Comparisons were made among Douglas fir forest, aspen forest and wheatgrass grassland for ecosystem-level water-use efficiency (WUE), defined as the ratio of photosynthetic CO2 assimilation rate and evapo-transpiration (ET) rate. The three sites used in this comparison spanned a range of plant functional types and environmental conditions within western Canada. When compared in the relative order Douglas fir (located on Vancouver Island, B.C), aspen (northern Saskatchewan), grassland (southern Alberta), the sites demonstrated a progressive decline in precipitation and a general increase in maximum air temperature and atmospheric saturation deficit (D) during the mid-summer. We made comparisons of ecosystem WUE measured by eddy covariance. The average WUE at the grassland site was 2.6 mmol/mol, which was much lower than the values observed for the two other sites (aspen: 5.4, Douglas fir: 8.1). The differences in WUE among sites were primarily due to variation in ET. The maximum ET values were approximately 5, 3.2 and 2.7 mm/day for the grassland, aspen and Douglas fir sites, respectively. There was a strong negative correlation between WUE and D for all sites. Excellent agreement was observed between WUE measured using eddy covariance with that calculated based on D and stable carbon isotope measurements. Our carbon isotope measurements indicated that ci/ca values were quite similar among the Douglas fir, aspen and grassland sites, despite large variation in environmental conditions among sites. This implied that the shorter-lived grass species had relatively high ci/ca values for the D of their habitat. By contrast, the longer-lived Douglas fir trees were more conservative in water-use with lower ci/ca values relative to their habitat D. This illustrates the interaction between biological and environmental characteristics influencing ecosystem-level water-use efficiency.