A humidity-free calculation of canopy conductance

A response of vegetation to humidity or saturation deficit appears in all calculations of canopy water vapor and CO2 exchange as represented in Fick’s First Law of Diffusion. Two issues with calculating canopy conductance as a function of humidity are first that the conductance is often back-calculated using humidity, therefore showing relationships between canopy conductance and humidity are circular; and second that vegetation responds to the rate of water vapor or CO2 exchange, not to the gradients and no physiological mechanism has even been found that senses humidity in a leaf.

The derivation of a new calculation of canopy conductance that does not directly use humidity or saturation deficit is presented. Above an aspen forest, the diurnal and seasonal patterns of the new canopy conductance compared well with values calculated using the Penman-Monteith or Ball-Berry-type equations. The new calculations predict a linear increase in canopy conductance with an increase in the saturation deficit. The new calculation also predicts that the water use efficiency increases non-linearly with decreasing canopy conductance, and that the net photosynthetic rate is low when the canopy conductance is high. Further implications and extensions of this new humidity-free calculation of canopy conductance are discussed.