The simulation of canopy transpiration under doubled CO2: The evidence and impact of feedbacks on transpiration in two 1-D soil-vegetation-atmosphere-transfer models

Increasing levels of atmospheric CO2 concentration [CO2] has caused a debate in the scientific community over how vegetation responds to this increase — specifically in the stomatal resistance increase (SRincrease) and the transpiration decrease (TRdecrease). This paper presents results in changes of stomatal resistance and transpiration from two canopy model simulations under varying environmental conditions for corn and soybeans for presentday and doubled atmospheric [CO2] conditions. The two canopy models used in the simulations were the off-line version of the land surface exchange parameterization of the GENESIS general circulation model (LSX) and the Penn State University Biosphere-Atmosphere Modeling Scheme (PSUBAMS). Simulations were performed with each model’s original stomatal resistance parameterization (SRP) and then with a field-derived SRP developed from corn and soybeans field measurements taken during 1993–1995. For corn using the original SRP, the percentage ratio (TRdecrease/SRincrease×100) calculated ranged from 20 to 37% for both models and values of Ω (a non-dimensional ‘decoupling coefficient’ that relates how closely the TRdecrease is correlated with SRincrease) ranged from 0.63 to 0.80. The results for corn, using the field derived SRP, showed that the percentage ratio and Ω ranged from 18 to 25% and 0.75 to 0.82, respectively. For soybeans, the values of the percentage ratio and Ω for the original SRP ranged from 20 to 39% and 0.61 to 0.80, respectively, while for the field derived SRP they were 12–20% and 0.80–0.87. The model derived values for the percentage ratio were substantially less than those reported in the literature for experiments conducted at the leaf scale. It is also demonstrated, using these models, how inter-canopy (LSX and PSUBAMS) and mixing layer (PSUBAMS) feedbacks modify transpiration beyond those initiated by changes in stomatal resistance alone.