Sensitivity of simulated terrestrial carbon assimilation and canopy transpiration to different stomatal conductance and carbon assimilation schemes

Accurate simulations of terrestrial carbon assimilation and canopy transpiration are needed for both climate modeling and vegetation dynamics. Coupled stomatal conductance and carbon assimilation (A - g_s) models have been widely used as part of land surface parameterizations in climate models to describe the biogeophysical and biogeochemical roles of terrestrial vegetation. Differences in various A - g_s schemes produce substantial differences in the estimation of carbon assimilation and canopy transpiration, as well as in other land-atmosphere fluxes. The terrestrial carbon assimilation and canopy transpiration simulated by two different representative A - g_s schemes, a simple A-g_s scheme adopted from the treatments of the NCAR model (Scheme I) and a two-big-leaf A - g_s scheme newly developed by Dai et al. (J Clim 17:2281-2299, 2004) (Scheme II), are compared via some sensitivity experiments to investigate impacts of different A - g_s schemes on the simulations. Major differences are found in the estimate of canopy carbon assimilation rate, canopy conductance and canopy transpiration between the two schemes, primarily due to differences in (a) functional forms used to estimate parameters for carbon assimilation sub-models, (b) co-limitation methods used to estimate carbon assimilation rate from the three limiting rates, and (c) leaf-to-canopy scaling schemes. On the whole, the differences in the scaling approach are the largest contributor to the simulation discrepancies, but the different methods of co-limitation of assimilation rate also impact the results. Except for a few biomes, the residual effects caused by the different parameter estimations in assimilation sub-models are relatively small. It is also noted that the two-leaf temperature scheme produces distinctly different sunlit and shaded leaf temperatures but has negligible impacts on the simulation of the carbon assimilation.