Background/Question/Methods Net ecosystem exchange of CO₂ (NEE) of terrestrial ecosystems varies seasonally and inter-annually partly because of climatic variability.   If we are to predict climate-driven variation in NEE, we must understand how climatic anomalies at different temporal scales influence NEE and its components, CO₂ uptake and CO₂ loss.  Variation in weekly values of growing season measurements of NEE, daytime ecosystem CO₂ exchange (Pd), and nighttime respiration (Re) from each of 8 rangeland sites in the western U.S. was partitioned into two components - variation between means of CO₂ fluxes for each week and the grand mean of fluxes across all weeks (seasonal variation) and variation between weekly values of fluxes and the average of flux measurements for each particular week (inter-annual variation).  We used regression analysis to determine contributions of variation in Pd and Re to seasonal and inter-annual variation in NEE and to identify climatic correlates of variability in fluxes.  

Results/Conclusions   Seasonal and inter-annual variability in NEE and Pd were highly correlated at all sites, indicating that within-year and among-year variability in NEE resulted mainly from shifts in net photosynthesis of rangeland vegetation rather than from shifts in ecosystem respiration.  Seasonal variation in rangeland Pd and NEE correlated highly with seasonal anomalies in climatic variables, including air temperature, net radiation, and precipitation.  In contrast, inter-annual variability in Pd and NEE was not well-explained by climatic anomalies.  Seasonal variability in rangeland Pd and, hence, in NEE responded predictably to variability in climate, but inter-annual differences in CO₂ fluxes of these rangelands apparently cannot be predicted from climatic variability alone.