Comparison of ecosystem water‐use efficiency among Douglas‐fir forest, aspen forest and grassland using eddy covariance and carbon isotope techniques

Abstract

Comparisons were made among Douglas-fir forest, aspen (broad leaf deciduous) forest and wheatgrass (C 3 ) grassland for ecosystem-level water-use efficiency (WUE). WUE was defined as the ratio of photosynthetic CO 2 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 ( AE SD) WUE at the grassland site was 2.6 AE 0.7 mmol mol À1 , which was much lower than the average values observed for the two other sites (aspen: 5.4 AE 2.3, Douglasfir: 8.1 AE 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 À1 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 CO 2 (d R ) in order to test for the expected correlation between shifts in environmental conditions and changes to the ecosystemintegrated ratio of leaf intercellular to ambient CO 2 concentration (c i /c a ). There was a consistent increase in d 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 d R measurements. There was excellent agreement between WUE values calculated using the two techniques. Our d R measurements indicated that c i /c a 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 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 CO 2 is a useful ecosystem-scale tool to help study constraints to photosynthesis and acclimation of ecosystems to environmental stress.

Soil n‐alkane δ13C along a mountain slope as an integrator of altitude effect on plant species δ13C

“…Surface litter and SOM d 13 C show consistent relationship with d 13 C of CO 2 respired by the ecosystem representing a spatially integrated estimate of isotope signals [Ponton et al, 2006]. Therefore more investigations on community-and landscape-level d 13 C possibly recorded in soils from different climate areas are of significance.…”

Progress and challenges in using stable isotopes to trace plant carbon and water relations across scales

“…4) was used widely as an indicator of water use efficiency at the leaf, plant and ecosystem scale (e.g. Bonal et al, 2000;Lauteri et al, 2004;Saurer et al, 2004;Ponton et al, 2006) in retrospective studies of carbon-water relations based on biological archives (see Sect. 2.6), and in breeding crop varieties for improved yield under water-limited conditions (Condon et al, 2002).…”

Evaluating Ecohydrological Impacts of Vegetation Activities on Climatological Perspectives Using MODIS Gross Primary Productivity and Evapotranspiration Products at Korean Regional Flux Network Site

“…Yu et al [18] analyzed the seasonal variation of the WUE in an ecosystem and demonstrated spatial patterns of ecosystem variables such as GPP, ET, and WUE at 1 km spatial resolution over three local flux tower sites in China between 2003 and 2005. Despite the completion of many studies focused on flux tower measurements, only a limited number of flux tower field measurements for WUE have been available in recent decades, and for only a few ecosystems, in comparison to the development of eddy covariance systems [24][25][26] …”