Physiological and environmental regulation of interannual variability in CO₂ exchange on rangelands in the western United States

Abstract

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 (A_max) 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, A_max, and α to environmental drivers would be explained by changes in leaf area index (LAI). As predicted, NEE was better correlated with A_max 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 (A_max, 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.