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

Increasing levels of atmospheric CO2 concentration [CO2] has caused a debat e in the scientific community over how vegetation responds to this increase - specifically in the stomatal resistance increase (SRincrease) and the tr anspiration decrease (TRdecrease). This paper presents results in changes o f stomatal resistance and transpiration from two canopy model simulations u nder 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 paramet erization of the GENESIS general circulation model (LSX) and the Penn State University Biosphere-Atmosphere Modeling Scheme (PSUBAMS). Simulations wer e performed with each model's original stomatal resistance parameterization (SRP) and then with a field-derived SRP developed from corn and soybeans f ield measurements taken during 1993-1995. For corn using the original SRP, the percentage ratio (TRdecrease/SRincreas e x 100) calculated ranged from 20 to 37% for both models and values of Ome ga (a non-dimensional 'decoupling coefficient' that relates how closely the TRdecrease is correlated with SRincrease) ranged from 0.63 to 0.80. The re sults for corn, using the field derived SRP, showed that the percentage rat io and Omega ranged from 18 to 25% and 0.75 to 0.82, respectively. For soyb eans, the values of the percentage ratio and Omega for the original SRP ran ged from 20 to 39% and 0.61 to 0.80, respectively, while for the field deri ved SRP they were 12-20% and 0.8-0.87. The model derived values for the per centage ratio were substantially less than those reported in the literature for experiments conducted at the leaf scale. It is also demonstrated, usin g these models, how inter-canopy (LSX and PSUBAMS) and mixing layer (PSUBAM S) feedbacks modify transpiration beyond those initiated by changes in stom atal resistance alone. (C) 2001 Elsevier Science B.V. All rights reserved.