Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/27/2009
Publication Date: 1/5/2010
Citation: Polley, H.W., Emmerich, W., Bradford, J.A., Sims, P.L., Johnson, D.A., Saliendra, N.Z., Svejcar, T., Angell, R., Frank, A.B., Phillips, R.L., Snyder, K.A., Morgan, J.A. 2010. Physiological and environmental regulation of interannual variability in CO2 exchange on rangelands in the western United States. Global Change Biology. 16:990-1002.
Interpretive Summary: The continuing increase in the concentration of carbon dioxide (CO2) gas in air may warm Earth’s climate. The rate of CO2 increase is regulated partly by the balance between CO2 uptake by growing plants and CO2 release by respiring plants and animals. The balance between CO2 uptake and release depends, in turn, on how variability in precipitation and other climatic variables affect the biological processes that regulate CO2 exchange between ecosystems and air. We used multi-year measurements of CO2 exchange from 8 native grazing-lands or rangelands in the western USA, including Great Plains grasslands, desert shrubland, desert grasslands, and sagebrush steppe, to determine how fluctuations in the environment affect CO2 uptake and release. Seasonal trends in the net CO2 balance of these ecosystems followed the seasonal pattern in light availability on rangelands with greatest precipitation and the seasonal pattern of precipitation on drier rangelands. Most of the overall variability in CO2 balance resulted from inter-annual differences in CO2 exchange, however. Inter-annual differences in CO2 balance, in turn, resulted from year-to-year differences in both the environment and the response of CO2 exchange to a given change in the environment. Our results demonstrate that the average CO2 balance for these rangelands is well described by variation in either light availability or precipitation. In any given year, however, CO2 balance may vary because of changes in both environmental factors and the response of CO2 exchange to a given change in the environment.
Technical Abstract: Arid and semi-arid grazing lands (rangelands) may regularly shift between functioning as a carbon (C) sink and a C source in response to variability in precipitation and other climatic or environmental variables. We analyzed measurements of carbon dioxide (CO2) exchange from 8 native rangeland ecosystems in the western USA, including Great Plains grasslands, desert shrubland, desert grasslands, and sagebrush steppe, to assess environmental controls on net ecosystem exchange of CO2 (NEE) and its diurnal components, daytime ecosystem CO2 exchange (PD) and night-time respiration (RE). We statistically partitioned variance in weekly values of CO2 exchange and climatic variables from each rangeland into a seasonal and inter-annual component. Seasonal variation results from week-to-week change in fluxes and climatic variables averaged for all years of data. Inter-annual variation reflects differences between observed values of a variable and the multi-year mean of the variable for each week of observation. Variability in weekly values of NEE resulted mostly from inter-annual anomalies in net CO2 uptake, rather than from seasonal variation in fluxes. For all rangelands, seasonal variability in NEE correlated strongly with variation in PD. Seasonal trends in NEE and PD followed the seasonal pattern in net radiation on rangelands with greatest precipitation and the seasonal pattern of precipitation on drier rangelands. Inter-annual variation in NEE resulted from year-to-year differences in the response of CO2 fluxes to short-term change in climatic variables rather than from the repeatable response of fluxes to climatic variation only. NEE-climate relationships differed among years partly because of inter-annual variability in assimilatory capacity of these rangelands. The average response of NEE for each rangeland was well described by variation in a single climatic variable. In order to accurately predict C balance, however, we require a better understanding of controls on processes that regulate flux responses to short-term fluctuations in climate.