PARENT SESSION
Oral Session #54: Vegetation: Controls on NPP, Global Climate Change. Presiding: W. Oechel.
Wednesday, August 8, 2001. 1:00 PM to 5:00 PM. Hall of Ideas G.

Estimating canopy CO₂ uptake under elevated and ambient atmospheric CO₂ concentrations with scaled sap-flux and 13C.

Schafer, Karina¹, Oren, Ram¹, Katul, Gabriel¹, Naumburg, Elke¹, Lai, Chun-Ta¹, Ellsworth, David², ^{1 2}

ABSTRACT- One limitation of elevated CO₂ experiments is the inability to measure canopy net CO₂ uptake (Anet). A process model was developed to estimate Anet in a homogenous Pinus taeda L. stand at the Duke Forest FACTS-I (Free Air Carbon Transfer and Storage) research site, NC, USA, over 3 and a half years by combining stomatal conductance of CO ₂ (gCO ₂) with the ratio of internal to external CO ₂ concentration (Ci/Ca). The gCO₂ of the stand was estimated from sap-flux scaled mean canopy stomatal conductance of water vapor (gw). The effective Ci/Ca and Anet were estimated from a scaling scheme driven by the vertical distribution of light. The gw was calculated from water flux measurements (Granier-type sensors) under ambient CO₂ (360 mol mol-1) and elevated CO₂ (ambient + 200 mol mol-1) conditions. Long-term mean Ci/ Ca was estimated from 13C. Under ambient conditions, modeled assimilation rates agreed well with: 1) instantaneous porometry Anet measurements conducted at different levels within the canopy; 2) daily and monthly estimates of daytime carbon fluxes measured with the eddy covariance system after accounting for respiration; and 3) annual biomass increment accounting for ecosystem respiration. Demonstrating the validity of this approach for ambient conditions, the method was used to estimate Anet for elevated CO₂ conditions to quantify the CO₂ enhancement of Anet. Additional assessments of the method under elevated CO₂ conditions included comparisons to annual biomass increments.

KEY WORDS: canopy stomatal conductance, net assimilation, elevated CO2