Comparison Of Ecosystem Water-Use Efficiency Among Douglas-Fir Forest, Aspen Forest And Grassland Using Eddy Covariance And Carbon Isotope Techniques

Comparisons were made among Douglas-fir forest, aspen (broad leaf deciduous) forest and wheatgrass (C₃) grassland for ecosystem-level water-use efficiency (WUE). WUE was defined as the ratio of photosynthetic CO₂ assimilation rate and evapotranspiration (ET) rate. The ET data measured by eddy covariance were screened so that they overwhelmingly represented transpiration. The three sites used in this comparison spanned a range of vegetation (plant functional) types and environmental conditions within western Canada. When compared in the relative order Douglas-fir (located on Vancouver Island, BC), 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_max) during the mid-summer. The average (±SD) WUE at the grassland site was 2.6±0.7 mmol mol⁻¹, which was much lower than the average values observed for the two other sites (aspen: 5.4±2.3, Douglas-fir: 8.1±2.4). The differences in WUE among sites were primarily because of variation in ET. The highest maximum ET rates were approximately 5, 3.2 and 2.7 mm day⁻¹ for the grassland, aspen and Douglas-fir sites, respectively. There was a strong negative correlation between WUE and D_max for all sites. We also made seasonal measurements of the carbon isotope ratio of ecosystem respired CO₂ (δ_R) in order to test for the expected correlation between shifts in environmental conditions and changes to the ecosystem-integrated ratio of leaf intercellular to ambient CO₂ concentration (c_i/c_a). There was a consistent increase in δ_R values in the grassland, aspen forest and Douglas-fir forest associated with a seasonal reduction in soil moisture. Comparisons were made between WUE measured using eddy covariance with that calculated based on D and δ_R measurements. There was excellent agreement between WUE values calculated using the two techniques. Our δ_R measurements indicated that c_i/c_avalues 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 c_i/c_a values for the D of their habitat. By contrast, the longer-lived Douglas-fir trees were more conservative in water-use with lower c_i/c_a values relative to their habitat D. This illustrates the interaction between biological and environmental characteristics influencing ecosystem-level WUE. The strong correlation we observed between the two independent measurements of WUE, indicates that the stable isotope composition of respired CO₂ is a useful ecosystem-scale tool to help study constraints to photosynthesis and acclimation of ecosystems to environmental stress.