out-weighs the cost of developing an
independent network of similar scope, skill and experience from scratch.
The FLUXNET Project Office has a 600 MHz personal computer to process data, a color
and inkjet printer and a CD ROM reader/writer to generate electronic copies of data sets.
d. Work Plan
The central project office that will coordinate activities for a network on measuring longterm fluxes of carbon, water and energy across a spectrum of biomes. Specific duties of
the FLUXNET office include:
1) constructing and analyzing integrated data sets for synthesis of field
data and the for the development and testing of Soil-AtmosphereVegetation-Transfer models.
2) organizing workshops for data synthesis and model testing activities.
3) interacting with investigators to ensure the documentation of data and
its submission to the data archive office
4) preparing peer-reviewed research papers and reports on FLUXNET
activities and analyzes.
5) executing and analyzing of site inter-comparison studies.
6) working with and providing scientific guidance to the FLUXNET Data
and Information System.
The duties of the Project Office are distinct from the FLUXNET Data and Information
system (FLUXNET DIS), which: 1) compiles and documents data in consistent formats
with known quality, 2) maintains the FLUXNET Web page for communications and data
exchange, and 3) transfers FLUXNET data and metadata to a long-term archive, currently
designated as the Oak Ridge National Laboratory Distributed Active Archive Center. To
ensure communication between the FLUXNET Project Office and FLUXNET DIS,
regular conference calls are conducted on a bi-weekly basis.
With regard to workshops, two classes will be planned and organized. Type one
activities will involve a cross-section of the flux measurement community. Goals will
include reporting recent results, discussions of methods, and using data from an array of
environments to aid the interpretation of results.
The type two workshop will involve a mixture of participants from the ecological
modeling, flux measurement and remote sensing communities. Aims of the workshops
will include constructing data sets for model testing, model testing and parameterization
and use of the models for synthesis, as how the biosphere is changing or may be
responding to external perturbations.
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MANAGEMENT PLAN
1. Personnel
Dennis Baldocchi will oversee the project, analyze data, conduct biophysical model tests,
develop algorithms and convene workshops. Dr. Baldocchi brings twenty years of
expertise and experience in measuring and modeling canopy scale CO2 and water vapor
fluxes. He, along with Dr. Valentini, were conveners of the La Thuile workshop 1995,
that formed the basis of the FLUXNET project.
Richard Olson, Steve Running, Ricardo Valentini, Ray Leuning and Susumu Yamamoto
will participate at no cost. Mr. Olson is a team leader of the EOS-DAAC facility at Oak
Ridge National Laboratory and principal investigator of the Fluxnet Data Information
System. He will bring expertise on data distribution to the project and will work closely
with Dr. Tom Boden of ORNL/ESD, on the generation and maintenance of the
FLUXNET database. The FLUXNET Project Office and FLUXNET DIS work closely
together and conduct bi-monthly teleconference calls to ensure collaboration.
Dr. Running will use FLUXNET data to validate selected EOS terrestrial data products
and use FLUXNET data to validate terrestrial biogeochemical models. He has over
twenty years of expertise in modeling net primary productivity and leaf area using
satellite products and is one of the leaders of efforts combining remote sensing and
tower-based flux information to assess regional and global carbon balances. Dr. Hollinger
will supervise and be responsible for the network inter-comparability studies. Finally,
Drs. Hollinger, Valentini, Yamamoto and Leuning are leaders of their respective regional
flux networks. Their participation and cooperation in this project will be key in obtaining
and synthesizing meaningful data sets of fluxes and ancillary variables for modelling
exercises. All participants have much experience making micrometeorological carbon
flux measurements for over ten years. .
A postdoctoral fellow/research associate will be responsible for day-to-day activities and
will serve as a liaison among the regional flux networks. At present Dr. Lianhong Gu fills
this position. His duties include: communicating with network investigators to ensure a
steady flow of data from each field site and ensuring that participants are documenting
data and measurement systems. The postdoctoral scientist analyzes and report results
from field inter-comparison studies, writes project reports and will be co-responsible for
organizing data sets for the workshops. The postdoctoral scientist is also involved in
constructing data sets for model tests and data synthesis.
We are also soliciting funds to support travel costs for short-term Visiting Scientist to the
FLUXNET project Office. Scientists in residence can work with the data sets to assess
model algorithms, perform model tests etc.
2. Timeline
Start-time
Jan ‘02
Summer
‘02
Jan ‘03
Summer
03
Jan ‘04
Summer
‘04
Dec ‘04
Activity
Start Project;; ensure data flow to network;
Prepare biophysical models for application
across functional types
Recruit and Host Visiting scientist;
International inter-calibration study; convene
International FLUXNET Conference
Compile metadata; develop carbon flux
algorithms from data
Recruit and Host Visiting scientist; conduct
international inter-calibration study; convene
Data Synthesis workshop
Test of biophysical models across functional
types
Recruit and Host Visiting scientist;
Conduct international intercalibration study;
convene Model Validation and
Intercomparison Workshop
Products
Edit Fluxnet
Synthesis Issue of
Ag. Forest Met
Inter-calibration
report, papers
Annual Report
Inter-calibration
report, papers
Annual Report
Final Report