STOMATAL CONTROL OF TRANSPIRATION - EXAMINATION OF MONTEITHS FORMULATION OF CANOPY RESISTANCE

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 b etween 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 mi nimum canopy resistance, obtained by extrapolation to zero transpirati on, and E, is a notional maximum transpiration rate, obtained by extra polation to infinite resistance. The exact significance and possible v alues of these parameters have not been specified yet. In this study w e show that this apparently new relation can be inferred from the comm on Jarvis-type models, in which canopy stomatal resistance is expresse d in the form of a minimal resistance multiplied by a product of indep endent stress functions (each one representing the influence of one fa ctor). This is made possible by replacing leaf water potential in the corresponding stress function by its dependence on transpiration and s oil water potential. The matching of the two formulations (Monteith an d Jarvis) allows one to express the two parameters r(sn) and E-x in te rms 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 pote ntial is equal to the soil water potential, all other conditions being equal. E-x depends upon soil water potential and represents the maxim um flux of water which can be extracted from the soil by the canopy.