Modeling CO₂ and water vapor turbulent flux distributions within a forest canopy

One-dimensional multilayer biosphere-atmosphere models (e.g., CANVEG) describe ecosystem carbon dioxide (CO₂) and water vapor (H₂O) fluxes well when cold temperatures or the hydrologic state of the ecosystem do not induce stomatal closure. To investigate the CANVEG model framework under such conditions, CO₂, H₂O, and sensible heat fluxes were measured with eddy-covariance methods together with xylem sap flux and leaf-level gas exchange in a 16-year-old (in 1999) southeastern loblolly pine forest. Leaf-level gas exchange measurements, collected over a 3-year period, provided all the necessary biochemical and physiological parameters for the CANVEG model. Using temperature-induced reductions of the biochemical kinetic rate constants, the CANVEG approach closely captures the diurnal patterns of the CO₂ and H₂O fluxes for two different formulations of the maximum Rubisco catalytic capacity (V_{c max}) - temperature function, suggesting that the CANVEG approach is not sensitive to V_{c max} variations for low temperatures. A soil moisture correction (w_r) to the Ball-Berry leaf-conductance approach was also proposed and tested. The w_r magnitude is consistent with values predicted by a root-xylem hydraulic approach and with leaf-level measurements. The w_r correction significantly improves the model's ability to capture diurnal patterns of H₂O fluxes for drought conditions. The modeled bulk canopy conductance (G_m) for pine foliage estimated from the CANVEG-modeled multilevel resistance values agreed well with canopy conductance (G_c) independently estimated from pine sap flux measurements. Detailed sensitivity analysis suggests that the leaf-level physiological parameters used in CANVEG are not static. The dynamic property of the conductance parameter, inferred from such sensitivity analysis, was further supported using 3 years of porometry measurements. The CANVEG model also reproduced basic biochemical processes as demonstrated by the agreement between modeled and leaf-level measured C_i/C_a, where C_i and C_a are the intercellular and atmospheric CO₂ concentration, respectively. The model estimated that vapor pressure deficit does not vary significantly within the canopy but that C_i/C_a varied by more than 15%. The broader implication of this variation is that "big-leaf" approaches that compress physiological and biochemical parameters into bulk canopy stomatal properties may be suitable for estimating water vapor flux but biased for CO₂ ecosystem fluxes.