Gross Primary Productivity of the True Steppe in Central Asia in Relation to NDVI: Scaling up CO2 Fluxes

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).

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