Stomatal control of transpiration: Examination of Monteith's formulation of canopy resistance

Abstract

The stomatal response to air humidity has been recently reinterpreted in the sense that stomata seem to respond to the rate of transpiration rather to air humidity per se. Monteith suggested that the relation between canopy stomatal resistance r(s) and canopy transpiration E can be written as r(s)/r(sn) = 1/(1 - E/E(x)), where r(sn) is a notional minimum canopy resistance, obtained by extrapolation to zero transpiration, and E(x) is a notional maximum transpiration rate, obtained by extrapolation to infinite resistance. The exact significance and possible values of these parameters have not been specified yet. In this study we show that this apparently new relation can be inferred from the common Jarvis-type models, in which canopy stomatal resistance is expressed in the form of a minimal resistance multiplied by a product of independent stress functions (each one representing the influence of one factor). This is made possible by replacing leaf water potential in the corresponding stress function by its dependence on transpiration and soil water potential. The matching of the two formulations (Monteith and Jarvis) allows one to express the two parameters r(sn) and E(x) in terms of the functions and parameters making up the Jarvis-type models; r(sn) appears to depend upon solar radiation and soil water potential: it represents the canopy stomatal resistance when the leaf water potential is equal to the soil water potential, all other conditions being equal. E(x) depends upon soil water potential and represents the maximum flux of water which can be extracted from the soil by the canopy.