Submitted to: Environmental Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/17/2003
Publication Date: 3/23/2004
Citation: Gilmanov, T.G., Johnson, D.A., Saliendra, N.Z., Akshalov, K., Wylie, B. 2004. Gross primary productivity of the true steppe in central asia in relation to ndvi: scaling up co2 fluxes. Environmental Management.
Interpretive Summary: Obtaining continuous measurements of carbon dioxide fluxes above rangeland ecosystems has become technologically feasible the last decade or so. However, these point measurements must be scaled up to broader areas to allow prediction at regional scales. One of the key tasks in this scaling-up process is to determine which flux measurement characteristics are most closely related to remotely sensed data. We conducted a study in northern Kazakhstan to evaluate if regional rangeland production can be estimated from carbon dioxide measurements. Estimates of plant production and ecosystem respiration were obtained in rangelands in northern Kazakhstan during four growing seasons (1998-2001). Computer models were developed from these data, and plant production characteristics from these models were compared to those developed for other ecosystems. This research showed that there were distinct advantages in using gross primary production to scale up carbon dioxide fluxes compared to other flux characteristics. Results from this study will not only improve regional estimates of carbon dioxide fluxes from rangelands, but also will provide insights into the drivers of carbon sink and source relationships such as drought, insect infestations, and fire.
Technical Abstract: Compared to other characteristics of CO2 exchange, gross primary productivity GPP is most directly related to photosynthetic activity. Until recently, it was considered difficult to obtain measurement-based GPP. The objective of our study was to evaluate if GPP can be estimated from continuous CO2 flux measurements. Estimates of GPP 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). Nonlinear identification of the nonrectangular hyperbolic model of ecosystem-scale, light-response curves were applied to the CO2 measurements. Maximum mean weekly apparent quantum yield was 0.0388 mol CO2 mol photons-1, and maximum mean weekly GPP was 28 g CO2 m-2 d-1 in July 2000. The highest mean weekly Re,max (20 g CO2 m-2 d-1) 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 four-year average GPP was 1,617 (range 1,308 to 1,957) g CO2 m-2 yr-1. Ten-day normalized difference vegetation index corrected for the start of the season (NDVIsos) was closely associated with 10-day average GPP (R2 of 0.66 to 0.83), which was higher than R2 values for regressions of mean 10-day net daytime fluxes on NDVIsos (0.55 to 0.72). This demonstrates the advantage of using GPP in scaling up flux-tower measurements compared to other characteristics (net daytime flux or net 24-hour flux).