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Abstract: We analyzed net CO2 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 CO2 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, σ 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 2100g CO2 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. [Copyright &y& Elsevier]

Sagebrush-steppe ecosystems cover more than 36 million ha in North America and represent an important economic and ecological resource. These ecosystems have a climate with an extended cold period that can last more than five months. The CO2 fluxes during this protracted cold period likely play an important role in determining annual fluxes in these ecosystems; however, few studies have measured continuous CO2 fluxes in sagebrush-steppe ecosystems during the winter. The objective of our study was to obtain continuous measurements of CO2 fluxes during winter at representative sagebrush-steppe sites in the western USA and to study their relationships to environmental factors. Measurements of CO2 fluxes were obtained using Bowen ratio/energy balance (BREB) techniques during the winter at two locations in Idaho and one location in Oregon. Average daily ecosystem respiration during the winter period (November 1–March 15) was 1.31 ± 0.80 g CO2 m-2 day-1 and 1.23 ± 1.19 g CO2 m-2 day-1 at the two Idaho sites and 0.68 ± 0.56 g CO2 m-2 day-1 at the Oregon site. These values are well within the range of previously published results for similar ecosystems. Multivariate analyses showed that soil temperature, wind speed, and snow depth were the environmental factors most closely related to winter CO2 effluxes. Based on testing of empirical flux models, additional research will be required to develop mathematical models that reliably predict winter CO2 effluxes across a wide range of sagebrush-steppe sites. [Copyright &y& Elsevier]

Algorithms that establish relationships between variables obtained through remote sensing and geographic information system (GIS) technologies are needed to allow the scaling up of site-specific CO2 flux measurements to regional levels. We obtained Bowen ratio–energy balance (BREB) flux tower measurements during the growing seasons of 1998–2000 above a grassland steppe in Kazakhstan. These BREB data were analyzed using ecosystem light–curve equations to quantify 10-day CO2 fluxes associated with gross primary production (GPP) and total respiration (R). Remotely sensed, temporally smoothed normalized difference vegetation index (NDVIsm) and environmental variables were used to develop multiple regression models for the mapping of 10-day CO2 fluxes for the Kazakh steppe. Ten-day GPP was estimated (R2 = 0.72) by day of year (DOY) and NDVIsm, and 10-day R was estimated (R2 = 0.48) with the estimated GPP and estimated 10-day photosynthetically active radiation (PAR). Regression tree analysis estimated 10-day PAR from latitude, NDVIsm, DOY, and precipitation (R2 = 0.81). Fivefold cross-validation indicated that these algorithms were reasonably robust. GPP, R, and resulting net ecosystem exchange (NEE) were mapped for the Kazakh steppe grassland every 10 days and summed to produce regional growing season estimates of GPP, R, and NEE. Estimates of 10-day NEE agreed well with BREB observations in 2000, showing a slight underestimation in the late summer. Growing season (May to October) mean NEE for Kazakh steppe grasslands was 1.27 Mg C/ha in 2000. Winter flux data were collected during the winter of 2001–2002 and are being analyzed to close the annual carbon budget for the Kazakh steppe. [ABSTRACT FROM AUTHOR]

Compared to other characteristics of CO2 exchange, gross primary productivity (Pg) is most directly related to photosynthetic activity. Until recently, it was considered difficult to obtain measurement-based Pg. The objective of our study was to evaluate if Pg can be estimated from continuous CO2 flux measurements using nonlinear identification of the nonrectangular hyperbolic model of ecosystem-scale, light-response curves. Estimates of Pg and ecosystem respiration (Re) were obtained using Bowen ratio– energy-balance measurements of CO2 exchange in a true-steppe ecosystem in northern Kazakhstan during four growing seasons (1998–2001). The maximum mean weekly apparent quantum yield (αmax) was 0.0388 mol CO2 mol photons and the maximum mean weekly Pg was 28 g CO2/m2/day in July 2000. The highest mean weekly Remax (20 g CO2 m2/day) was observed in July of both 1999 and 2000. Nighttime respiration calculated from daily respiration corrected for length of the dark period and temperature (using Q10 = 2) was closely associated with measured nighttime respiration (R2 = 0.67 to 0.93). The 4-year average annual gross primary production (GPP) was 1617 g CO2/m2/ year (range = 1308–1957). Ten-day normalized difference vegetation index corrected for the start of the season (NDVIsos) was closely associated with 10-day average Pg (R2 = 0.66 to 0.83), which was higher than R2 values for regressions of mean 10-day net daytime fluxes on NDVIsos (0.55–0.72). This demonstrates the advantage of using Pg in scaling up flux-tower measurements compared to other characteristics (net daytime flux or net 24-h flux). [ABSTRACT FROM AUTHOR]

Abstract: The sagebrush-steppe ecosystem covers more than 36 million ha and could play an important role in the global carbon cycle; however, quantitative estimates of CO2 fluxes on these extensive ecosystems are not available. The Bowen ratio/energy balance technique (BREB) was used to continuously monitor CO2 fluxes during the 1996 to 1999 growing seasons at a sagebrush-steppe site near Dubois, Idaho, USA. The daytime and night-time CO2 fluxes were modeled to provide estimates of occasionally missing or aberrant data points so that daily (24-h) integrals across the entire growing season could be quantified. Depending on the particular time of the season, daytime fluxes were best described by a rectangular hyberbolic, nonrectangular hyperbolic, or hysteresis-type functions that included radiation, relative humidity, and soil temperature. Night-time CO2 fluxes exhibited greater variability than daytime fluxes and were not closely correlated with any single meteorological characteristic. Night-time fluxes were predicted using a nonlinear parameter identification technique that estimated values of daytime respiration, which were significantly correlated with night-time fluxes. For the four growing seasons of our study, the integrated seasonal fluxes ranged from 284 to 1,103 g CO2 m−2 with an overall average of 635 g CO2 m−2. Respiratory losses during the non-growing season were estimated to be about 1.5 g CO2 m−2 day−1 or a total of 270 g CO2 m−2. This gives an annual net positive flux (carbon sequestration) estimate of 365 g CO2 m−2 (or 1.0 t C ha−1). These results suggest that the combination of BREB measurements and modeling techniques can be used to provide estimates of CO2 fluxes on important rangeland ecosystems. Das Wüstenbeifuß-Steppenökosystem erstreckt sich über mehr als 36 Millionen Hektar und könnte eine wichtige Rolle im globalen Kohlenstoffkreislauf spielen. Quantitative Schätzungen der CO2-Flüsse dieser ausgedehnten Ökosysteme sind jedoch nicht verfügbar. Die “Bowen-Verhältnis”-Energiebilanz-Technik (BREB) wurde benutzt, um die CO2-Flüsse während der Wachstumssaisonen 1996 bis 1999 in einer Wüstenbeifuß-Steppe bei Dubois, Idaho, USA, kontinuierlich zu erfassen. Die Tages- und Nacht-CO2-Flüsse wurden modelliert um Schätzwerte für gelegentlich fehlende oder abweichende Datenpunkte zu bekommen, so dass Tagesintegrale (24h) über die gesamte Wachstumssaison quantifiziert werden konnten. Abhängig vom jeweiligen Zeitpunkt der Saison wurden die Tagesflüsse am Besten durch rechtwinklig hyperbolische, nicht-rechtwinklig hyperbolische oder hystereseähnliche Funktionen beschrieben, die Strahlung, relative Luftfeuchtigkeit und Bodentemperatur enthielten. Die Nacht-CO2-Flüsse zeigten eine größere Variabilität als Tagesflüsse und waren mit keiner einzelnen meteorologischen Kenngröße eng korreliert. Die Nachtflüsse wurden vorhergesagt, indem eine nichtlineare Parameter-Identifizierungstechnik verwendet wurde, mit der die Werte der Tagesrespiration geschätzt wurden, die wiederum mit den Nachtflüssen signifikant korreliert waren. Für die vier Wachstumssaisonen unserer Studie bewegten sich die integrierten saisonalen Flüsse zwischen 284 und 1103 g CO2 m−2 mit einem Gesamtmittelwert von 635 g CO2 m−2. Die Respirationsverluste außerhalb der Wachstumssaison wurden auf ungefähr 1.5 g CO2 m−2 Tag−1 bzw. insgesamt auf 270 g CO2 m−2 geschätzt. Dies ergibt einen geschätzten positiven jährlichen Nettofluss (Kohlenstofffixierung) von 365 g CO2 m−2 (oder 1 t C ha−1). Diese Ergebnisse lassen erkennen, dass eine Kombination von BREB-Messungen und Modellierungstechniken verwendet werden kann, um Schätzungen des CO2-Flusses in wichtigen Weideland-Ökosystemen zur Verfügung zu stellen. [Copyright &y& Elsevier]