Rangelands account for almost half of the earth's land surface and may play an important role in the global carbon (C) cycle. We studied net ecosystem exchange (NEE) of C on eight North American rangeland sites over a 6-yr period. Management practices and disturbance regimes can influence NEE; for consistency, we compared ungrazed and undisturbed rangelands including four Great Plains sites from Texas to North Dakota, two Southwestern hot desert sites in New Mexico and Arizona, and two Northwestern sagebrush steppe sites in Idaho and Oregon. We used the Bowen ratio-energy balance system for continuous measurements of energy, water vapor, and carbon dioxide (CO 2) fluxes at each study site during the measurement period (1996 to 2001 for most sites). Data were processed and screened using standardized procedures, which facilitated across-location comparisons. Although almost any site could be either a sink or source for C depending on yearly weather patterns, five of the eight native rangelands typically were sinks for atmospheric CO 2 during the study period. Both sagebrush steppe sites were sinks and three of four Great Plains grasslands were sinks, but the two Southwest hot desert sites were sources of C on an annual basis. Most rangelands were characterized by short periods of high C uptake (2 mo to 3 mo) and long periods of C balance or small respiratory losses of C. Weather patterns during the measurement period strongly influenced conclusions about NEE on any given rangeland site. Droughts tended to limit periods of high C uptake and thus cause even the most productive sites to become sources of C on an annual basis. Our results show that native rangelands are a potentially important terrestrial sink for atmospheric CO 2 , and maintaining the period of active C uptake will be critical if we are to manage rangelands for C sequestration. Resumen Los pastizales nativos constituyen casi la mitad de la superficie terrestre y pueden desempeñ ar un papel importante en el ciclo global del carbón (C). El objetivo de esta investigación fue estudiar el intercambio neto de carbono dentro del ecosistema (NEE) en ocho sitios de pastizales de Norteamérica durante un período de seis añ os. Las prácticas de manejo y grados de disturbio pueden influenciar el NEE, pero para consistencia, se compararon pastizales con y sin pastoreo. Se usaron cuatro sitios de las Grandes Planicies desde Texas a Dakota del Norte, dos sitios del desierto al sudoeste de New Me´xico y Arizona y dos sitios del noroeste del desierto de arbustivas en Idaho y Oregon. Se utilizo el sistema proporción-energía de Bowen para las medidas continuas de energía, de vapor de agua y de los flujos del CO 2 en cada sitio (1996 a 2001 para la mayoría de los sitios). Se examinaron los datos usando procedimientos estandarizados que facilitaron comparaciones entre sitios. Aunque casi cualquier sitio podría actuar como reservorio o fuente de C dependiendo de los patrones anuales. Cinco de los ocho pastizales nativos típicamente demandaron CO 2 atmosfe´rico durante el p...
Phillips, Rebecca L.; Snyder, Keirith A.; and Morgan, Jack A., "Physiological and environmental regulation of interannual variability in CO2 exchange on rangelands in the western United States" (2009 Physiological and environmental regulation of interannual variability in CO 2 exchange on rangelands in the western United States AbstractFor most ecosystems, net ecosystem exchange of CO 2 (NEE) varies within and among years in response to environmental change. We analyzed measurements of CO 2 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 CO 2 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 (a) or ecosystem respiration at 10 1C (R 10 ) and (2) among-year variation in the responses of NEE, A max , and a to environmental drivers would be explained by changes in leaf area index (LAI). As predicted, NEE was better correlated with A max than a or R 10 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 , a) 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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