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

One-dimensional multilayer biosphere-atmosphere models (e.g., CANVEG) descr ibe ecosystem carbon dioxide (CO2) and water vapor (H2O) fluxes well when c old temperatures or the hydrologic state of the ecosystem do not induce sto matal closure. To investigate the CANVEG model framework under such conditi ons, 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-ye ar-old (in 1999) southeastern loblolly pine forest. Leaf-level gas exchange measurements, collected over a 3-year period, provided all the necessary b iochemical and physiological parameters for the CANVEG model. Using tempera ture-induced reductions of the biochemical kinetic rate constants, the CANV EG 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 corr ection (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 ro ot-xylem hydraulic approach and with leaf-level measurements. The w(r) corr ection significantly improves the model's ability to capture diurnal patter ns of H2O fluxes for drought conditions. The modeled bulk canopy conductanc e (G(m)) for pine foliage estimated from the CANVEG-modeled multilevel resi stance values agreed well with canopy conductance (G(c)) independently esti mated from pine sap flux measurements. Detailed sensitivity analysis sugges ts that the leaf-level physiological parameters used in CANVEG are not stat ic. The dynamic property of the conductance parameter, inferred from such s ensitivity analysis, was further supported using 3 years of porometry measu rements. The CANVEG model also reproduced basic biochemical processes as de monstrated 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 va ry 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 t hat compress physiological and biochemical parameters into bulk canopy stom atal properties may be suitable for estimating water vapor flux but biased for CO2 ecosystem fluxes.