CO2 uptake and ecophysiological parameters of the grain crops of midcontinent North America: Estimates from flux tower measurements

Abstract

a b s t r a c tWe analyzed net CO 2 exchange data from 13 flux tower sites with 27 site-years of measurements over maize and wheat fields across midcontinent North America. A numerically robust "light-soil temperature-VPD"-based method was used to partition the data into photosynthetic assimilation and ecosystem respiration components. Year-round ecosystem-scale ecophysiological parameters of apparent quantum yield, photosynthetic capacity, convexity of the light response, respiration rate parameters, ecological light-use efficiency, and the curvature of the VPD-response of photosynthesis for maize and wheat crops were numerically identified and interpolated/extrapolated. This allowed us to gap-fill CO 2 exchange components and calculate annual totals and budgets. VPD-limitation of photosynthesis was systematically observed in grain crops of the region (occurring from 20 to 120 days during the growing season, depending on site and year), determined by the VPD regime and the numerical value of the curvature parameter of the photosynthesis-VPD-response, VPD . In 78% of the 27 site-years of observations, annual gross photosynthesis in these crops significantly exceeded ecosystem respiration, resulting in a net ecosystem production of up to 2100 g CO 2 m −2 year −1 . The measurement-based photosynthesis, respiration, and net ecosystem production data, as well as the estimates of the ecophysiological parameters, provide an empirical basis for parameterization and validation of mechanistic models of grain crop production in this economically and ecologically important region of North America.

Grassland and Cropland Net Ecosystem Production of the U.S. Great Plains: Regression Tree Model Development and Comparative Analysis

“…We used NEP at the 30 min time step from the flux towers to quality control for possible instrument related outliers. The quality controlled carbon flux data sets were partitioned into gross primary production (GPP) and ecosystem respiration (Re) components utilizing light response and vapor pressure-based partitioning of carbon flux into GPP and Re components [2][3][4]. Partitioning the fluxes into GPP and Re facilitated a more functional filling of temporary flux tower data gaps [24].…”

“…Partitioning the fluxes into GPP and Re facilitated a more functional filling of temporary flux tower data gaps [24]. Partitioned and gap-filled GPP and Re and NEP used in this mapping effort were largely from focused regional flux tower synthesis for grain crops [4], leguminous crops [3], and grasslands [2]. We selected the carbon flux light response and vapor pressure, or "non-rectangular hyperbolic method" flux partitioning models [2][3][4] over Q 10 and short-term exponential fit models (which model Re as a function of temperature based on night-time data) and rectangular hyperbolic fit models (which use the relationship between photosynthetic active radiation (PAR) and daytime NEP to model Re) because it produces the most reasonable C flux estimates and data gap filling [25], particularly in non-forest, low canopy height systems.…”

“…Regional syntheses of carbon flux tower data [2][3][4] provide geographically referenced estimates from multiple flux towers and offer detailed summarization of carbon flux properties.…”

“…Throughout modern history, scientists have been developing models to formulate generalized algorithms to expand geographically and temporally sparse field observations to a much larger landscape means [2][3][4] or to make up-scaled maps of the measured field characteristics [5]. Understanding, mapping, and quantifying regional carbon flux magnitudes and variability of terrestrial non-point carbon sinks (removal of atmospheric carbon) and sources (emission of carbon to the atmosphere) is important in the promotion of ecosystem sustainability.…”

“…The primary objective of this study was to combine measurements acquired at flux towers with applicable remote sensing, weather, and other biogeophysical data to develop separate grassland and cropland ecologically-based net ecosystem production (NEP) models and derive mean daily NEP maps at a weekly time step from 2000 to 2008 for the study area of interest, the U.S. Great Plains (Figure 1). Although a flux tower measures only its surrounding vicinity, numerous studies have shown that these recordings can be used in regional or land cover specific synthesis [3,4] or up-scaled to much greater levels through analyses of geospatial data relationships using computer modeling [9,13,18]. Of particular interest is the use of data mining regression tree algorithms for carbon flux mapping [5,10,11,13,14,18].…”