climate change. Several methods including inventory approaches (e.g., Pacala
et al. 2001; Goodale et al. 2002; SOCCR 2007), ecosystem modeling (e.g., Potter
et al. 1993; Running and Hunt 1993; Xiao et al. 2009), and atmospheric inversions
(e.g., Tans et al. 1990; Deng et al. 2007) have been widely used to estimate NEE
over broad regions. The resulting flux estimates, however, exhibit large differences
in both patterns and magnitude (e.g., Huntzinger et al. 2012) despite the general
agreement that the terrestrial ecosystems in the northern hemisphere provide a
large carbon sink (e.g., Pacala et al. 2001; Goodale et al. 2002; Deng et al. 2007;
Pan et al. 2011a).
The eddy covariance technique provides an alternative approach for estimating
NEE. Eddy covariance flux towers have been providing continuous measurements
of ecosystem-level exchange of carbon, water, and energy spanning diurnal,
synoptic, seasonal, and interannual time scales since the early 1990s (Wofsy et al.
1993; Baldocchi et al. 2001). These flux towers provides probably the best estimates of ecosystem-level carbon fluxes. These NEE measurements are routinely
partitioned into its two major components: gross primary productivity (GPP) and
ecosystem respiration (R e ) (Reichstein et al. 2005; Desai et al. 2008).
At present, over 500 eddy covariance flux towers are operating on a long-term
and continuous basis around the world (FLUXNET, http://daac.ornl.gov/
FLUXNET). This global network encompasses a large range of climate and
biome types (Baldocchi et al. 2001). AmeriFlux is the regional network that
coordinates analyses of observations from flux towers within the U.S. Despite the
larger number of flux towers, the flux observations only represent fluxes at the
scale of the tower footprint with longitudinal dimensions ranging between a
hundred meters and several kilometers depending on homogeneous vegetation and
fetch (Schmid 1994; Göckede et al. 2008). To quantify the net exchange of CO 2
between the terrestrial biosphere and the atmosphere over regions, continents, or
the globe, significant efforts are needed to upscale flux observations from towers to
these broad regions (Xiao et al. 2008).
Considerable advances have been made in the upscaling of flux observations
during recent years (e.g., Xiao et al. 2008, 2010, 2011a; Jung et al. 2009; Sun et al.
2011; Zhang et al. 2011). For instance, a data-driven approach has been used to
upscale carbon fluxes from the AmeriFlux network to the continental scale and
to produce gridded fields of GPP and NEE with high spatial (1 km) and temporal
(8-day) resolutions for the conterminous U.S. over the period 2000–2006 (Xiao
et al. 2008, 2010, 2011a). The GPP and NEE fields were derived from eddy
covariance (EC) flux measurements and MODIS data, and are referred to as
EC-MOD. The continuous EC-MOD flux fields were used to assess the magnitude,
distribution, and interannual variability of recent U.S. ecosystem carbon exchange
(Xiao et al. 2010, 2011a). One of the main innovations in the EC-MOD estimates
compared to traditional approaches is the use of daily NEE measurements from flux
towers. These measurements represent direct samples of net CO 2 exchange from
sites encompassing a wide variety of U.S. biomes and climate types, which have not
been previously utilized in U.S. carbon budget studies (e.g., Houghton et al. 1999;
Caspersen et al. 2000; Schimel et al. 2000; Pacala et al. 2001; SOCCR 2007).
150
J. Xiao
et al. 2001; Goodale et al. 2002; SOCCR 2007), ecosystem modeling (e.g., Potter
et al. 1993; Running and Hunt 1993; Xiao et al. 2009), and atmospheric inversions
(e.g., Tans et al. 1990; Deng et al. 2007) have been widely used to estimate NEE
over broad regions. The resulting flux estimates, however, exhibit large differences
in both patterns and magnitude (e.g., Huntzinger et al. 2012) despite the general
agreement that the terrestrial ecosystems in the northern hemisphere provide a
large carbon sink (e.g., Pacala et al. 2001; Goodale et al. 2002; Deng et al. 2007;
Pan et al. 2011a).
The eddy covariance technique provides an alternative approach for estimating
NEE. Eddy covariance flux towers have been providing continuous measurements
of ecosystem-level exchange of carbon, water, and energy spanning diurnal,
synoptic, seasonal, and interannual time scales since the early 1990s (Wofsy et al.
1993; Baldocchi et al. 2001). These flux towers provides probably the best estimates of ecosystem-level carbon fluxes. These NEE measurements are routinely
partitioned into its two major components: gross primary productivity (GPP) and
ecosystem respiration (R e ) (Reichstein et al. 2005; Desai et al. 2008).
At present, over 500 eddy covariance flux towers are operating on a long-term
and continuous basis around the world (FLUXNET, http://daac.ornl.gov/
FLUXNET). This global network encompasses a large range of climate and
biome types (Baldocchi et al. 2001). AmeriFlux is the regional network that
coordinates analyses of observations from flux towers within the U.S. Despite the
larger number of flux towers, the flux observations only represent fluxes at the
scale of the tower footprint with longitudinal dimensions ranging between a
hundred meters and several kilometers depending on homogeneous vegetation and
fetch (Schmid 1994; Göckede et al. 2008). To quantify the net exchange of CO 2
between the terrestrial biosphere and the atmosphere over regions, continents, or
the globe, significant efforts are needed to upscale flux observations from towers to
these broad regions (Xiao et al. 2008).
Considerable advances have been made in the upscaling of flux observations
during recent years (e.g., Xiao et al. 2008, 2010, 2011a; Jung et al. 2009; Sun et al.
2011; Zhang et al. 2011). For instance, a data-driven approach has been used to
upscale carbon fluxes from the AmeriFlux network to the continental scale and
to produce gridded fields of GPP and NEE with high spatial (1 km) and temporal
(8-day) resolutions for the conterminous U.S. over the period 2000–2006 (Xiao
et al. 2008, 2010, 2011a). The GPP and NEE fields were derived from eddy
covariance (EC) flux measurements and MODIS data, and are referred to as
EC-MOD. The continuous EC-MOD flux fields were used to assess the magnitude,
distribution, and interannual variability of recent U.S. ecosystem carbon exchange
(Xiao et al. 2010, 2011a). One of the main innovations in the EC-MOD estimates
compared to traditional approaches is the use of daily NEE measurements from flux
towers. These measurements represent direct samples of net CO 2 exchange from
sites encompassing a wide variety of U.S. biomes and climate types, which have not
been previously utilized in U.S. carbon budget studies (e.g., Houghton et al. 1999;
Caspersen et al. 2000; Schimel et al. 2000; Pacala et al. 2001; SOCCR 2007).
150
J. Xiao