The analysis based on EC-MOD flux estimates provides an alternative, independent, and novel perspective on recent U.S. ecosystem carbon exchange (Xiao et al.
2010, 2011a).
The gridded flux estimates are also valuable for evaluating simulations of ecosystem models and atmospheric inversions. EC-MOD fluxes have been used to
evaluate GPP and NEE (Sun et al. 2011). The North American Carbon Program
(NACP) regional interim synthesis also compared EC-MOD fluxes with simulations
from a number of ecosystem models for temperate North America (Huntzinger
et al. 2012). EC-MOD fluxes have also been used to evaluate the flux estimates
derived from a boundary layer model at regional scales (Dang et al. 2011) and
inversions for North America derived from a nested inversion model at the continental scale (Deng et al. 2013).
A number of upscaling studies have been reported at recent professional
meetings and journals, following the early work by Xiao et al. (2008). The advances
in the upscaling of flux observations were summarized in plenary talks at the 2nd
North American Carbon Program (NACP) All-Investigators Meeting in San Diego,
California (February 2009) and the AmeriFlux Science Meeting and 3rd NACP
All-Investigators Meeting in New Orleans, Louisiana (January 31–February 4,
2011). A recent special issue in Journal of Geophysical Research—Biogeosciences
is devoted to the upscaling of flux observations. This special issue consists of seven
research articles on different topics of upscaling science and reflects the most recent
advances in the upscaling of flux observations (Xiao et al. 2012).
Here I use a data-driven approach (Xiao et al. 2008) and satellite remote
sensing to extend the NEE estimates of EC-MOD for the conterminous U.S.
(2000–2006) (Xiao et al. 2011a) to the 10-year period from 2000 to 2009. Flux
observations from the AmeriFlux network and various satellite data streams are
combined to develop a predictive model for NEE, and the predictive model is then
used to produce gridded NEE estimates with 1 km spatial resolution and 8-day
time step for the 10-year period. The extended gridded flux estimates (EC-MOD)
are then used to examine the spatial and temporal dynamics of NEE for U.S.
terrestrial ecosystems at seasonal, annual, and interannual scales.
6.2 Theoretical Basis
NEE is the difference between GPP and ecosystem respiration (R e ). NEE is
influenced by a variety of meteorological, physiological, atmospheric, hydrologic,
and edaphic variables (Xiao et al. 2008). GPP is the amount of carbon fixed by
vegetation through photosynthesis, and is influenced by incoming solar radiation,
air temperature, vapor pressure deficit, soil moisture, and nitrogen availability
(Clark et al. 1999, 2004). GPP is also regulated by leaf area index (LAI) and canopy
phenology at the ecosystem level (Richardson et al. 2010). Ecosystem respiration
(R e ) includes autotrophic (R a ) and heterotrophic respiration (R h ). The controlling
6 Assessing Net Ecosystem Exchange
151
2010, 2011a).
The gridded flux estimates are also valuable for evaluating simulations of ecosystem models and atmospheric inversions. EC-MOD fluxes have been used to
evaluate GPP and NEE (Sun et al. 2011). The North American Carbon Program
(NACP) regional interim synthesis also compared EC-MOD fluxes with simulations
from a number of ecosystem models for temperate North America (Huntzinger
et al. 2012). EC-MOD fluxes have also been used to evaluate the flux estimates
derived from a boundary layer model at regional scales (Dang et al. 2011) and
inversions for North America derived from a nested inversion model at the continental scale (Deng et al. 2013).
A number of upscaling studies have been reported at recent professional
meetings and journals, following the early work by Xiao et al. (2008). The advances
in the upscaling of flux observations were summarized in plenary talks at the 2nd
North American Carbon Program (NACP) All-Investigators Meeting in San Diego,
California (February 2009) and the AmeriFlux Science Meeting and 3rd NACP
All-Investigators Meeting in New Orleans, Louisiana (January 31–February 4,
2011). A recent special issue in Journal of Geophysical Research—Biogeosciences
is devoted to the upscaling of flux observations. This special issue consists of seven
research articles on different topics of upscaling science and reflects the most recent
advances in the upscaling of flux observations (Xiao et al. 2012).
Here I use a data-driven approach (Xiao et al. 2008) and satellite remote
sensing to extend the NEE estimates of EC-MOD for the conterminous U.S.
(2000–2006) (Xiao et al. 2011a) to the 10-year period from 2000 to 2009. Flux
observations from the AmeriFlux network and various satellite data streams are
combined to develop a predictive model for NEE, and the predictive model is then
used to produce gridded NEE estimates with 1 km spatial resolution and 8-day
time step for the 10-year period. The extended gridded flux estimates (EC-MOD)
are then used to examine the spatial and temporal dynamics of NEE for U.S.
terrestrial ecosystems at seasonal, annual, and interannual scales.
6.2 Theoretical Basis
NEE is the difference between GPP and ecosystem respiration (R e ). NEE is
influenced by a variety of meteorological, physiological, atmospheric, hydrologic,
and edaphic variables (Xiao et al. 2008). GPP is the amount of carbon fixed by
vegetation through photosynthesis, and is influenced by incoming solar radiation,
air temperature, vapor pressure deficit, soil moisture, and nitrogen availability
(Clark et al. 1999, 2004). GPP is also regulated by leaf area index (LAI) and canopy
phenology at the ecosystem level (Richardson et al. 2010). Ecosystem respiration
(R e ) includes autotrophic (R a ) and heterotrophic respiration (R h ). The controlling
6 Assessing Net Ecosystem Exchange
151