For most ecosystems, net ecosystem exchange of CO2 (NEE) varies within and among years in response to environmental change. We analyzed measurements of CO2 exchange from eight native rangeland ecosystems in the western United States (58 site-years of data) in order to determine the contributions of photosynthetic and respiratory (physiological) components of CO2 exchange to environmentally caused variation in NEE. Rangelands included Great Plains grasslands, desert shrubland, desert grasslands, and sagebrush steppe. We predicted that (1) week-to-week change in NEE and among-year variation in the response of NEE to temperature, net radiation, and other environmental drivers would be better explained by change in maximum rates of ecosystem photosynthesis (Amax) than by change in apparent light-use efficiency (α) or ecosystem respiration at 10 °C (R10) and (2) among-year variation in the responses of NEE, Amax, and α to environmental drivers would be explained by changes in leaf area index (LAI). As predicted, NEE was better correlated with Amax than α or R10 for six of the eight rangelands. Week-to-week variation in NEE and physiological parameters correlated mainly with time-lagged indices of precipitation and water-related environmental variables, like potential evapotranspiration, for desert sites and with net radiation and temperature for Great Plains grasslands. For most rangelands, the response of NEE to a given change in temperature, net radiation, or evaporative demand differed among years because the response of photosynthetic parameters (Amax, α) to environmental drivers differed among years. Differences in photosynthetic responses were not explained by variation in LAI alone. A better understanding of controls on canopy photosynthesis will be required to predict variation in NEE of rangeland ecosystems.
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