HR: 1340h
AN: B13B-1192    [Abstracts]
TI: Temporal Dynamics and Environmental Controls on Carbon Isotope Discrimination at the Canopy Scale
AU: * Billmark, K A
EM: kaycie@umn.edu
AF: Department of Soil, Water, and Climate, University of Minnesota - Twin Cities Borlaug Hall 1991 Upper Buford Circle, St. Paul, MN 55108, United States
AU: Griffis, T J
EM: tgriffis@umn.edu
AF: Department of Soil, Water, and Climate, University of Minnesota - Twin Cities Borlaug Hall 1991 Upper Buford Circle, St. Paul, MN 55108, United States
AU: Lee, X
EM: xuhui.lee@yale.edu
AF: School of Forestry and Environmental Studies, Yale University 21 Sachem Street, New Haven, CT 06520, United States
AU: Welp, L R
EM: lisa.welp@yale.edu
AF: School of Forestry and Environmental Studies, Yale University 21 Sachem Street, New Haven, CT 06520, United States
AU: Baker, J M
EM: jbaker@umn.edu
AF: Department of Soil, Water, and Climate, University of Minnesota - Twin Cities Borlaug Hall 1991 Upper Buford Circle, St. Paul, MN 55108, United States
AU: Baker, J M
EM: jbaker@umn.edu
AF: USDA-ARS, University of Minnesota - Twin Cities Borlaug Hall 1991 Upper Buford Circle, St. Paul, MN 55108, United States
AB: Much is currently known about ¹³C isotopic discrimination by C₃ plants at the leaf scale. Multidisciplinary techniques from micrometeorology and the stable isotope community have exploited this knowledge to better understand the dynamic processes and environmental controls on atmosphere/biosphere exchange. Unfortunately, there remains a dearth of measurements relating carbon isotope discrimination at the canopy scale (Δ_canopy) with the net carbon ecosystem flux. Our goals here are to evaluate temporal fluctuations in Δ_canopy as a result of variable environmental conditions and to critically assess the efficacy of leaf-level assumptions applied at the canopy scale. At the University of Minnesota's Rosemount Research and Outreach Center (RROC), the exchange of ¹²CO₂ and ¹³CO₂ isotopologues are continuously measured using tunable diode laser (TDL) and micrometeorological techniques (eddy covariance-TDL and gradient-TDL methods). We utilize these data in conjunction with eddy flux and ancillary meteorological measurements to estimate Δ_canopy, a key parameter for understanding ecosystem carbon source/sink behavior. Traditionally, Δ_canopy is estimated using stomatal conductance models and leaf level isotopic discrimination parameters. In this study, we similarly calculated Δ_canopy (Big-Leaf approach), where stomatal conductance was obtained through inversion of the Penman-Monteith equation. Additionally, given the high resolution of eddy flux and isoflux measurements at the RROC site, we were able to calculate Δ_canopy using an inverse flux approach. For this approach, we partitioned the net ecosystem flux using eddy covariance measurements and a nighttime temperature regression method, and then calculated Δ_canopy from the isoflux mass balance. Both calculations of Δ_canopy emphasized the diurnal, daily and seasonal variability of this important parameter. In particular, atypically hot weather strongly influenced canopy isotope discrimination. Trends in the two Δ_canopy calculations were often similar; however, the Big-Leaf approach showed a strong dependence on the stomatal conductance calculation and was highly sensitive to the leaf-level discrimination parameters. Although the inverse flux approach also has limitations, especially regarding estimation of the daytime respiration flux, this method avoids uncertainties associated with canopy conductance and leaf-to-canopy discrimination assumptions inherent in the Big-Leaf approach. Moreover, validation of daytime respiration estimates through, for example, chamber measurements, should further improve the Δ_canopy calculation. The inverse flux approach is, therefore, a simple and robust constraint on traditional Δ_canopy estimates, and may be easily applied to isotope and flux data collected at worldwide flux network sites, such as AmeriFlux locations. This added spatial and temporal resolution in Δ_canopy made available to the modeling community may serve to improve global carbon cycle budgets.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0428 Carbon cycling (4806)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 1843 Land/atmosphere interactions (1218, 1631, 3322)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting