HR: 13:40h
AN: B13B-01 INVITED     [Abstracts]
TI: Comparison of Ecosystem Water-use Efficiency Among Douglas fir Forest, Aspen Forest and Grassland Using Eddy Covariance and Carbon Isotope Techniques
AU: * Flanagan, L B
EM: larry.flanagan@uleth.ca
AF: University of Lethbridge, Department of Biological Sciences, 4401 University Drive, Lethbridge, AB T1K 3M4 Canada
AU: Ponton, S
EM: stephane.ponton@cirad.fr
AF: University of Lethbridge, Department of Biological Sciences, 4401 University Drive, Lethbridge, AB T1K 3M4 Canada
AU: Alstad, K P
EM: kalstad@uvic.ca
AF: University of Lethbridge, Department of Biological Sciences, 4401 University Drive, Lethbridge, AB T1K 3M4 Canada
AU: Johnson, B G
EM: bruce.johnson@uleth.ca
AF: University of Lethbridge, Department of Biological Sciences, 4401 University Drive, Lethbridge, AB T1K 3M4 Canada
AU: Morgenstern, K
EM: kai.morgenstern@ubc.ca
AF: University of British Columbia, Faculty of Agricultural Sciences, 135-2357 Main Mall, Vancouver, BC V6T 1Z4 Canada
AU: Kljun, N
EM: natascha.kljun@env.ethz.ch
AF: Institute for Atmospheric and Climate Science, ETH Zurich, Winterhurerstr. 190, Zurich, CH-8057 Switzerland
AU: Black, T A
EM: andrew.black@ubc.ca
AF: University of British Columbia, Faculty of Agricultural Sciences, 135-2357 Main Mall, Vancouver, BC V6T 1Z4 Canada
AU: Barr, A G
EM: Alan.Barr@ec.gc.ca
AF: Climate Research Branch, Meteorological Service of Canada, 11 Innovation Blvd., Saskatoon, SK S7N 3H5 Canada
AB: Comparisons were made among Douglas fir forest, aspen (broad leaf deciduous) forest and wheatgrass (C3) grassland for ecosystem-level water-use efficiency. Water-use efficiency (WUE) was defined as the ratio of photosynthetic CO₂ assimilation rate and evapo-transpiration (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, 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. The average WUE at the grassland site was 2.6 mmol mol^-1, which was much lower than the average 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 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 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 atmospheric saturation deficit 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_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 water-use efficiency.
DE: 0400 BIOGEOSCIENCES
DE: 0428 Carbon cycling (4806)
DE: 0476 Plant ecology (1851)
DE: 1615 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912)
SC: Biogeosciences [B]
MN: Fall Meeting 2005