Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2009
Publication Date: 3/1/2010
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., Sanabria, J., Mielnick, P.C., Dugas, W.A. 2010. Precipitation regulates the response of net ecosystem CO2 exchange to environmental variation on U.S. rangelands. Rangeland Ecology and Management. 63:176-186.
Interpretive Summary: The concentration of carbon dioxide (CO2) gas in air is increasing with possible consequences for Earth’s climate. Growing plants remove CO2 from air, whereas respiration by plants and animals adds CO2 to air. The balance between these CO2 fluxes depends partly on variability in precipitation and other climatic variables. We measured CO2 exchange and climatic variables like temperature and precipitation on 8 native rangeland ecosystems in the western USA to determine how fluctuations in climate affected CO2 uptake and release. Rangelands studied included Great Plains grasslands, desert shrubland, desert grasslands, and sagebrush steppe. Variability in the CO2 balance at each site resulted mostly from year-to-year differences in how fluxes responded to changes in light and water availability. This source of variability in CO2 exchange was more important on typically dry than wetter rangelands. In any given year, therefore, the CO2 balance of these ecosystems, and especially of dry rangelands, may differ because of differences in the response of CO2 fluxes to a given shift in climatic variables.
Technical Abstract: For virtually all terrestrial ecosystems, the balance between photosynthetic uptake of carbon dioxide (CO2) and CO2 loss to respiration varies among years in response to inter-annual variation in climate. Variability in CO2 exchange results from inter-annual differences in (1) climatic variables at a given point in the annual cycle (direct effects of the environment) and in (2) the response of CO2 flux to a given change in the environment because of climate-caused changes in biological processes that regulate photosynthesis and respiration (functional change). Functional change complicates climate-based predictions of CO2 exchange, yet its causes and contribution to flux variability remain poorly defined. We used a regression procedure to distinguish contributions of functional change and direct effects of the environment to inter-annual variation in net ecosystem exchange of CO2 (NEE) of 8 rangeland ecosystems in the western USA (58 site-years of data). We predicted that 1) functional change is caused by inter-annual change in precipitation on each rangeland and 2) the contribution of functional change to variance in NEE increases among rangelands as mean precipitation increases. Functional change explained 10-40% of the variance in NEE and accounted for more than twice the variance in fluxes of direct effects of environmental variability for 6 of the 8 ecosystems. Functional change was associated with inter-annual variation in precipitation on most rangelands but, contrary to prediction, contributed proportionally more to variance in NEE on arid than more mesic ecosystems. In order to accurately predict NEE from climatic variables, we must account for precipitation and other effects on flux-environment relationships.