Leaf gas exchange measurement for steady-state stomatal conductance model calibration.

Stomatal conductance models are essential components of crop and land surface models, but collecting data to calibrate them remains challenging due to large leaf-to-leaf variability, slow stomatal kinetics, and a lack of consistent measurement protocols, leading to unknown reliability and representativeness of calibrated model parameter estimates. We combined field measurements, 3D biophysical simulations, and statistical power analyses to quantify parameter calibration discrepancies with different instruments under different conditions to provide recommendations for protocol development. Leaf-to-leaf physiological variability in measured steady-state stomatal conductance exceeded threefold under identical conditions, calling into question the use of few steady-state response curves to represent a canopy. Stomatal kinetics introduce systematic error in parameter calibration, and slower stomatal response times necessitated larger survey sample sizes to recover known stomatal model parameters of simulated data. Primary recommendations are as follows: survey measurements (c. 100 samples) are needed to sample leaf-to-leaf variability and can be supplemented by steady-state measurements to better represent environmental responses, survey measurements should maximize the range of leaf-level environmental conditions while minimizing transient effects, and steady-state measurements with controlled environmental conditions should maintain constant conditions for 15-45 min before measurement to allow for true stomatal steady state and not just instrument equilibrium.