Fig. 1.

Monthly precipitation during 2010 and 2011 and 9-yr average (2000–2009) at the study site.

 


Fig. 2.

Daily average values of meteorological variables recorded at the pasture eddy covariance flux site near Lockney, TX, for Days of the Year (DOY) 152–360: (a) air temperature, (b) soil temperature at 4 cm below the surface, and (c) vapor pressure deficit.

 


Fig. 3.

Daily accumulated net ecosystem exchange (NEE) of CO2 during the measurement period in (a) 2010 and (b) 2011. Negative values indicate a gain of C by the ecosystem and positive values indicate ecosystem loss of C.

 


Fig. 4.

Diurnal variations in half-hourly net ecosystem exchange (NEE) of CO2 on Day of the Year 195 in 2010 and 2011.

 


Fig. 5.

The cumulative gross primary production (GPP), net ecosystem exchange (NEE), and ecosystem respiration (Reco) since Day of the Year 169 for 2010 and 2011. Negative values indicate a gain of C by the ecosystem and positive values indicate ecosystem loss of C.

 


Fig. 6.

Response of ecosystem respiration (Reco) to soil temperature measurements (Tsoil) at the 4-cm depth in 2010 for (a) all days (r2 = 0.66, n = 197, P < 0.001) and (b) all days except days when volumetric water content was <0.15 m3 m–3 (r2 = 0.92, n = 156, P < 0.001) and in 2011for (c) all days (r2 = 0.35, n = 171, P < 0.001) and (d) all days except days when volumetric water content was < 0.15 m3 m–3 (r2 = 0.90, n = 86, P < 0.001). The solid lines represent the regression functions.

 


Fig. 7.

Daily gross primary production (GPP) plotted against corresponding daily ecosystem respiration (Reco): (a) data for the whole study period and the grazing period in 2010 (Days of the Year [DOY] 199–227); (b,c) data for 2010 and 2011 (n = 197, P < 0.001). Solid lines represent the regression line and dashed lines represent the 1:1 line.

 


Fig. 8.

Light response curves generated by plotting daytime half-hourly net ecosystem exchange (NEE) against corresponding photosynthetic photon flux density (PPFD) for different periods of growth in 2010 and 2011: (a) Days of the Year (DOY) 190 to 198 (peak growth) in 2010, (b) (DOY 200 to 209 (beginning of grazing) in 2010, (c) DOY 218 to 227 (end of grazing) in 2010, (d) DOY 244 to 254 (flowering) in 2010, (e) DOY 190 to 199 in 2011 and (f) DOY 244 to 254 in 2011. The curves (solid lines) were fitted using the modified Michaelis–Menten equation (Eq. [3]) except for (e).